diff --git a/tdutils/td/utils/port/wstring_convert.cpp b/tdutils/td/utils/port/wstring_convert.cpp
index 81d95fea..ffe7e15e 100644
--- a/tdutils/td/utils/port/wstring_convert.cpp
+++ b/tdutils/td/utils/port/wstring_convert.cpp
@@ -21,10 +21,7 @@ Result<std::wstring> to_wstring(CSlice slice) {
     return Status::Error("Wrong encoding");
   }
 
-  size_t wstring_len = 0;
-  for (auto c : slice) {
-    wstring_len += ((c & 0xc0) != 0x80) + ((c & 0xf8) == 0xf0);
-  }
+  size_t wstring_len = utf8_utf16_length(slice);
 
   std::wstring result(wstring_len, static_cast<wchar_t>(0));
   if (wstring_len) {
diff --git a/tdutils/td/utils/utf8.h b/tdutils/td/utils/utf8.h
index 398dbb09..059416cd 100644
--- a/tdutils/td/utils/utf8.h
+++ b/tdutils/td/utils/utf8.h
@@ -28,6 +28,15 @@ inline size_t utf8_length(Slice str) {
   return result;
 }
 
+/// returns length of UTF-8 string in UTF-16 code units
+inline size_t utf8_utf16_length(Slice str) {
+  size_t result = 0;
+  for (auto c : str) {
+    result += is_utf8_character_first_code_unit(c) + ((c & 0xf8) == 0xf0);
+  }
+  return result;
+}
+
 /// appends a Unicode character using UTF-8 encoding
 void append_utf8_character(string &str, uint32 ch);