diff --git a/README.md b/README.md
index dd717300..9638e348 100644
--- a/README.md
+++ b/README.md
@@ -243,7 +243,7 @@ The `add_option_function<type>(...` function will typically require the template
 🚧 Flag options specified through the `add_flag*` functions allow a syntax for the option names to default particular options to a false value or any other value if some flags are passed.  For example:
 
 ```cpp
-app.add_flag("--flag,!--no-flag,result,"help for flag"); // 🚧
+app.add_flag("--flag,!--no-flag",result,"help for flag"); // 🚧
 ```
 
 specifies that if `--flag` is passed on the command line result will be true or contain a value of 1. If `--no-flag` is
@@ -341,6 +341,8 @@ CLI11 has several Validators built-in that perform some common checks
 -   `CLI::IsMember(...)`: 🚧 Require an option be a member of a given set.  See [Transforming Validators](#transforming-validators) for more details.
 -   `CLI::Transformer(...)`: 🚧 Modify the input using a map.  See [Transforming Validators](#transforming-validators) for more details.
 -   `CLI::CheckedTransformer(...)`: 🚧 Modify the input using a map, and require that the input is either in the set or already one of the outputs of the set. See [Transforming Validators](#transforming-validators) for more details.
+-   `CLI::AsNumberWithUnit(...)`: Modify the `<NUMBER> <UNIT>` pair by matching the unit and multiplying the number by the corresponding factor. It can be used as a base for transformers, that accept things like size values (`1 KB`) or durations (`0.33 ms`).
+-   `CLI::AsSizeValue(...)`: Convert inputs like `100b`, `42 KB`, `101 Mb`, `11 Mib` to absolute values. `KB` can be configured to be interpreted as 10^3 or 2^10.
 -   `CLI::ExistingFile`: Requires that the file exists if given.
 -   `CLI::ExistingDirectory`: Requires that the directory exists.
 -   `CLI::ExistingPath`: Requires that the path (file or directory) exists.
diff --git a/include/CLI/StringTools.hpp b/include/CLI/StringTools.hpp
index aee833ab..bb4944f7 100644
--- a/include/CLI/StringTools.hpp
+++ b/include/CLI/StringTools.hpp
@@ -193,6 +193,11 @@ inline bool valid_name_string(const std::string &str) {
     return true;
 }
 
+/// Verify that str consists of letters only
+inline bool isalpha(const std::string &str) {
+    return std::all_of(str.begin(), str.end(), [](char c) { return std::isalpha(c, std::locale()); });
+}
+
 /// Return a lower case version of a string
 inline std::string to_lower(std::string str) {
     std::transform(std::begin(str), std::end(str), std::begin(str), [](const std::string::value_type &x) {
diff --git a/include/CLI/Validators.hpp b/include/CLI/Validators.hpp
index 54294743..ee4011ee 100644
--- a/include/CLI/Validators.hpp
+++ b/include/CLI/Validators.hpp
@@ -6,8 +6,10 @@
 #include "CLI/StringTools.hpp"
 #include "CLI/TypeTools.hpp"
 
+#include <cmath>
 #include <functional>
 #include <iostream>
+#include <map>
 #include <memory>
 #include <string>
 
@@ -443,14 +445,17 @@ template <typename T> std::string generate_set(const T &set) {
 }
 
 /// Generate a string representation of a map
-template <typename T> std::string generate_map(const T &map) {
+template <typename T> std::string generate_map(const T &map, bool key_only = false) {
     using element_t = typename detail::element_type<T>::type;
     using iteration_type_t = typename detail::pair_adaptor<element_t>::value_type; // the type of the object pair
     std::string out(1, '{');
     out.append(detail::join(detail::smart_deref(map),
-                            [](const iteration_type_t &v) {
-                                return detail::as_string(detail::pair_adaptor<element_t>::first(v)) + "->" +
-                                       detail::as_string(detail::pair_adaptor<element_t>::second(v));
+                            [key_only](const iteration_type_t &v) {
+                                auto res = detail::as_string(detail::pair_adaptor<element_t>::first(v));
+                                if(!key_only) {
+                                    res += "->" + detail::as_string(detail::pair_adaptor<element_t>::second(v));
+                                }
+                                return res;
                             },
                             ","));
     out.push_back('}');
@@ -505,6 +510,31 @@ auto search(const T &set, const V &val, const std::function<V(V)> &filter_functi
     return {(it != std::end(setref)), it};
 }
 
+/// Performs a *= b; if it doesn't cause integer overflow. Returns false otherwise.
+template <typename T> typename std::enable_if<std::is_integral<T>::value, bool>::type checked_multiply(T &a, T b) {
+    if(a == 0 || b == 0) {
+        a *= b;
+        return true;
+    }
+    T c = a * b;
+    if(c / a != b) {
+        return false;
+    }
+    a = c;
+    return true;
+}
+
+/// Performs a *= b; if it doesn't equal infinity. Returns false otherwise.
+template <typename T>
+typename std::enable_if<std::is_floating_point<T>::value, bool>::type checked_multiply(T &a, T b) {
+    T c = a * b;
+    if(std::isinf(c) && !std::isinf(a) && !std::isinf(b)) {
+        return false;
+    }
+    a = c;
+    return true;
+}
+
 } // namespace detail
 /// Verify items are in a set
 class IsMember : public Validator {
@@ -707,7 +737,7 @@ class CheckedTransformer : public Validator {
         : CheckedTransformer(std::forward<T>(mapping),
                              [filter_fn_1, filter_fn_2](std::string a) { return filter_fn_2(filter_fn_1(a)); },
                              other...) {}
-}; // namespace CLI
+};
 
 /// Helper function to allow ignore_case to be passed to IsMember or Transform
 inline std::string ignore_case(std::string item) { return detail::to_lower(item); }
@@ -722,6 +752,197 @@ inline std::string ignore_space(std::string item) {
     return item;
 }
 
+/// Multiply a number by a factor using given mapping.
+/// Can be used to write transforms for SIZE or DURATION inputs.
+///
+/// Example:
+///   With mapping = `{"b"->1, "kb"->1024, "mb"->1024*1024}`
+///   one can recognize inputs like "100", "12kb", "100 MB",
+///   that will be automatically transformed to 100, 14448, 104857600.
+///
+/// Output number type matches the type in the provided mapping.
+/// Therefore, if it is required to interpret real inputs like "0.42 s",
+/// the mapping should be of a type <string, float> or <string, double>.
+class AsNumberWithUnit : public Validator {
+  public:
+    /// Adjust AsNumberWithUnit behavior.
+    /// CASE_SENSITIVE/CASE_INSENSITIVE controls how units are matched.
+    /// UNIT_OPTIONAL/UNIT_REQUIRED throws ValidationError
+    ///   if UNIT_REQUIRED is set and unit literal is not found.
+    enum Options {
+        CASE_SENSITIVE = 0,
+        CASE_INSENSITIVE = 1,
+        UNIT_OPTIONAL = 0,
+        UNIT_REQUIRED = 2,
+        DEFAULT = CASE_INSENSITIVE | UNIT_OPTIONAL
+    };
+
+    template <typename Number>
+    explicit AsNumberWithUnit(std::map<std::string, Number> mapping,
+                              Options opts = DEFAULT,
+                              const std::string &unit_name = "UNIT") {
+        description(generate_description<Number>(unit_name, opts));
+        validate_mapping(mapping, opts);
+
+        // transform function
+        func_ = [mapping, opts](std::string &input) -> std::string {
+            Number num;
+
+            detail::rtrim(input);
+            if(input.empty()) {
+                throw ValidationError("Input is empty");
+            }
+
+            // Find split position between number and prefix
+            auto unit_begin = input.end();
+            while(unit_begin > input.begin() && std::isalpha(*(unit_begin - 1), std::locale())) {
+                --unit_begin;
+            }
+
+            std::string unit{unit_begin, input.end()};
+            input.resize(std::distance(input.begin(), unit_begin));
+            detail::trim(input);
+
+            if(opts & UNIT_REQUIRED && unit.empty()) {
+                throw ValidationError("Missing mandatory unit");
+            }
+            if(opts & CASE_INSENSITIVE) {
+                unit = detail::to_lower(unit);
+            }
+
+            bool converted = detail::lexical_cast(input, num);
+            if(!converted) {
+                throw ValidationError("Value " + input + " could not be converted to " + detail::type_name<Number>());
+            }
+
+            if(unit.empty()) {
+                // No need to modify input if no unit passed
+                return {};
+            }
+
+            // find corresponding factor
+            auto it = mapping.find(unit);
+            if(it == mapping.end()) {
+                throw ValidationError(unit +
+                                      " unit not recognized. "
+                                      "Allowed values: " +
+                                      detail::generate_map(mapping, true));
+            }
+
+            // perform safe multiplication
+            bool ok = detail::checked_multiply(num, it->second);
+            if(!ok) {
+                throw ValidationError(detail::as_string(num) + " multiplied by " + unit +
+                                      " factor would cause number overflow. Use smaller value.");
+            }
+            input = detail::as_string(num);
+
+            return {};
+        };
+    }
+
+  private:
+    /// Check that mapping contains valid units.
+    /// Update mapping for CASE_INSENSITIVE mode.
+    template <typename Number> static void validate_mapping(std::map<std::string, Number> &mapping, Options opts) {
+        for(auto &kv : mapping) {
+            if(kv.first.empty()) {
+                throw ValidationError("Unit must not be empty.");
+            }
+            if(!detail::isalpha(kv.first)) {
+                throw ValidationError("Unit must contain only letters.");
+            }
+        }
+
+        // make all units lowercase if CASE_INSENSITIVE
+        if(opts & CASE_INSENSITIVE) {
+            std::map<std::string, Number> lower_mapping;
+            for(auto &kv : mapping) {
+                auto s = detail::to_lower(kv.first);
+                if(lower_mapping.count(s)) {
+                    throw ValidationError("Several matching lowercase unit representations are found: " + s);
+                }
+                lower_mapping[detail::to_lower(kv.first)] = kv.second;
+            }
+            mapping = std::move(lower_mapping);
+        }
+    }
+
+    /// Generate description like this: NUMBER [UNIT]
+    template <typename Number> static std::string generate_description(const std::string &name, Options opts) {
+        std::stringstream out;
+        out << detail::type_name<Number>() << ' ';
+        if(opts & UNIT_REQUIRED) {
+            out << name;
+        } else {
+            out << '[' << name << ']';
+        }
+        return out.str();
+    }
+};
+
+/// Converts a human-readable size string (with unit literal) to uin64_t size.
+/// Example:
+///   "100" => 100
+///   "1 b" => 100
+///   "10Kb" => 10240 // you can configure this to be interpreted as kilobyte (*1000) or kibibyte (*1024)
+///   "10 KB" => 10240
+///   "10 kb" => 10240
+///   "10 kib" => 10240 // *i, *ib are always interpreted as *bibyte (*1024)
+///   "10kb" => 10240
+///   "2 MB" => 2097152
+///   "2 EiB" => 2^61 // Units up to exibyte are supported
+class AsSizeValue : public AsNumberWithUnit {
+  public:
+    using result_t = uint64_t;
+
+    /// If kb_is_1000 is true,
+    /// interpret 'kb', 'k' as 1000 and 'kib', 'ki' as 1024
+    /// (same applies to higher order units as well).
+    /// Otherwise, interpret all literals as factors of 1024.
+    /// The first option is formally correct, but
+    /// the second interpretation is more wide-spread
+    /// (see https://en.wikipedia.org/wiki/Binary_prefix).
+    explicit AsSizeValue(bool kb_is_1000) : AsNumberWithUnit(get_mapping(kb_is_1000)) {
+        if(kb_is_1000) {
+            description("SIZE [b, kb(=1000b), kib(=1024b), ...]");
+        } else {
+            description("SIZE [b, kb(=1024b), ...]");
+        }
+    }
+
+  private:
+    /// Get <size unit, factor> mapping
+    static std::map<std::string, result_t> init_mapping(bool kb_is_1000) {
+        std::map<std::string, result_t> m;
+        result_t k_factor = kb_is_1000 ? 1000 : 1024;
+        result_t ki_factor = 1024;
+        result_t k = 1;
+        result_t ki = 1;
+        m["b"] = 1;
+        for(std::string p : {"k", "m", "g", "t", "p", "e"}) {
+            k *= k_factor;
+            ki *= ki_factor;
+            m[p] = k;
+            m[p + "b"] = k;
+            m[p + "i"] = ki;
+            m[p + "ib"] = ki;
+        }
+        return m;
+    }
+
+    /// Cache calculated mapping
+    static std::map<std::string, result_t> get_mapping(bool kb_is_1000) {
+        if(kb_is_1000) {
+            static auto m = init_mapping(true);
+            return m;
+        } else {
+            static auto m = init_mapping(false);
+            return m;
+        }
+    }
+};
+
 namespace detail {
 /// Split a string into a program name and command line arguments
 /// the string is assumed to contain a file name followed by other arguments
diff --git a/tests/HelpersTest.cpp b/tests/HelpersTest.cpp
index ca376b84..6f2e0234 100644
--- a/tests/HelpersTest.cpp
+++ b/tests/HelpersTest.cpp
@@ -105,6 +105,18 @@ TEST(StringTools, flagValues) {
     EXPECT_EQ(CLI::detail::to_flag_value("475555233"), 475555233);
 }
 
+TEST(StringTools, Validation) {
+    EXPECT_TRUE(CLI::detail::isalpha(""));
+    EXPECT_TRUE(CLI::detail::isalpha("a"));
+    EXPECT_TRUE(CLI::detail::isalpha("abcd"));
+    EXPECT_FALSE(CLI::detail::isalpha("_"));
+    EXPECT_FALSE(CLI::detail::isalpha("2"));
+    EXPECT_FALSE(CLI::detail::isalpha("test test"));
+    EXPECT_FALSE(CLI::detail::isalpha("test "));
+    EXPECT_FALSE(CLI::detail::isalpha(" test"));
+    EXPECT_FALSE(CLI::detail::isalpha("test2"));
+}
+
 TEST(Trim, Various) {
     std::string s1{"  sdlfkj sdflk sd s  "};
     std::string a1{"sdlfkj sdflk sd s"};
@@ -360,6 +372,194 @@ TEST(Validators, ProgramNameSplit) {
     EXPECT_TRUE(res.second.empty());
 }
 
+TEST(CheckedMultiply, Int) {
+    int a = 10;
+    int b = -20;
+    ASSERT_TRUE(CLI::detail::checked_multiply(a, b));
+    ASSERT_EQ(a, -200);
+
+    a = 0;
+    b = -20;
+    ASSERT_TRUE(CLI::detail::checked_multiply(a, b));
+    ASSERT_EQ(a, 0);
+
+    a = 20;
+    b = 0;
+    ASSERT_TRUE(CLI::detail::checked_multiply(a, b));
+    ASSERT_EQ(a, 0);
+
+    a = std::numeric_limits<int>::max();
+    b = 1;
+    ASSERT_TRUE(CLI::detail::checked_multiply(a, b));
+    ASSERT_EQ(a, std::numeric_limits<int>::max());
+
+    a = std::numeric_limits<int>::max();
+    b = 2;
+    ASSERT_FALSE(CLI::detail::checked_multiply(a, b));
+    ASSERT_EQ(a, std::numeric_limits<int>::max());
+
+    a = std::numeric_limits<int>::max();
+    b = -1;
+    ASSERT_TRUE(CLI::detail::checked_multiply(a, b));
+    ASSERT_EQ(a, -std::numeric_limits<int>::max());
+
+    a = std::numeric_limits<int>::max();
+    b = std::numeric_limits<int>::max();
+    ASSERT_FALSE(CLI::detail::checked_multiply(a, b));
+    ASSERT_EQ(a, std::numeric_limits<int>::max());
+
+    a = std::numeric_limits<int>::min();
+    b = std::numeric_limits<int>::max();
+    ASSERT_FALSE(CLI::detail::checked_multiply(a, b));
+    ASSERT_EQ(a, std::numeric_limits<int>::min());
+
+    a = std::numeric_limits<int>::min();
+    b = 1;
+    ASSERT_TRUE(CLI::detail::checked_multiply(a, b));
+    ASSERT_EQ(a, std::numeric_limits<int>::min());
+
+    a = std::numeric_limits<int>::min();
+    b = -1;
+    ASSERT_FALSE(CLI::detail::checked_multiply(a, b));
+    ASSERT_EQ(a, std::numeric_limits<int>::min());
+
+    a = std::numeric_limits<int>::min() / 100;
+    b = 99;
+    ASSERT_TRUE(CLI::detail::checked_multiply(a, b));
+    ASSERT_EQ(a, std::numeric_limits<int>::min() / 100 * 99);
+}
+
+TEST(CheckedMultiply, SizeT) {
+    size_t a = 10;
+    size_t b = 20;
+    ASSERT_TRUE(CLI::detail::checked_multiply(a, b));
+    ASSERT_EQ(a, 200u);
+
+    a = 0u;
+    b = 20u;
+    ASSERT_TRUE(CLI::detail::checked_multiply(a, b));
+    ASSERT_EQ(a, 0u);
+
+    a = 20u;
+    b = 0u;
+    ASSERT_TRUE(CLI::detail::checked_multiply(a, b));
+    ASSERT_EQ(a, 0u);
+
+    a = std::numeric_limits<size_t>::max();
+    b = 1u;
+    ASSERT_TRUE(CLI::detail::checked_multiply(a, b));
+    ASSERT_EQ(a, std::numeric_limits<size_t>::max());
+
+    a = std::numeric_limits<size_t>::max();
+    b = 2u;
+    ASSERT_FALSE(CLI::detail::checked_multiply(a, b));
+    ASSERT_EQ(a, std::numeric_limits<size_t>::max());
+
+    a = std::numeric_limits<size_t>::max();
+    b = std::numeric_limits<size_t>::max();
+    ASSERT_FALSE(CLI::detail::checked_multiply(a, b));
+    ASSERT_EQ(a, std::numeric_limits<size_t>::max());
+
+    a = std::numeric_limits<size_t>::max() / 100;
+    b = 99u;
+    ASSERT_TRUE(CLI::detail::checked_multiply(a, b));
+    ASSERT_EQ(a, std::numeric_limits<size_t>::max() / 100u * 99u);
+}
+
+TEST(CheckedMultiply, Float) {
+    float a = 10;
+    float b = 20;
+    ASSERT_TRUE(CLI::detail::checked_multiply(a, b));
+    ASSERT_FLOAT_EQ(a, 200);
+
+    a = 0;
+    b = 20;
+    ASSERT_TRUE(CLI::detail::checked_multiply(a, b));
+    ASSERT_FLOAT_EQ(a, 0);
+
+    a = INFINITY;
+    b = 20;
+    ASSERT_TRUE(CLI::detail::checked_multiply(a, b));
+    ASSERT_FLOAT_EQ(a, INFINITY);
+
+    a = 2;
+    b = -INFINITY;
+    ASSERT_TRUE(CLI::detail::checked_multiply(a, b));
+    ASSERT_FLOAT_EQ(a, -INFINITY);
+
+    a = std::numeric_limits<float>::max() / 100;
+    b = 1;
+    ASSERT_TRUE(CLI::detail::checked_multiply(a, b));
+    ASSERT_FLOAT_EQ(a, std::numeric_limits<float>::max() / 100);
+
+    a = std::numeric_limits<float>::max() / 100;
+    b = 99;
+    ASSERT_TRUE(CLI::detail::checked_multiply(a, b));
+    ASSERT_FLOAT_EQ(a, std::numeric_limits<float>::max() / 100 * 99);
+
+    a = std::numeric_limits<float>::max() / 100;
+    b = 101;
+    ASSERT_FALSE(CLI::detail::checked_multiply(a, b));
+    ASSERT_FLOAT_EQ(a, std::numeric_limits<float>::max() / 100);
+
+    a = std::numeric_limits<float>::max() / 100;
+    b = -99;
+    ASSERT_TRUE(CLI::detail::checked_multiply(a, b));
+    ASSERT_FLOAT_EQ(a, std::numeric_limits<float>::max() / 100 * -99);
+
+    a = std::numeric_limits<float>::max() / 100;
+    b = -101;
+    ASSERT_FALSE(CLI::detail::checked_multiply(a, b));
+    ASSERT_FLOAT_EQ(a, std::numeric_limits<float>::max() / 100);
+}
+
+TEST(CheckedMultiply, Double) {
+    double a = 10;
+    double b = 20;
+    ASSERT_TRUE(CLI::detail::checked_multiply(a, b));
+    ASSERT_DOUBLE_EQ(a, 200);
+
+    a = 0;
+    b = 20;
+    ASSERT_TRUE(CLI::detail::checked_multiply(a, b));
+    ASSERT_DOUBLE_EQ(a, 0);
+
+    a = INFINITY;
+    b = 20;
+    ASSERT_TRUE(CLI::detail::checked_multiply(a, b));
+    ASSERT_DOUBLE_EQ(a, INFINITY);
+
+    a = 2;
+    b = -INFINITY;
+    ASSERT_TRUE(CLI::detail::checked_multiply(a, b));
+    ASSERT_DOUBLE_EQ(a, -INFINITY);
+
+    a = std::numeric_limits<double>::max() / 100;
+    b = 1;
+    ASSERT_TRUE(CLI::detail::checked_multiply(a, b));
+    ASSERT_DOUBLE_EQ(a, std::numeric_limits<double>::max() / 100);
+
+    a = std::numeric_limits<double>::max() / 100;
+    b = 99;
+    ASSERT_TRUE(CLI::detail::checked_multiply(a, b));
+    ASSERT_DOUBLE_EQ(a, std::numeric_limits<double>::max() / 100 * 99);
+
+    a = std::numeric_limits<double>::max() / 100;
+    b = 101;
+    ASSERT_FALSE(CLI::detail::checked_multiply(a, b));
+    ASSERT_DOUBLE_EQ(a, std::numeric_limits<double>::max() / 100);
+
+    a = std::numeric_limits<double>::max() / 100;
+    b = -99;
+    ASSERT_TRUE(CLI::detail::checked_multiply(a, b));
+    ASSERT_DOUBLE_EQ(a, std::numeric_limits<double>::max() / 100 * -99);
+
+    a = std::numeric_limits<double>::max() / 100;
+    b = -101;
+    ASSERT_FALSE(CLI::detail::checked_multiply(a, b));
+    ASSERT_DOUBLE_EQ(a, std::numeric_limits<double>::max() / 100);
+}
+
 // Yes, this is testing an app_helper :)
 TEST(AppHelper, TempfileCreated) {
     std::string name = "TestFileNotUsed.txt";
diff --git a/tests/TransformTest.cpp b/tests/TransformTest.cpp
index ba0e11a9..8bb055f2 100644
--- a/tests/TransformTest.cpp
+++ b/tests/TransformTest.cpp
@@ -431,3 +431,406 @@ TEST_F(TApp, BoundTests) {
     EXPECT_TRUE(help.find("bounded to") != std::string::npos);
     EXPECT_TRUE(help.find("[3.4 - 5.9]") != std::string::npos);
 }
+
+TEST_F(TApp, NumberWithUnitCorrecltySplitNumber) {
+    std::map<std::string, int> mapping{{"a", 10}, {"b", 100}, {"cc", 1000}};
+
+    int value = 0;
+    app.add_option("-n", value)->transform(CLI::AsNumberWithUnit(mapping));
+
+    args = {"-n", "42"};
+    run();
+    EXPECT_EQ(value, 42);
+
+    args = {"-n", "42a"};
+    run();
+    EXPECT_EQ(value, 420);
+
+    args = {"-n", "  42  cc  "};
+    run();
+    EXPECT_EQ(value, 42000);
+    args = {"-n", "  -42  cc  "};
+    run();
+    EXPECT_EQ(value, -42000);
+}
+
+TEST_F(TApp, NumberWithUnitFloatTest) {
+    std::map<std::string, double> mapping{{"a", 10}, {"b", 100}, {"cc", 1000}};
+    double value = 0;
+    app.add_option("-n", value)->transform(CLI::AsNumberWithUnit(mapping));
+
+    args = {"-n", "42"};
+    run();
+    EXPECT_DOUBLE_EQ(value, 42);
+
+    args = {"-n", ".5"};
+    run();
+    EXPECT_DOUBLE_EQ(value, .5);
+
+    args = {"-n", "42.5 a"};
+    run();
+    EXPECT_DOUBLE_EQ(value, 425);
+
+    args = {"-n", "42.cc"};
+    run();
+    EXPECT_DOUBLE_EQ(value, 42000);
+}
+
+TEST_F(TApp, NumberWithUnitCaseSensitive) {
+    std::map<std::string, int> mapping{{"a", 10}, {"A", 100}};
+
+    int value = 0;
+    app.add_option("-n", value)->transform(CLI::AsNumberWithUnit(mapping, CLI::AsNumberWithUnit::CASE_SENSITIVE));
+
+    args = {"-n", "42a"};
+    run();
+    EXPECT_EQ(value, 420);
+
+    args = {"-n", "42A"};
+    run();
+    EXPECT_EQ(value, 4200);
+}
+
+TEST_F(TApp, NumberWithUnitCaseInsensitive) {
+    std::map<std::string, int> mapping{{"a", 10}, {"B", 100}};
+
+    int value = 0;
+    app.add_option("-n", value)->transform(CLI::AsNumberWithUnit(mapping, CLI::AsNumberWithUnit::CASE_INSENSITIVE));
+
+    args = {"-n", "42a"};
+    run();
+    EXPECT_EQ(value, 420);
+
+    args = {"-n", "42A"};
+    run();
+    EXPECT_EQ(value, 420);
+
+    args = {"-n", "42b"};
+    run();
+    EXPECT_EQ(value, 4200);
+
+    args = {"-n", "42B"};
+    run();
+    EXPECT_EQ(value, 4200);
+}
+
+TEST_F(TApp, NumberWithUnitMandatoryUnit) {
+    std::map<std::string, int> mapping{{"a", 10}, {"A", 100}};
+
+    int value;
+    app.add_option("-n", value)
+        ->transform(CLI::AsNumberWithUnit(mapping,
+                                          CLI::AsNumberWithUnit::Options(CLI::AsNumberWithUnit::UNIT_REQUIRED |
+                                                                         CLI::AsNumberWithUnit::CASE_SENSITIVE)));
+
+    args = {"-n", "42a"};
+    run();
+    EXPECT_EQ(value, 420);
+
+    args = {"-n", "42A"};
+    run();
+    EXPECT_EQ(value, 4200);
+
+    args = {"-n", "42"};
+    EXPECT_THROW(run(), CLI::ValidationError);
+}
+
+TEST_F(TApp, NumberWithUnitMandatoryUnit2) {
+    std::map<std::string, int> mapping{{"a", 10}, {"B", 100}};
+
+    int value;
+    app.add_option("-n", value)
+        ->transform(CLI::AsNumberWithUnit(mapping,
+                                          CLI::AsNumberWithUnit::Options(CLI::AsNumberWithUnit::UNIT_REQUIRED |
+                                                                         CLI::AsNumberWithUnit::CASE_INSENSITIVE)));
+
+    args = {"-n", "42A"};
+    run();
+    EXPECT_EQ(value, 420);
+
+    args = {"-n", "42b"};
+    run();
+    EXPECT_EQ(value, 4200);
+
+    args = {"-n", "42"};
+    EXPECT_THROW(run(), CLI::ValidationError);
+}
+
+TEST_F(TApp, NumberWithUnitBadMapping) {
+    EXPECT_THROW(CLI::AsNumberWithUnit(std::map<std::string, int>{{"a", 10}, {"A", 100}},
+                                       CLI::AsNumberWithUnit::CASE_INSENSITIVE),
+                 CLI::ValidationError);
+    EXPECT_THROW(CLI::AsNumberWithUnit(std::map<std::string, int>{{"a", 10}, {"9", 100}}), CLI::ValidationError);
+    EXPECT_THROW(CLI::AsNumberWithUnit(std::map<std::string, int>{{"a", 10}, {"AA A", 100}}), CLI::ValidationError);
+    EXPECT_THROW(CLI::AsNumberWithUnit(std::map<std::string, int>{{"a", 10}, {"", 100}}), CLI::ValidationError);
+}
+
+TEST_F(TApp, NumberWithUnitBadInput) {
+    std::map<std::string, int> mapping{{"a", 10}, {"b", 100}};
+
+    int value;
+    app.add_option("-n", value)->transform(CLI::AsNumberWithUnit(mapping));
+
+    args = {"-n", "13 a b"};
+    EXPECT_THROW(run(), CLI::ValidationError);
+    args = {"-n", "13 c"};
+    EXPECT_THROW(run(), CLI::ValidationError);
+    args = {"-n", "a"};
+    EXPECT_THROW(run(), CLI::ValidationError);
+    args = {"-n", "12.0a"};
+    EXPECT_THROW(run(), CLI::ValidationError);
+    args = {"-n", "a5"};
+    EXPECT_THROW(run(), CLI::ValidationError);
+    args = {"-n", ""};
+    EXPECT_THROW(run(), CLI::ValidationError);
+    args = {"-n", "13 a-"};
+    EXPECT_THROW(run(), CLI::ValidationError);
+}
+
+TEST_F(TApp, NumberWithUnitIntOverflow) {
+    std::map<std::string, int> mapping{{"a", 1000000}, {"b", 100}, {"c", 101}};
+
+    int32_t value;
+    app.add_option("-n", value)->transform(CLI::AsNumberWithUnit(mapping));
+
+    args = {"-n", "1000 a"};
+    run();
+    EXPECT_EQ(value, 1000000000);
+
+    args = {"-n", "1000000 a"};
+    EXPECT_THROW(run(), CLI::ValidationError);
+
+    args = {"-n", "-1000000 a"};
+    EXPECT_THROW(run(), CLI::ValidationError);
+
+    args = {"-n", "21474836 b"};
+    run();
+    EXPECT_EQ(value, 2147483600);
+
+    args = {"-n", "21474836 c"};
+    EXPECT_THROW(run(), CLI::ValidationError);
+}
+
+TEST_F(TApp, NumberWithUnitFloatOverflow) {
+    std::map<std::string, float> mapping{{"a", 2}, {"b", 1}, {"c", 0}};
+
+    float value;
+    app.add_option("-n", value)->transform(CLI::AsNumberWithUnit(mapping));
+
+    args = {"-n", "3e+38 a"};
+    EXPECT_THROW(run(), CLI::ValidationError);
+
+    args = {"-n", "3e+38 b"};
+    run();
+    EXPECT_FLOAT_EQ(value, 3e+38);
+
+    args = {"-n", "3e+38 c"};
+    run();
+    EXPECT_FLOAT_EQ(value, 0);
+}
+
+TEST_F(TApp, AsSizeValue1000_1024) {
+    uint64_t value;
+    app.add_option("-s", value)->transform(CLI::AsSizeValue(true));
+
+    args = {"-s", "10240"};
+    run();
+    EXPECT_EQ(value, 10240u);
+
+    args = {"-s", "1b"};
+    run();
+    EXPECT_FLOAT_EQ(value, 1);
+
+    uint64_t k_value = 1000u;
+    uint64_t ki_value = 1024u;
+    args = {"-s", "1k"};
+    run();
+    EXPECT_EQ(value, k_value);
+    args = {"-s", "1kb"};
+    run();
+    EXPECT_EQ(value, k_value);
+    args = {"-s", "1 Kb"};
+    run();
+    EXPECT_EQ(value, k_value);
+    args = {"-s", "1ki"};
+    run();
+    EXPECT_EQ(value, ki_value);
+    args = {"-s", "1kib"};
+    run();
+    EXPECT_EQ(value, ki_value);
+
+    k_value = 1000ull * 1000u;
+    ki_value = 1024ull * 1024u;
+    args = {"-s", "1m"};
+    run();
+    EXPECT_EQ(value, k_value);
+    args = {"-s", "1mb"};
+    run();
+    EXPECT_EQ(value, k_value);
+    args = {"-s", "1mi"};
+    run();
+    EXPECT_EQ(value, ki_value);
+    args = {"-s", "1mib"};
+    run();
+    EXPECT_EQ(value, ki_value);
+
+    k_value = 1000ull * 1000u * 1000u;
+    ki_value = 1024ull * 1024u * 1024u;
+    args = {"-s", "1g"};
+    run();
+    EXPECT_EQ(value, k_value);
+    args = {"-s", "1gb"};
+    run();
+    EXPECT_EQ(value, k_value);
+    args = {"-s", "1gi"};
+    run();
+    EXPECT_EQ(value, ki_value);
+    args = {"-s", "1gib"};
+    run();
+    EXPECT_EQ(value, ki_value);
+
+    k_value = 1000ull * 1000u * 1000u * 1000u;
+    ki_value = 1024ull * 1024u * 1024u * 1024u;
+    args = {"-s", "1t"};
+    run();
+    EXPECT_EQ(value, k_value);
+    args = {"-s", "1tb"};
+    run();
+    EXPECT_EQ(value, k_value);
+    args = {"-s", "1ti"};
+    run();
+    EXPECT_EQ(value, ki_value);
+    args = {"-s", "1tib"};
+    run();
+    EXPECT_EQ(value, ki_value);
+
+    k_value = 1000ull * 1000u * 1000u * 1000u * 1000u;
+    ki_value = 1024ull * 1024u * 1024u * 1024u * 1024u;
+    args = {"-s", "1p"};
+    run();
+    EXPECT_EQ(value, k_value);
+    args = {"-s", "1pb"};
+    run();
+    EXPECT_EQ(value, k_value);
+    args = {"-s", "1pi"};
+    run();
+    EXPECT_EQ(value, ki_value);
+    args = {"-s", "1pib"};
+    run();
+    EXPECT_EQ(value, ki_value);
+
+    k_value = 1000ull * 1000u * 1000u * 1000u * 1000u * 1000u;
+    ki_value = 1024ull * 1024u * 1024u * 1024u * 1024u * 1024u;
+    args = {"-s", "1e"};
+    run();
+    EXPECT_EQ(value, k_value);
+    args = {"-s", "1eb"};
+    run();
+    EXPECT_EQ(value, k_value);
+    args = {"-s", "1ei"};
+    run();
+    EXPECT_EQ(value, ki_value);
+    args = {"-s", "1eib"};
+    run();
+    EXPECT_EQ(value, ki_value);
+}
+
+TEST_F(TApp, AsSizeValue1024) {
+    uint64_t value;
+    app.add_option("-s", value)->transform(CLI::AsSizeValue(false));
+
+    args = {"-s", "10240"};
+    run();
+    EXPECT_EQ(value, 10240u);
+
+    args = {"-s", "1b"};
+    run();
+    EXPECT_FLOAT_EQ(value, 1);
+
+    uint64_t ki_value = 1024u;
+    args = {"-s", "1k"};
+    run();
+    EXPECT_EQ(value, ki_value);
+    args = {"-s", "1kb"};
+    run();
+    EXPECT_EQ(value, ki_value);
+    args = {"-s", "1 Kb"};
+    run();
+    EXPECT_EQ(value, ki_value);
+    args = {"-s", "1ki"};
+    run();
+    EXPECT_EQ(value, ki_value);
+    args = {"-s", "1kib"};
+    run();
+    EXPECT_EQ(value, ki_value);
+
+    ki_value = 1024ull * 1024u;
+    args = {"-s", "1m"};
+    run();
+    EXPECT_EQ(value, ki_value);
+    args = {"-s", "1mb"};
+    run();
+    EXPECT_EQ(value, ki_value);
+    args = {"-s", "1mi"};
+    run();
+    EXPECT_EQ(value, ki_value);
+    args = {"-s", "1mib"};
+    run();
+    EXPECT_EQ(value, ki_value);
+
+    ki_value = 1024ull * 1024u * 1024u;
+    args = {"-s", "1g"};
+    run();
+    EXPECT_EQ(value, ki_value);
+    args = {"-s", "1gb"};
+    run();
+    EXPECT_EQ(value, ki_value);
+    args = {"-s", "1gi"};
+    run();
+    EXPECT_EQ(value, ki_value);
+    args = {"-s", "1gib"};
+    run();
+    EXPECT_EQ(value, ki_value);
+
+    ki_value = 1024ull * 1024u * 1024u * 1024u;
+    args = {"-s", "1t"};
+    run();
+    EXPECT_EQ(value, ki_value);
+    args = {"-s", "1tb"};
+    run();
+    EXPECT_EQ(value, ki_value);
+    args = {"-s", "1ti"};
+    run();
+    EXPECT_EQ(value, ki_value);
+    args = {"-s", "1tib"};
+    run();
+    EXPECT_EQ(value, ki_value);
+
+    ki_value = 1024ull * 1024u * 1024u * 1024u * 1024u;
+    args = {"-s", "1p"};
+    run();
+    EXPECT_EQ(value, ki_value);
+    args = {"-s", "1pb"};
+    run();
+    EXPECT_EQ(value, ki_value);
+    args = {"-s", "1pi"};
+    run();
+    EXPECT_EQ(value, ki_value);
+    args = {"-s", "1pib"};
+    run();
+    EXPECT_EQ(value, ki_value);
+
+    ki_value = 1024ull * 1024u * 1024u * 1024u * 1024u * 1024u;
+    args = {"-s", "1e"};
+    run();
+    EXPECT_EQ(value, ki_value);
+    args = {"-s", "1eb"};
+    run();
+    EXPECT_EQ(value, ki_value);
+    args = {"-s", "1ei"};
+    run();
+    EXPECT_EQ(value, ki_value);
+    args = {"-s", "1eib"};
+    run();
+    EXPECT_EQ(value, ki_value);
+}