diff --git a/include/spdlog/details/file_helper.h b/include/spdlog/details/file_helper.h
index 4c5fe237..c5923358 100644
--- a/include/spdlog/details/file_helper.h
+++ b/include/spdlog/details/file_helper.h
@@ -96,24 +96,13 @@ public:
             std::fflush(_fd);
     }
 
-    long size()
+    size_t size()
     {
         if (!_fd)
             throw spdlog_ex("Cannot use size() on closed file " + os::filename_to_str(_filename));
 
-        auto pos = ftell(_fd);
-        if (fseek(_fd, 0, SEEK_END) != 0)
-            throw spdlog_ex("fseek failed on file " + os::filename_to_str(_filename), errno);
-
-        auto file_size = ftell(_fd);
-
-        if(fseek(_fd, pos, SEEK_SET) !=0)
-            throw spdlog_ex("fseek failed on file " + os::filename_to_str(_filename), errno);
-
-        if (file_size == -1)
-            throw spdlog_ex("ftell failed on file " + os::filename_to_str(_filename), errno);
-
-        return file_size;
+		return os::filesize(_fd);
+        
     }
 
     const filename_t& filename() const
diff --git a/include/spdlog/details/os.h b/include/spdlog/details/os.h
index 384477da..d2f7e5c8 100644
--- a/include/spdlog/details/os.h
+++ b/include/spdlog/details/os.h
@@ -10,7 +10,10 @@
 #include <ctime>
 #include <functional>
 #include <string>
+#include <stdio.h>
 #include <string.h>
+#include <sys/stat.h>
+
 
 #ifdef _WIN32
 
@@ -27,10 +30,11 @@
 #include <share.h>
 #endif
 
+#include <sys/types.h>
+
 #elif __linux__
 
 #include <sys/syscall.h> //Use gettid() syscall under linux to get thread id
-#include <sys/stat.h>
 #include <unistd.h>
 #include <chrono>
 
@@ -182,6 +186,48 @@ inline bool file_exists(const filename_t& filename)
 #endif
 }
 
+//Return file size according to open FILE* object
+inline size_t filesize(FILE *f)
+{
+#ifdef _WIN32
+#if _WIN64 //64 bits
+	int fd = _fileno(f);
+	struct _stat64 st;	
+	if (_fstat64(fd, &st) == 0)
+		return st.st_size;
+	else
+		throw spdlog_ex("Failed getting file size from fd", errno);
+#else //windows 32 bits
+	int fd = _fileno(f);
+	struct _stat st;
+	if (_fstat(fd, &st) == 0)
+		return st.st_size;
+	else
+		throw spdlog_ex("Failed getting file size from fd", errno);
+#endif
+
+#else// common unix
+	#if __x86_64__ || __ppc64__ //64 bits
+	int fd = fileno(f)
+	struct stat64 st;
+	if (fstat64(fd, &st) == 0)
+		return st.st_size;
+	else
+		throw spdlog_ex("Failed getting file size from fd", errno);
+#else //unix 32 bits
+	int fd = fileno(f)
+		struct stat st;
+	if (fstat(fd, &st) == 0)
+		return st.st_size;
+	else
+		throw spdlog_ex("Failed getting file size from fd", errno);
+#endif
+#endif
+}
+
+
+
+
 //Return utc offset in minutes or throw spdlog_ex on failure
 inline int utc_minutes_offset(const std::tm& tm = details::os::localtime())
 {
diff --git a/tests/file_helper.cpp b/tests/file_helper.cpp
index e59cb88d..54d54cf1 100644
--- a/tests/file_helper.cpp
+++ b/tests/file_helper.cpp
@@ -63,7 +63,7 @@ TEST_CASE("file_helper_reopen", "[file_helper::reopen()]]")
 TEST_CASE("file_helper_reopen2", "[file_helper::reopen(false)]]")
 {
     prepare_logdir();
-    auto expected_size = 14;
+    size_t expected_size = 14;
     file_helper helper(true);
     helper.open(target_filename);
     write_with_helper(helper, expected_size);