Fit a video under a file-size limit with ffmpeg on Windows: work out the video bit rate from the…

Guide ID: art_01M7CBPH7V7RH1CG3QMPJ7QM8N

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Guide title

Fit a video under a file-size limit with ffmpeg on Windows: work out the video bit rate from the length and the sound, encode in two passes with x264, and check the real size

Coverage

For anyone who must send a video under a size limit and wants it to fit the first time. A PowerShell script works out the video bit rate from the length, the sound and the limit, encodes with x264 in two passes, measures the real size, lowers the rate and encodes again when the first attempt is over, and checks the streams and length. It ran on generated files with ffmpeg 9.0.2, including fourteen refusal cases, and shows what the one-line alternatives did.

Example

With a 20 s test clip and a 2 MB limit the plan was 640 kbit/s for the picture and 128 kbit/s for the sound; the result was 1,791,148 bytes. With a 0.5 MB limit the first attempt was 7,026 bytes over and the second, after lowering the video rate by the excess, was 10,610 bytes under. Plain ffmpeg exited with code 0 each time while cutting the clip to about 3 s of its 20 s, or giving a file far over the limit.

Useful starting points

• Work out the video bit rate from the limit, the length and the sound before encoding; do not guess a quality setting.
• Measure the real size and encode again at a lower rate if it is over; a small margin can still overshoot.
• Check how the place you send the file to counts its limit (MB or MiB), then look at the start, the middle and the end of the result.