diff --git a/include/spdlog/sinks/rotating_file_sink.h b/include/spdlog/sinks/rotating_file_sink.h
index 6fc1627c..cd5450eb 100644
--- a/include/spdlog/sinks/rotating_file_sink.h
+++ b/include/spdlog/sinks/rotating_file_sink.h
@@ -23,7 +23,7 @@ namespace sinks {
 //
 // Rotating file sink based on size
 //
-template<typename Mutex>
+template<typename Mutex, typename FileNameCalc = details::filename_calculator>
 class rotating_file_sink final : public base_sink<Mutex>
 {
 public:
@@ -32,28 +32,10 @@ public:
         , max_size_(max_size)
         , max_files_(max_files)
     {
-        file_helper_.open(calc_filename(base_filename_, 0));
+        file_helper_.open(FileNameCalc::calc_filename(base_filename_, 0));
         current_size_ = file_helper_.size(); // expensive. called only once
     }
 
-    // calc filename according to index and file extension if exists.
-    // e.g. calc_filename("logs/mylog.txt, 3) => "logs/mylog.3.txt".
-    static filename_t calc_filename(const filename_t &filename, std::size_t index)
-    {
-        typename std::conditional<std::is_same<filename_t::value_type, char>::value, fmt::memory_buffer, fmt::wmemory_buffer>::type w;
-        if (index != 0u)
-        {
-            filename_t basename, ext;
-            std::tie(basename, ext) = details::file_helper::split_by_extenstion(filename);
-            fmt::format_to(w, SPDLOG_FILENAME_T("{}.{}{}"), basename, index, ext);
-        }
-        else
-        {
-            fmt::format_to(w, SPDLOG_FILENAME_T("{}"), filename);
-        }
-        return fmt::to_string(w);
-    }
-
 protected:
     void sink_it_(const details::log_msg &msg) override
     {
@@ -85,20 +67,20 @@ private:
         file_helper_.close();
         for (auto i = max_files_; i > 0; --i)
         {
-            filename_t src = calc_filename(base_filename_, i - 1);
+            filename_t src = FileNameCalc::calc_filename(base_filename_, i - 1);
             if (!details::file_helper::file_exists(src))
             {
                 continue;
             }
-            filename_t target = calc_filename(base_filename_, i);
+            filename_t target = FileNameCalc::calc_filename(base_filename_, i);
 
-            if (!rename_file(src, target))
+            if (!FileNameCalc::rename_file(src, target))
             {
                 // if failed try again after a small delay.
                 // this is a workaround to a windows issue, where very high rotation
                 // rates can cause the rename to fail with permission denied (because of antivirus?).
                 details::os::sleep_for_millis(100);
-                if (!rename_file(src, target))
+                if (!FileNameCalc::rename_file(src, target))
                 {
                     file_helper_.reopen(true); // truncate the log file anyway to prevent it to grow beyond its limit!
                     throw spdlog_ex(
@@ -109,15 +91,6 @@ private:
         file_helper_.reopen(true);
     }
 
-    // delete the target if exists, and rename the src file  to target
-    // return true on success, false otherwise.
-    bool rename_file(const filename_t &src_filename, const filename_t &target_filename)
-    {
-        // try to delete the target file in case it already exists.
-        (void)details::os::remove(target_filename);
-        return details::os::rename(src_filename, target_filename) == 0;
-    }
-
     filename_t base_filename_;
     std::size_t max_size_;
     std::size_t max_files_;