diff --git a/libavformat/avformat.h b/libavformat/avformat.h
index 2427f84077..6308940ce5 100644
--- a/libavformat/avformat.h
+++ b/libavformat/avformat.h
@@ -265,6 +265,7 @@ typedef struct AVFormatParameters {
 #define AVFMT_NOTIMESTAMPS  0x0080 /**< Format does not need / have any timestamps. */
 #define AVFMT_GENERIC_INDEX 0x0100 /**< Use generic index building code. */
 #define AVFMT_TS_DISCONT    0x0200 /**< Format allows timestamp discontinuities. */
+#define AVFMT_VARIABLE_FPS  0x0400 /**< Format allows variable fps. */
 
 typedef struct AVOutputFormat {
     const char *name;
diff --git a/libavformat/avienc.c b/libavformat/avienc.c
index 8d51fa2901..0e55953850 100644
--- a/libavformat/avienc.c
+++ b/libavformat/avienc.c
@@ -606,4 +606,5 @@ AVOutputFormat avi_muxer = {
     avi_write_packet,
     avi_write_trailer,
     .codec_tag= (const AVCodecTag* const []){codec_bmp_tags, codec_wav_tags, 0},
+    .flags= AVFMT_VARIABLE_FPS,
 };
diff --git a/libavformat/matroskaenc.c b/libavformat/matroskaenc.c
index e5e87f90d0..44787be2d2 100644
--- a/libavformat/matroskaenc.c
+++ b/libavformat/matroskaenc.c
@@ -909,7 +909,7 @@ AVOutputFormat matroska_muxer = {
     mkv_write_header,
     mkv_write_packet,
     mkv_write_trailer,
-    .flags = AVFMT_GLOBALHEADER,
+    .flags = AVFMT_GLOBALHEADER | AVFMT_VARIABLE_FPS,
     .codec_tag = (const AVCodecTag* const []){codec_bmp_tags, codec_wav_tags, 0},
     .subtitle_codec = CODEC_ID_TEXT,
 };
diff --git a/libavformat/movenc.c b/libavformat/movenc.c
index 95b881aec2..b73bb15630 100644
--- a/libavformat/movenc.c
+++ b/libavformat/movenc.c
@@ -1841,7 +1841,7 @@ AVOutputFormat mov_muxer = {
     mov_write_header,
     mov_write_packet,
     mov_write_trailer,
-    .flags = AVFMT_GLOBALHEADER,
+    .flags = AVFMT_GLOBALHEADER | AVFMT_VARIABLE_FPS,
     .codec_tag = (const AVCodecTag* const []){codec_movvideo_tags, codec_movaudio_tags, 0},
 };
 #endif
@@ -1873,7 +1873,7 @@ AVOutputFormat mp4_muxer = {
     mov_write_header,
     mov_write_packet,
     mov_write_trailer,
-    .flags = AVFMT_GLOBALHEADER,
+    .flags = AVFMT_GLOBALHEADER | AVFMT_VARIABLE_FPS,
     .codec_tag = (const AVCodecTag* const []){ff_mp4_obj_type, 0},
 };
 #endif
diff --git a/libavformat/nutenc.c b/libavformat/nutenc.c
index cbea85da39..3482d5234a 100644
--- a/libavformat/nutenc.c
+++ b/libavformat/nutenc.c
@@ -820,6 +820,6 @@ AVOutputFormat nut_muxer = {
     write_header,
     write_packet,
     write_trailer,
-    .flags = AVFMT_GLOBALHEADER,
+    .flags = AVFMT_GLOBALHEADER | AVFMT_VARIABLE_FPS,
     .codec_tag= (const AVCodecTag* const []){codec_bmp_tags, codec_wav_tags, ff_nut_subtitle_tags, 0},
 };