diff --git a/packager/media/formats/webvtt/webvtt_file_buffer.cc b/packager/media/formats/webvtt/webvtt_file_buffer.cc
index d73a4657d6..02507c1858 100644
--- a/packager/media/formats/webvtt/webvtt_file_buffer.cc
+++ b/packager/media/formats/webvtt/webvtt_file_buffer.cc
@@ -73,10 +73,12 @@ void WebVttFileBuffer::Append(const TextSample& sample) {
   buffer_.append("\n");  // end of sample
 }
 
-bool WebVttFileBuffer::WriteTo(File* file) {
+bool WebVttFileBuffer::WriteTo(File* file, uint64_t* size) {
   DCHECK(file);
   DCHECK_GT(buffer_.size(), 0u) << "The buffer should at least have a header";
 
+  if (size)
+    *size = buffer_.size();
   const int written = file->Write(buffer_.c_str(), buffer_.size());
   if (written < 0) {
     return false;
diff --git a/packager/media/formats/webvtt/webvtt_file_buffer.h b/packager/media/formats/webvtt/webvtt_file_buffer.h
index 04f8cede61..5656ba93d5 100644
--- a/packager/media/formats/webvtt/webvtt_file_buffer.h
+++ b/packager/media/formats/webvtt/webvtt_file_buffer.h
@@ -27,7 +27,7 @@ class WebVttFileBuffer {
   void Reset();
   void Append(const TextSample& sample);
 
-  bool WriteTo(File* file);
+  bool WriteTo(File* file, uint64_t* size);
 
   // Get the number of samples that have been appended to this file.
   size_t sample_count() const { return sample_count_; }
diff --git a/packager/media/formats/webvtt/webvtt_muxer.cc b/packager/media/formats/webvtt/webvtt_muxer.cc
index b3cb7865b6..b4c031ff51 100644
--- a/packager/media/formats/webvtt/webvtt_muxer.cc
+++ b/packager/media/formats/webvtt/webvtt_muxer.cc
@@ -45,15 +45,12 @@ Status WebVttMuxer::WriteToFile(const std::string& filename, uint64_t* size) {
     return Status(error::FILE_FAILURE, "Failed to open " + filename);
   }
 
-  buffer_->WriteTo(file.get());
+  buffer_->WriteTo(file.get(), size);
   buffer_->Reset();
   if (!file.release()->Close()) {
     return Status(error::FILE_FAILURE, "Failed to close " + filename);
   }
 
-  if (size) {
-    *size = File::GetFileSize(filename.c_str());
-  }
   return Status::OK;
 }