How a size target becomes a bitrate
File size is bitrate multiplied by duration. If you know two of the three, the third falls
out. Ask for 25 MB and hand over a 90-second clip, and the total budget is
(25 × 8 × 1024) / 90 ≈ 2275 kbps. Subtract the audio allowance and what
remains is the video bitrate.
Two corrections get applied. First, roughly 8% is reserved for container overhead — MP4 headers, the index, and muxing slack. Without it, a "25 MB" request lands at 26 MB. Second, if the first encode still overshoots, the file is re-encoded once at a rate corrected by the measured overshoot.
When a target size is the wrong tool
A bitrate target does not know what your video contains. A static talking-head clip looks fine at 800 kbps; a fast pan over foliage does not. If you set a small target on demanding footage you get visible blocking, and the tool will not warn you — it hit your number, which is what you asked for.
For quality-driven work, prefer the compression level presets, which use constant quality (CRF) and let the file size land where it lands. Use a size target when you have an external constraint — an upload limit, an email cap, a storage quota.
Frequently asked
Can I go below 1 MB?
You can ask, but a very small target on a normal-length video forces a bitrate so low that the result is unwatchable. The tool will still produce a file; it just will not look good. For very short clips there is a floor — below roughly 100 kbps the encoder refuses and the tool falls back to quality-based encoding.
Why is the output slightly under my target?
Deliberately. Bitrate control is approximate, so the encoder aims below the ceiling rather than at it. Being 5% under is better than being 2% over when the destination enforces a hard cap.