How Do You Compress a Video to Fit a Size Limit?
When a platform says “attachments up to 25 MB”, you don’t care about quality parameters — you care whether the result fits. Target size is for exactly that: you give the limit, the tool works backwards.
How the numbers work
A video’s size is essentially bitrate × duration. Fix the limit and the duration, and the usable bitrate is fixed too. The tool keeps about 3% of the target as headroom (container overhead and small encoder overshoot), subtracts what the audio needs, and gives everything else to the picture.
| Target size | 1-minute video | 5-minute video | 10-minute video |
|---|---|---|---|
| 10 MB | about 1.2 Mbps | about 160 kbps | does not fit — you are asked to raise the target |
| 25 MB | about 3.1 Mbps | about 550 kbps | about 230 kbps |
| 50 MB | about 6.4 Mbps | about 1.2 Mbps | about 550 kbps |
Those are the bitrates left for the picture. In target-size mode audio is budgeted at 96 kbps at most; if the source’s AAC track is already at a low bitrate it is kept as-is instead of being re-encoded.
When the resolution is lowered automatically
The same bitrate is very thin spread over every pixel of 1080p, and comfortable at 480p. Forcing a high resolution at a starved bitrate produces a blocky mess that looks far worse than a clean lower resolution. So when the calculated bitrate cannot support the current resolution, the tool steps down through 1080p, 720p, 540p, 480p, and 360p until each pixel gets enough data.
For example: squeezing a 5-minute 1080p video into 25 MB leaves only about 550 kbps, so it is brought down to roughly 360p; the same video at 50 MB can stay around 540p. The result card shows the actual output resolution and notes that it was lowered to hit the target size.
When it still overshoots, or cannot run at all
| What you see | Why | What to do |
|---|---|---|
| “Target size is too small” | At this duration even the lowest usable bitrate does not fit | Raise the target, or trim the video first |
| Result slightly over the limit, with a warning | Encoder rate control is approximate, and it was still over after one retry | Pick a smaller resolution, or go one target size down |
| “Already under the target size — kept the original” | The source was within the limit to begin with | Nothing to do; use the original |
| Duration could not be read | A few damaged or unusual files carry no duration | Set Target size back to “No limit” and compress by quality |
When the first encode overshoots, the tool lowers the bitrate in proportion and encodes again, so an occasional video takes close to twice as long.
How this differs from the compression level
The compression level (Smallest, Balanced, High quality) works by quality: a simple video comes out tiny, a complex one larger, and you do not know the size in advance. Target size is the reverse: the size is fixed and the quality follows from it. Once a target is set, the picture bitrate is decided by the target, and the quality setting of the compression level no longer applies.
- Just want the video smaller, with no hard limit: use the compression level — quality is more consistent.
- The platform has an explicit size limit: use Target size and skip the trial and error.
- Want both: compress with a target first; if the result lands far below the limit, Balanced would have been enough for that clip.
References
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