What was tested
Six files were made with FFmpeg 9.0.1 from its built-in test sources: a moving 30 fps test pattern for the picture and a 220 Hz tone mixed with quiet pink noise for the sound. They are not captures from any particular recorder. Each one copies a container and track layout that screen recordings commonly use, so the results describe how this tool handles those layouts, not how a given recording app behaves.
| File | Picture | Audio as written | Size |
|---|---|---|---|
| A. 60 min MP4 | H.264, 1920x1080 | AAC, stereo, 48 kHz, 160 kbps | 4,388,816,051 bytes (4.39 GB) |
| B. 10 min MOV | H.264, 1920x1080 | AAC, stereo, 48 kHz, 128 kbps | 728,935,720 bytes |
| C. 10 min MKV | H.264, 1920x1080 | Track 1 named System audio: AAC stereo, default. Track 2 named Microphone: Opus mono | 736,844,134 bytes |
| D. 10 min WebM | VP8, 1280x720 | Opus, stereo, 48 kHz, 96 kbps requested | 81,204,433 bytes |
| E. 1 min MP4 | H.264, 1920x1080 | None | 71,877,497 bytes |
| F. 30 s MKV | H.264, 640x360 | AAC, 5.1 (six channels), 48 kHz | 7,999,438 bytes |
The built site was served locally and driven in Chrome 153.0.8010.53 on Windows 11, on an Intel Core i5-13600KF with 32 GB of memory, using Playwright. Advertising and analytics requests were blocked so the test did not count as visits. Every saved file was read back with ffprobe to confirm codec, sample rate, channels and duration. Run date: 5 October 2026.
What the File check panel reported
The panel appears under the editor once a file opens. Its bitrate is measured from the track's packets, because containers rarely store one.
| File | Panel summary | Ready to edit after |
|---|---|---|
| A | MP4, 1 audio track, 1 video track. AAC, stereo, 48 kHz, 160 kbps | 12.0 s |
| B | QuickTime File Format, 1 audio, 1 video. AAC, stereo, 48 kHz, 128 kbps | 1.3 s |
| C | Matroska, 2 audio, 1 video. 1. System audio, eng, AAC, stereo, 160 kbps, default. 2. Microphone, eng, Opus, mono, 73 kbps | 1.8 s |
| D | WebM, 1 audio, 1 video. Opus, stereo, 48 kHz, 69 kbps | 1.8 s |
| E | Refused at open: This file has no audio track, so there is nothing to extract. | 0.1 s |
| F | Matroska, 1 audio, 1 video. AAC, 6 ch, 48 kHz, 186 kbps | 0.2 s |
Ready means the waveform for the whole timeline was drawn. That time follows the length of the audio, not the size of the file: the 4.39 GB hour took 12 seconds, because the picture is never decoded. File D shows why the bitrate is measured. FFmpeg was asked for 96 kbps Opus, but Opus varies its rate with the signal, and the simple test sound needed only 69 kbps.
Export times and sizes
| File | Output | Time | Saved size | Panel estimate |
|---|---|---|---|---|
| A (60 min) | MP3 | 61.4 s | 86,400,744 bytes | About 86 MB |
| A | M4A, unchanged copy | 6.1 s | 72,676,105 bytes | About 72 MB |
| A | WAV | 29.4 s | 691,200,044 bytes | About 691 MB |
| B (10 min) | MP3 | 8.8 s | 14,400,744 bytes | About 14 MB |
| B | M4A, unchanged copy | 1.2 s | 9,739,957 bytes | About 9.6 MB |
| B | WAV | 5.0 s | 115,200,044 bytes | About 115 MB |
| C (10 min, track 1) | M4A, unchanged copy | 1.3 s | 12,263,586 bytes | About 12 MB |
| D (10 min) | MP3 | 9.6 s | 14,401,320 bytes | About 14 MB |
| D | M4A, AAC encode | 3.9 s | 14,518,706 bytes | About 14 MB |
| F (30 s, 5.1) | MP3, folded to stereo | 0.3 s | 721,320 bytes | About 721 KB |
| F | WAV, six channels kept | 0.3 s | 17,292,140 bytes | About 17 MB |
Every estimate landed within 2 percent of the saved file. On this machine MP3 ran at roughly 60 times real time, so an hour of audio took about a minute. The unchanged M4A copy was about ten times faster again, because nothing is decoded or encoded. WAV sat between the two. A phone will be slower; these are desktop figures.
What ffprobe found in the outputs
- Every MP3 was 48 kHz stereo at 192 kbps, as the bitrate guide describes. MP3 durations ran 48 to 72 ms longer than the source, which is encoder padding at the end.
- Every unchanged M4A copy kept the source bitrate (128 or 160 kbps) and ran 21.3 ms longer than the source. That is the 1024 samples of AAC encoder priming that MP4 and MOV hide with an edit list; the copy keeps them at the start rather than altering any packet.
- WAV was 16-bit PCM at the source rate and channel count, including all six channels of file F.
- The two tracks of file C exported separately. The Microphone track is Opus, so an M4A from it is a new AAC encode, and the panel says so before Convert is pressed.
Two defects this run found and fixed
The first pass of this test failed in two places, and both failures were reaching visitors.
The unchanged M4A copy failed on every MP4 and MOV source. That priming offset gives the first packet a negative timestamp (-0.0213 s), and the MP4 writer refused it. Matroska files were unaffected, which is why it went unnoticed. The copy now shifts every packet by that offset without changing its bytes. The same failure also left the page on Converting with no error message; a failed job now reports the stage where it stopped.
A 5.1 track to MP3 stopped with a message saying the browser lacked a decoder. The decoder was fine: MP3 cannot hold six channels. All 13 conversion failures recorded between July and October 2026 happened on the MKV page, where six-channel tracks are common, so this is the likely cause; the failure count over the next two weeks will show whether it was. MP3 export now folds such a track to stereo and says so; WAV keeps every channel.
Reading these numbers for your own recording
For an hour-long lecture or meeting saved as MP4 or MOV with AAC audio, the unchanged M4A copy is the quickest and smallest: about 72 MB at 160 kbps, with no generation loss. MP3 at 192 kbps costs 86.4 MB per hour and plays almost everywhere. WAV costs 691 MB per hour for 48 kHz stereo and is only worth it when the next step is an audio editor. If the File check shows a recording with no audio track, the sound was not captured; no converter can recover it. For opening problems, the troubleshooting guide lists what each message means, and the screen recording page covers where Mac, Windows, iPhone and Android save their recordings.