diff --git a/posix.cc b/posix.cc
index 3b02dd0b..7ef6b7b9 100644
--- a/posix.cc
+++ b/posix.cc
@@ -53,6 +53,20 @@
 
 #endif  // _WIN32
 
+#if FMT_GCC_VERSION >= 407
+# define FMT_UNUSED __attribute__((unused))
+#else
+# define FMT_UNUSED
+#endif
+
+#if FMT_USE_STATIC_ASSERT
+# define FMT_STATIC_ASSERT(cond, message) static_assert(cond, message)
+#else
+# define FMT_CONCAT_(a, b) FMT_CONCAT(a, b)
+# define FMT_STATIC_ASSERT(cond, message) \
+  typedef int FMT_CONCAT_(Assert, __LINE__)[(cond) ? 1 : -1] FMT_UNUSED
+#endif
+
 namespace {
 #ifdef _WIN32
 // Return type of read and write functions.
@@ -98,13 +112,13 @@ int fmt::BufferedFile::fileno() const {
   return fd;
 }
 
-fmt::File::File(const char *path, int oflag) {
+fmt::File::File(fmt::StringRef path, int oflag) {
   int mode = S_IRUSR | S_IWUSR;
 #ifdef _WIN32
   fd_ = -1;
-  FMT_POSIX_CALL(sopen_s(&fd_, path, oflag, _SH_DENYNO, mode));
+  FMT_POSIX_CALL(sopen_s(&fd_, path.c_str(), oflag, _SH_DENYNO, mode));
 #else
-  FMT_RETRY(fd_, FMT_POSIX_CALL(open(path, oflag, mode)));
+  FMT_RETRY(fd_, FMT_POSIX_CALL(open(path.c_str(), oflag, mode)));
 #endif
   if (fd_ == -1)
     throw SystemError(errno, "cannot open file {}", path);
@@ -128,6 +142,25 @@ void fmt::File::close() {
     throw SystemError(errno, "cannot close file");
 }
 
+fmt::LongLong fmt::File::size() const {
+#ifdef _WIN32
+  LARGE_INTEGER size = {};
+  if (!GetFileSizeEx(_get_osfhandle(fd_), &size))
+    throw WindowsError(GetLastError(), "cannot get file size");
+  FMT_STATIC_ASSERT(sizeof(fmt::LongLong) >= sizeof(size.QuadPart),
+      "return type of File::size is not large enough");
+  return size.QuadPart;
+#else
+  typedef struct stat Stat;
+  Stat file_stat = Stat();
+  if (fstat(fd_, &file_stat) == -1)
+    throw SystemError(errno, "cannot get file attributes");
+  FMT_STATIC_ASSERT(sizeof(fmt::LongLong) >= sizeof(file_stat.st_size),
+      "return type of File::size is not large enough");
+  return file_stat.st_size;
+#endif
+}
+
 std::size_t fmt::File::read(void *buffer, std::size_t count) {
   RWResult result = 0;
   FMT_RETRY(result, FMT_POSIX_CALL(read(fd_, buffer, convert_rwcount(count))));
diff --git a/posix.h b/posix.h
index 0b7965a4..96a4e973 100644
--- a/posix.h
+++ b/posix.h
@@ -206,7 +206,7 @@ class File {
   File() FMT_NOEXCEPT(true) : fd_(-1) {}
 
   // Opens a file and constructs a File object representing this file.
-  File(const char *path, int oflag);
+  File(fmt::StringRef path, int oflag);
 
 #if !FMT_USE_RVALUE_REFERENCES
   // Emulate a move constructor and a move assignment operator if rvalue
@@ -277,6 +277,9 @@ class File {
   // Closes the file.
   void close();
 
+  // Returns the file size.
+  fmt::LongLong size() const;
+
   // Attempts to read count bytes from the file into the specified buffer.
   std::size_t read(void *buffer, std::size_t count);
 
diff --git a/test/posix-test.cc b/test/posix-test.cc
index af9f06e8..7b6c3f2e 100644
--- a/test/posix-test.cc
+++ b/test/posix-test.cc
@@ -194,6 +194,17 @@ TEST(FileTest, CloseNoRetry) {
   close_count = 0;
 }
 
+TEST(FileTest, Size) {
+  BufferedFile bf("test", "w");
+  std::string content = "top secret, destroy before reading";
+  bf.print(content);
+  bf.close();
+  File f("test", File::RDONLY);
+  EXPECT_EQ(content.size(), f.size());
+  // TODO: test if size can handle large file sizes
+  // TODO: test FMT_STATIC_ASSERT
+}
+
 TEST(FileTest, ReadRetry) {
   File read_end, write_end;
   File::pipe(read_end, write_end);