diff --git a/libavformat/ivfdec.c b/libavformat/ivfdec.c
index 511f2387ed..8eeaba38ac 100644
--- a/libavformat/ivfdec.c
+++ b/libavformat/ivfdec.c
@@ -51,11 +51,18 @@ static int read_header(AVFormatContext *s)
     st->codecpar->codec_id   = ff_codec_get_id(ff_codec_bmp_tags, st->codecpar->codec_tag);
     st->codecpar->width      = avio_rl16(s->pb);
     st->codecpar->height     = avio_rl16(s->pb);
-    time_base.den         = avio_rl32(s->pb);
-    time_base.num         = avio_rl32(s->pb);
-    st->duration          = avio_rl32(s->pb);
+    time_base.den            = avio_rl32(s->pb);
+    time_base.num            = avio_rl32(s->pb);
+    st->nb_frames            = avio_rl32(s->pb);
     avio_skip(s->pb, 4); // unused
 
+    // Infer duration from nb_frames, in order to be backward compatible with
+    // previous IVF demuxer.
+    // It is popular to configure time_base to 1/frame_rate by IVF muxer, that
+    // the duration is happen to be the same with nb_frames. See
+    // `https://chromium.googlesource.com/webm/vp8-test-vectors/+/refs/heads/main`
+    st->duration             = st->nb_frames;
+
     ffstream(st)->need_parsing = AVSTREAM_PARSE_HEADERS;
 
     if (!time_base.den || !time_base.num) {
diff --git a/libavformat/ivfenc.c b/libavformat/ivfenc.c
index 47b4efbcd1..88399099d4 100644
--- a/libavformat/ivfenc.c
+++ b/libavformat/ivfenc.c
@@ -72,7 +72,8 @@ static int ivf_write_header(AVFormatContext *s)
     avio_wl16(pb, par->height);
     avio_wl32(pb, s->streams[0]->time_base.den);
     avio_wl32(pb, s->streams[0]->time_base.num);
-    avio_wl64(pb, 0xFFFFFFFFFFFFFFFFULL); // length is overwritten at the end of muxing
+    avio_wl32(pb, 0xFFFFFFFF); // "number of frames" is overwritten at the end of muxing
+    avio_wl32(pb, 0); // unused
 
     return 0;
 }
@@ -99,16 +100,12 @@ static int ivf_write_trailer(AVFormatContext *s)
     AVIOContext *pb = s->pb;
     IVFEncContext *ctx = s->priv_data;
 
-    if ((pb->seekable & AVIO_SEEKABLE_NORMAL) &&
-        (ctx->frame_cnt > 1 || (ctx->frame_cnt == 1 && ctx->last_pkt_duration))) {
+    // overwrite the "number of frames"
+    if ((pb->seekable & AVIO_SEEKABLE_NORMAL)) {
         int64_t end = avio_tell(pb);
 
         avio_seek(pb, 24, SEEK_SET);
-        // overwrite the "length" field (duration)
-        avio_wl32(pb, ctx->last_pkt_duration ?
-                  ctx->sum_delta_pts + ctx->last_pkt_duration :
-                  ctx->frame_cnt * ctx->sum_delta_pts / (ctx->frame_cnt - 1));
-        avio_wl32(pb, 0); // zero out unused bytes
+        avio_wl32(pb, ctx->frame_cnt);
         avio_seek(pb, end, SEEK_SET);
     }
 
diff --git a/tests/ref/fate/cbs-vp9-vp90-2-03-deltaq b/tests/ref/fate/cbs-vp9-vp90-2-03-deltaq
index db09cfd5e0..f621d7a480 100644
--- a/tests/ref/fate/cbs-vp9-vp90-2-03-deltaq
+++ b/tests/ref/fate/cbs-vp9-vp90-2-03-deltaq
@@ -1 +1 @@
-bb630ef560f83951fa6547a664fdb636
+fe62460fe28202e0666e628afd8602ca
diff --git a/tests/ref/fate/cbs-vp9-vp90-2-06-bilinear b/tests/ref/fate/cbs-vp9-vp90-2-06-bilinear
index f579459179..9359e21e40 100644
--- a/tests/ref/fate/cbs-vp9-vp90-2-06-bilinear
+++ b/tests/ref/fate/cbs-vp9-vp90-2-06-bilinear
@@ -1 +1 @@
-2ca9d012c7212e38f5e2727ac66ec6c5
+179e228004c396a301c89f34b6c72f68
diff --git a/tests/ref/fate/cbs-vp9-vp90-2-09-lf_deltas b/tests/ref/fate/cbs-vp9-vp90-2-09-lf_deltas
index e0b5686d0b..5b21675c76 100644
--- a/tests/ref/fate/cbs-vp9-vp90-2-09-lf_deltas
+++ b/tests/ref/fate/cbs-vp9-vp90-2-09-lf_deltas
@@ -1 +1 @@
-78f5e46bfaecbcd62b9126697a0d97b7
+1d1f0768c547461ae2abef57f0aabc24
diff --git a/tests/ref/fate/cbs-vp9-vp90-2-10-show-existing-frame b/tests/ref/fate/cbs-vp9-vp90-2-10-show-existing-frame
index 4a4d752428..19b7a78dd8 100644
--- a/tests/ref/fate/cbs-vp9-vp90-2-10-show-existing-frame
+++ b/tests/ref/fate/cbs-vp9-vp90-2-10-show-existing-frame
@@ -1 +1 @@
-eea9d10a696c6ed971e4fae9fb619b10
+13fa042ee1b4079c227a5c5c96e2db38
diff --git a/tests/ref/fate/cbs-vp9-vp90-2-10-show-existing-frame2 b/tests/ref/fate/cbs-vp9-vp90-2-10-show-existing-frame2
index 6da8999114..e7bf2a078d 100644
--- a/tests/ref/fate/cbs-vp9-vp90-2-10-show-existing-frame2
+++ b/tests/ref/fate/cbs-vp9-vp90-2-10-show-existing-frame2
@@ -1 +1 @@
-abf4c7d4be7d3576d96b6f92166b5894
+2ab7c95e4637fb6a15efd8c0a8d6af98
diff --git a/tests/ref/fate/cbs-vp9-vp90-2-segmentation-aq-akiyo b/tests/ref/fate/cbs-vp9-vp90-2-segmentation-aq-akiyo
index 12dfb10d40..f30889dbdc 100644
--- a/tests/ref/fate/cbs-vp9-vp90-2-segmentation-aq-akiyo
+++ b/tests/ref/fate/cbs-vp9-vp90-2-segmentation-aq-akiyo
@@ -1 +1 @@
-86cd3750cc9a0672717643c9b9f87fd5
+b5be66a6a8792f7aac090beb9f3b4555
diff --git a/tests/ref/fate/cbs-vp9-vp90-2-segmentation-sf-akiyo b/tests/ref/fate/cbs-vp9-vp90-2-segmentation-sf-akiyo
index c2b1b8723a..dca77f2113 100644
--- a/tests/ref/fate/cbs-vp9-vp90-2-segmentation-sf-akiyo
+++ b/tests/ref/fate/cbs-vp9-vp90-2-segmentation-sf-akiyo
@@ -1 +1 @@
-5d12fbe6220aae9e62b1d79785a83387
+7bde6532fc682bfa3f5170cf9d607865
diff --git a/tests/ref/fate/cbs-vp9-vp90-2-tiling-pedestrian b/tests/ref/fate/cbs-vp9-vp90-2-tiling-pedestrian
index f9cab39bd6..0aa3cc8ce6 100644
--- a/tests/ref/fate/cbs-vp9-vp90-2-tiling-pedestrian
+++ b/tests/ref/fate/cbs-vp9-vp90-2-tiling-pedestrian
@@ -1 +1 @@
-4c51f3c796baa7c2baa4b7ec0d011406
+1e40e8b48e4682e8b8004b9e0e60a5b6
diff --git a/tests/ref/fate/cbs-vp9-vp91-2-04-yuv440 b/tests/ref/fate/cbs-vp9-vp91-2-04-yuv440
index 6289930070..947e1229eb 100644
--- a/tests/ref/fate/cbs-vp9-vp91-2-04-yuv440
+++ b/tests/ref/fate/cbs-vp9-vp91-2-04-yuv440
@@ -1 +1 @@
-293bdc92851ca1105e27f04737d8c5f3
+9bb416c0304a13c4f66c56aef8431cd4
diff --git a/tests/ref/fate/cbs-vp9-vp91-2-04-yuv444 b/tests/ref/fate/cbs-vp9-vp91-2-04-yuv444
index 628ea9a4d9..bf251138ed 100644
--- a/tests/ref/fate/cbs-vp9-vp91-2-04-yuv444
+++ b/tests/ref/fate/cbs-vp9-vp91-2-04-yuv444
@@ -1 +1 @@
-911eafd8e442e646c5ce97d781757ca8
+3a7ed001d30f96d4888f5ca16e6263ce
diff --git a/tests/ref/fate/cbs-vp9-vp92-2-20-10bit-yuv420 b/tests/ref/fate/cbs-vp9-vp92-2-20-10bit-yuv420
index eeb7580d74..2cad8b947c 100644
--- a/tests/ref/fate/cbs-vp9-vp92-2-20-10bit-yuv420
+++ b/tests/ref/fate/cbs-vp9-vp92-2-20-10bit-yuv420
@@ -1 +1 @@
-16198c32c29228e0513004ed1bf6fcee
+7315bb7b55693a87c350b48cd2ee9811
diff --git a/tests/ref/fate/cbs-vp9-vp93-2-20-10bit-yuv422 b/tests/ref/fate/cbs-vp9-vp93-2-20-10bit-yuv422
index b25bc1166e..bb1c0f7ea7 100644
--- a/tests/ref/fate/cbs-vp9-vp93-2-20-10bit-yuv422
+++ b/tests/ref/fate/cbs-vp9-vp93-2-20-10bit-yuv422
@@ -1 +1 @@
-4bceedef4aa6a663a09761971e43b5a8
+1a7b5bf86bf0bbef10c9a1b2c799b276
diff --git a/tests/ref/fate/cbs-vp9-vp93-2-20-12bit-yuv444 b/tests/ref/fate/cbs-vp9-vp93-2-20-12bit-yuv444
index 8d122d1370..9b7b358d04 100644
--- a/tests/ref/fate/cbs-vp9-vp93-2-20-12bit-yuv444
+++ b/tests/ref/fate/cbs-vp9-vp93-2-20-12bit-yuv444
@@ -1 +1 @@
-0f413b840633bfcfcc78b4c9fab933bf
+9b7a0b7fc081542d9be1074b23054861
