diff --git a/include/boost/core/bit.hpp b/include/boost/core/bit.hpp
index 22f098d..613e71b 100644
--- a/include/boost/core/bit.hpp
+++ b/include/boost/core/bit.hpp
@@ -17,6 +17,7 @@
 
 #include <boost/config.hpp>
 #include <boost/static_assert.hpp>
+#include <boost/cstdint.hpp>
 #include <limits>
 #include <cstring>
 
@@ -28,8 +29,9 @@ namespace core
 {
 
 // bit_cast
+
 template<class To, class From>
-To bit_cast( From const & from) BOOST_NOEXCEPT
+To bit_cast( From const & from ) BOOST_NOEXCEPT
 {
     BOOST_STATIC_ASSERT( sizeof(To) == sizeof(From) );
 
@@ -39,6 +41,17 @@ To bit_cast( From const & from) BOOST_NOEXCEPT
 }
 
 // counting
+
+namespace detail
+{
+
+BOOST_CXX14_CONSTEXPR int countl_impl( boost::uint32_t x ) BOOST_NOEXCEPT
+{
+    return 0;
+}
+
+} // namespace detail
+
 template<class T>
 BOOST_CORE_BIT_CONSTEXPR int countl_zero( T x ) BOOST_NOEXCEPT;
 
@@ -48,8 +61,56 @@ BOOST_CORE_BIT_CONSTEXPR int countl_one( T x ) BOOST_NOEXCEPT
     return boost::core::countl_zero( ~x );
 }
 
+namespace detail
+{
+
+inline int countr_impl( boost::uint32_t x ) BOOST_NOEXCEPT
+{
+    static unsigned char const mod37[] = { 32, 0, 1, 26, 2, 23, 27, 0, 3, 16, 24, 30, 28, 11, 0, 13, 4, 7, 17, 0, 25, 22, 31, 15, 29, 10, 12, 6, 0, 21, 14, 9, 5, 20, 8, 19, 18 };
+    return mod37[ ( -(boost::int32_t)x & x ) % 37 ];
+}
+
+inline int countr_impl( boost::uint64_t x ) BOOST_NOEXCEPT
+{
+    return static_cast<boost::uint32_t>( x ) != 0?
+        boost::core::detail::countr_impl( static_cast<boost::uint32_t>( x ) ):
+        boost::core::detail::countr_impl( static_cast<boost::uint32_t>( x >> 32 ) ) + 32;
+}
+
+inline int countr_impl( boost::uint8_t x ) BOOST_NOEXCEPT
+{
+    return boost::core::detail::countr_impl( static_cast<boost::uint32_t>( x ) | 0x100 );
+}
+
+inline int countr_impl( boost::uint16_t x ) BOOST_NOEXCEPT
+{
+    return boost::core::detail::countr_impl( static_cast<boost::uint32_t>( x ) | 0x10000 );
+}
+
+} // namespace detail
+
 template<class T>
-BOOST_CORE_BIT_CONSTEXPR int countr_zero( T x ) BOOST_NOEXCEPT;
+BOOST_CORE_BIT_CONSTEXPR int countr_zero( T x ) BOOST_NOEXCEPT
+{
+    BOOST_STATIC_ASSERT( sizeof(T) == sizeof(boost::uint8_t) || sizeof(T) == sizeof(boost::uint16_t) || sizeof(T) == sizeof(boost::uint32_t) || sizeof(T) == sizeof(boost::uint64_t) );
+
+    if( sizeof(T) == sizeof(boost::uint8_t) )
+    {
+        return boost::core::detail::countr_impl( static_cast<boost::uint8_t>( x ) );
+    }
+    else if( sizeof(T) == sizeof(boost::uint16_t) )
+    {
+        return boost::core::detail::countr_impl( static_cast<boost::uint16_t>( x ) );
+    }
+    else if( sizeof(T) == sizeof(boost::uint32_t) )
+    {
+        return boost::core::detail::countr_impl( static_cast<boost::uint32_t>( x ) );
+    }
+    else
+    {
+        return boost::core::detail::countr_impl( static_cast<boost::uint64_t>( x ) );
+    }
+}
 
 template<class T>
 BOOST_CORE_BIT_CONSTEXPR int countr_one( T x ) BOOST_NOEXCEPT
@@ -61,6 +122,7 @@ template<class T>
 BOOST_CORE_BIT_CONSTEXPR int popcount( T x ) BOOST_NOEXCEPT;
 
 // rotating
+
 template<class T>
 BOOST_CXX14_CONSTEXPR T rotl( T x, int s ) BOOST_NOEXCEPT
 {
@@ -76,6 +138,7 @@ BOOST_CXX14_CONSTEXPR T rotr( T x, int s ) BOOST_NOEXCEPT
 }
 
 // integral powers of 2
+
 template<class T>
 BOOST_CONSTEXPR bool has_single_bit( T x ) BOOST_NOEXCEPT
 {
diff --git a/test/Jamfile.v2 b/test/Jamfile.v2
index 18856f7..a927a37 100644
--- a/test/Jamfile.v2
+++ b/test/Jamfile.v2
@@ -224,6 +224,7 @@ run cmath_test.cpp ;
 
 run bit_cast_test.cpp ;
 run bit_rotate_test.cpp ;
+run bit_countr_test.cpp ;
 
 use-project /boost/core/swap : ./swap ;
 build-project ./swap ;
diff --git a/test/bit_cast_test.cpp b/test/bit_cast_test.cpp
index c9fba44..3d3408f 100644
--- a/test/bit_cast_test.cpp
+++ b/test/bit_cast_test.cpp
@@ -1,4 +1,4 @@
-// Test for boost/core/bit.hpp
+// Test for boost/core/bit.hpp (bit_cast)
 //
 // Copyright 2020 Peter Dimov
 // Distributed under the Boost Software License, Version 1.0.
diff --git a/test/bit_countr_test.cpp b/test/bit_countr_test.cpp
new file mode 100644
index 0000000..dedcf9e
--- /dev/null
+++ b/test/bit_countr_test.cpp
@@ -0,0 +1,46 @@
+// Test for boost/core/bit.hpp (countr_zero, countr_one)
+//
+// Copyright 2020 Peter Dimov
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/core/bit.hpp>
+#include <boost/core/lightweight_test.hpp>
+#include <boost/core/detail/splitmix64.hpp>
+#include <boost/cstdint.hpp>
+#include <limits>
+
+template<class T> void test_countr( T x )
+{
+    x |= 1;
+
+    for( int i = 0; i <= std::numeric_limits<T>::digits; ++i, x <<= 1 )
+    {
+        BOOST_TEST_EQ( boost::core::countr_zero( x ), i );
+        BOOST_TEST_EQ( boost::core::countr_one( static_cast<T>( ~x ) ), i );
+    }
+}
+
+int main()
+{
+    test_countr( static_cast<unsigned char>( 0 ) );
+    test_countr( static_cast<unsigned short>( 0 ) );
+    test_countr( static_cast<unsigned int>( 0 ) );
+    test_countr( static_cast<unsigned long>( 0 ) );
+    test_countr( static_cast<unsigned long long>( 0 ) );
+
+    boost::detail::splitmix64 rng;
+
+    for( int i = 0; i < 1000; ++i )
+    {
+        boost::uint64_t x = rng();
+
+        test_countr( static_cast<unsigned char>( x ) );
+        test_countr( static_cast<unsigned short>( x ) );
+        test_countr( static_cast<unsigned int>( x ) );
+        test_countr( static_cast<unsigned long>( x ) );
+        test_countr( static_cast<unsigned long long>( x ) );
+    }
+
+    return boost::report_errors();
+}
diff --git a/test/bit_rotate_test.cpp b/test/bit_rotate_test.cpp
index a81ea04..2fbc226 100644
--- a/test/bit_rotate_test.cpp
+++ b/test/bit_rotate_test.cpp
@@ -1,4 +1,4 @@
-// Test for boost/core/bit.hpp
+// Test for boost/core/bit.hpp (rotl, rotr)
 //
 // Copyright 2020 Peter Dimov
 // Distributed under the Boost Software License, Version 1.0.