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Javid Jamae
Javid Jamae

Posted on • Originally published at ffmpeg-micro.com

FFmpeg Segment Muxer: segment_time, strftime, and reset_timestamps

#ffmpeg #video #cli #tutorial

Originally published at ffmpeg-micro.com

The FFmpeg segment muxer splits a video into multiple files based on duration. Three flags cause most of the confusion: segment_time, strftime, and reset_timestamps. The official FFmpeg docs define each in one sentence. This guide shows what they actually do, with verified examples and the gotchas that trip people up.

Quick answer: Split a video into 5-second segments with timestamped filenames and clean playback timestamps:

ffmpeg -i input.mp4 -c copy -f segment -segment_time 5 -reset_timestamps 1 -strftime 1 "segment_%Y%m%d_%H%M%S.mp4"
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What segment_time Does (and Why Your Segments Are the Wrong Length)

segment_time sets the target duration for each segment in seconds. With -c copy (stream copy, no re-encoding), FFmpeg can only cut at keyframe boundaries. If your video has keyframes every 8 seconds and you set -segment_time 3, your segments will be roughly 8 seconds each.

Actual segment durations from a 13-second test video with -segment_time 3 -c copy:

Segment Expected Actual
output_000.mp4 ~3s 8.34s
output_001.mp4 ~3s 5.01s

The muxer waited for the next keyframe after the 3-second mark. Stream copying can't split mid-GOP because there's no re-encoding to create a new keyframe at the cut point.

Fix: force keyframes at your split points. This requires re-encoding:

ffmpeg -i input.mp4 -c:v libx264 -preset ultrafast -crf 23 \
  -force_key_frames "expr:gte(t,n_forced*3)" \
  -f segment -segment_time 3 -reset_timestamps 1 "output_%03d.mp4"
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Now the segments are exactly 3 seconds each.

reset_timestamps: Why Your Player Shows the Wrong Time

Without reset_timestamps, each segment inherits its position from the original video. Check with ffprobe: start_time=8.408000. Add -reset_timestamps 1 and it becomes start_time=0.000000.

If you're generating segments for standalone playback, always use -reset_timestamps 1.

strftime: Timestamped Filenames

With -strftime 1, use date/time tokens in the filename: segment_%Y%m%d_%H%M%S.mp4 produces files like segment_20260530_050835.mp4. You can't mix %03d and strftime in the same pattern.

Common Pitfalls

  • Segments way longer than segment_time: Using -c copy with a GOP interval larger than segment time. Re-encode with -force_key_frames or accept keyframe-aligned cuts.
  • Player shows black frames: Missing -reset_timestamps 1.
  • "Could not write header" error: Use -segment_format mp4 explicitly.

FAQ

Does segment_time guarantee exact segment durations?

Only when re-encoding. With -c copy, FFmpeg cuts at the nearest keyframe after the target time.

What's the difference between the segment muxer and HLS muxer?

The HLS muxer (-f hls) is purpose-built for Apple HLS streaming. The segment muxer (-f segment) is a general-purpose splitter. Use HLS muxer for streaming delivery, segment muxer for batch splitting.

Last verified: 2026-05-30 against FFmpeg 4.3

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