Video File Size Calculator
File size is bitrate multiplied by duration. That is the whole formula, and the calculator below works it in both directions: how big a file will be at a given bitrate, and what bitrate fits a size you have been given.
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The formulas
Size from bitrate:
size (MB) = (video kbps + audio kbps) × seconds ÷ 8,192
Bitrate from a target size:
total kbps = target MB × 8,192 ÷ seconds, then subtract the audio bitrate to get the video bitrate.
8,192 rather than 8,000 because a megabyte here is 1,024 × 1,024 bytes and a kilobit is 1,000 bits. Mixing the two conventions is where most hand calculations go wrong by about 5%.
Leave headroom
Container headers, the index and per-frame overhead add a small amount that no formula predicts exactly, and rate control aims at an average rather than a ceiling. Target 3–5% below any hard limit: 24 MB for a 25 MB cap, 9 MB for 10 MB. The compressor on this site applies that margin for you.
Sanity-checking the answer
Divide the video bitrate by width × height × fps to get bits per pixel per frame. Below 0.04 the result will look starved no matter which encoder you use — that is the number telling you to drop the resolution rather than the quality.
Compress a video to 25 MB → Runs in your browser. Nothing is uploaded.