Key takeaways
- The formula is exact for a constant bitrate and close for a real encode: file size = (video kbps + audio kbps) × 1,000 × seconds ÷ 8. In our two-pass tests at a target bitrate, the real file landed -1.75% to 0.36% from the formula.
- A quality-based encode (CRF) ignores your bitrate and came out 7.9% to 11.3% under the same target, so the estimate is an upper bound for it.
- At 8,000 kbps video and 128 kbps audio, a minute is 58.1 MB; an hour is 3.41 GB.
How to use the estimate
Decide the bitrate first, then check the size against your limit. One megabit per second is 7.5 million bytes a minute, which this calculator shows as 7.15 MB because it counts a megabyte as 1,048,576 bytes. Audio is small next to video, so a few extra kilobits there barely move the total.
Treat the figure as an upper bound for a quality-based export and as the target for a two-pass export. Phone and camera recordings are usually variable bitrate, so their files drift from the formula by several percent.
Scenarios
Each row is the calculator run on the inputs shown.
| Scenario | Estimated file size | Total bitrate | Audio share |
|---|---|---|---|
| 4 Mbps video, 1 minute | 29.5 MB | 4,128 kbps | 3.1% |
| 8 Mbps video, 1 minute | 58.1 MB | 8,128 kbps | 1.6% |
| 8 Mbps video, 10 minutes | 581.4 MB | 8,128 kbps | 1.6% |
| 16 Mbps video, 1 minute | 115.4 MB | 16,128 kbps | 0.8% |
| 45 Mbps video, 1 minute | 323.7 MB | 45,256 kbps | 0.6% |
| 2 Mbps video, 1 hour | 899.5 MB | 2,096 kbps | 4.6% |
Measured: formula against real files
We encoded a 10 second 1080p excerpt (CC BY 3.0, Big Buck Bunny) with x264 in two passes at a bitrate the formula gave for a target size, plus 128 kbps AAC audio, and compared the size we got with the target. The one-pass column is the same clip encoded with a quality setting (CRF 28), which does not aim at a size.
| Output size | Target size | Video bitrate from the formula | Two-pass result | Error | CRF 28 result | Error |
|---|---|---|---|---|---|---|
| 720p | 1.00 MB | 672 kbps | 1.00 MB | +0.36% | 897 KB | -10.3% |
| 720p | 2.00 MB | 1472 kbps | 1.99 MB | -0.62% | 1.81 MB | -9.4% |
| 720p | 5.00 MB | 3872 kbps | 4.93 MB | -1.48% | 4.60 MB | -7.9% |
| 1080p | 1.00 MB | 672 kbps | 1.00 MB | +0.31% | 887 KB | -11.3% |
| 1080p | 2.00 MB | 1472 kbps | 1.98 MB | -0.83% | 1.79 MB | -10.3% |
| 1080p | 5.00 MB | 3872 kbps | 4.91 MB | -1.75% | 4.55 MB | -9.0% |
Two-pass encoding aims at the average bitrate and gets within about one percent; a quality-based encode lands where the picture needs it. Container overhead adds a fraction of a percent on top of both.