diff --git a/src/http/modules/ngx_http_mp4_module.c b/src/http/modules/ngx_http_mp4_module.c
index a675212..a4a47ee 100644
--- a/src/http/modules/ngx_http_mp4_module.c
+++ b/src/http/modules/ngx_http_mp4_module.c
@@ -1066,7 +1066,6 @@
 
     atom_data_size = mp4->mdat_data.buf->file_last - start_offset;
     mp4->mdat_data.buf->file_pos = start_offset;
-    mp4->content_length += atom_data_size;
 
     ngx_log_debug2(NGX_LOG_DEBUG_HTTP, mp4->file.log, 0,
                    "mdat new offset @%O:%O", start_offset, atom_data_size);
@@ -1083,6 +1082,8 @@
         atom_header_size = sizeof(ngx_mp4_atom_header_t);
     }
 
+    mp4->content_length += atom_header_size + atom_data_size;
+
     ngx_mp4_set_32value(atom_header, atom_size);
     ngx_mp4_set_atom_name(atom_header, 'm', 'd', 'a', 't');