diff --git a/libcxx/include/__format/buffer.h b/libcxx/include/__format/buffer.h
--- a/libcxx/include/__format/buffer.h
+++ b/libcxx/include/__format/buffer.h
@@ -257,6 +257,34 @@
   __output_buffer<_CharT> __output_;
   typename __writer_selector<_OutIt, _CharT>::type __writer_;
 };
+
+/// A buffer that counts the number of insertions.
+///
+/// Since \ref formatted_size only needs to know the size the output itself is
+/// discarded.
+template <__formatter::__char_type _CharT>
+class _LIBCPP_TEMPLATE_VIS __formatted_size_buffer {
+public:
+  _LIBCPP_HIDE_FROM_ABI auto make_output_iterator() {
+    return __output_.make_output_iterator();
+  }
+
+  _LIBCPP_HIDE_FROM_ABI void flush(_CharT*, size_t __size) {
+    __size_ += __size;
+  }
+
+  _LIBCPP_HIDE_FROM_ABI size_t result() && {
+    __output_.flush();
+    return __size_;
+  }
+
+private:
+  __internal_storage<_CharT> __storage_;
+  __output_buffer<_CharT> __output_{__storage_.begin(), __storage_.capacity(),
+                                    this};
+  size_t __size_{0};
+};
+
 } // namespace __format
 
 #endif // !defined(_LIBCPP_HAS_NO_CONCEPTS)
diff --git a/libcxx/include/format b/libcxx/include/format
--- a/libcxx/include/format
+++ b/libcxx/include/format
@@ -576,6 +576,28 @@
 }
 #endif
 
+template <class... _Args>
+_LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_FORMAT size_t
+formatted_size(string_view __fmt, const _Args&... __args) {
+  __format::__formatted_size_buffer<char> __buffer;
+  _VSTD::__vformat_to(__buffer.make_output_iterator(), __fmt,
+                      basic_format_args{_VSTD::make_format_args(__args...)});
+
+  return _VSTD::move(__buffer).result();
+}
+
+#ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
+template <class... _Args>
+_LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_FORMAT size_t
+formatted_size(wstring_view __fmt, const _Args&... __args) {
+  __format::__formatted_size_buffer<wchar_t> __buffer;
+  _VSTD::__vformat_to(__buffer.make_output_iterator(), __fmt,
+                      basic_format_args{_VSTD::make_wformat_args(__args...)});
+
+  return _VSTD::move(__buffer).result();
+}
+#endif
+
 template <class _OutIt>
 struct _LIBCPP_TEMPLATE_VIS format_to_n_result {
   _OutIt out;
@@ -610,22 +632,6 @@
 }
 #endif
 
-template <class... _Args>
-_LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_FORMAT size_t
-formatted_size(string_view __fmt, const _Args&... __args) {
-  // TODO FMT Improve PoC: using std::string is inefficient.
-  return _VSTD::vformat(__fmt, _VSTD::make_format_args(__args...)).size();
-}
-
-#ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
-template <class... _Args>
-_LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_FORMAT size_t
-formatted_size(wstring_view __fmt, const _Args&... __args) {
-  // TODO FMT Improve PoC: using std::string is inefficient.
-  return _VSTD::vformat(__fmt, _VSTD::make_wformat_args(__args...)).size();
-}
-#endif
-
 #ifndef _LIBCPP_HAS_NO_LOCALIZATION
 
 template <class _OutIt, class _CharT, class _FormatOutIt>
@@ -714,6 +720,27 @@
 }
 #endif
 
+template <class... _Args>
+_LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_FORMAT size_t
+formatted_size(locale __loc, string_view __fmt, const _Args&... __args) {
+  __format::__formatted_size_buffer<char> __buffer;
+  _VSTD::__vformat_to(__buffer.make_output_iterator(), _VSTD::move(__loc),__fmt,
+                      basic_format_args{_VSTD::make_format_args(__args...)});
+  return _VSTD::move(__buffer).result();
+}
+
+#ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
+template <class... _Args>
+_LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_FORMAT size_t
+formatted_size(locale __loc, wstring_view __fmt, const _Args&... __args) {
+  __format::__formatted_size_buffer<wchar_t> __buffer;
+  _VSTD::__vformat_to(__buffer.make_output_iterator(), _VSTD::move(__loc),__fmt,
+                      basic_format_args{_VSTD::make_wformat_args(__args...)});
+
+  return _VSTD::move(__buffer).result();
+}
+#endif
+
 template <output_iterator<const char&> _OutIt, class... _Args>
 _LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_FORMAT format_to_n_result<_OutIt>
 format_to_n(_OutIt __out_it, iter_difference_t<_OutIt> __n, locale __loc,
@@ -744,26 +771,6 @@
 }
 #endif
 
-template <class... _Args>
-_LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_FORMAT size_t
-formatted_size(locale __loc, string_view __fmt, const _Args&... __args) {
-  // TODO FMT Improve PoC: using std::string is inefficient.
-  return _VSTD::vformat(_VSTD::move(__loc), __fmt,
-                        _VSTD::make_format_args(__args...))
-      .size();
-}
-
-#ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
-template <class... _Args>
-_LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_FORMAT size_t
-formatted_size(locale __loc, wstring_view __fmt, const _Args&... __args) {
-  // TODO FMT Improve PoC: using std::string is inefficient.
-  return _VSTD::vformat(_VSTD::move(__loc), __fmt,
-                        _VSTD::make_wformat_args(__args...))
-      .size();
-}
-#endif
-
 #endif // _LIBCPP_HAS_NO_LOCALIZATION
 
 #endif // !defined(_LIBCPP_HAS_NO_CONCEPTS)