For a known average bitrate, file payload size is bitrate multiplied by duration, divided by eight. Include audio in the total and distinguish decimal storage units from binary ones. Container metadata and variable bitrate mean an export preset alone is not an exact file-size guarantee.
On this page
Use consistent units
| Unit | Meaning |
|---|---|
| 1 Mbps | 1,000,000 bits per second |
| 1 kbps | 1,000 bits per second |
| 1 MB / 1 GB | 1,000,000 / 1,000,000,000 bytes |
| 1 MiB / 1 GiB | 1,048,576 / 1,073,741,824 bytes |
Work one example
Take a 60-second video with an average video bitrate of 10 Mbps and audio at 320 kbps. Total rate is 10.32 Mbps. Multiplying 10,320,000 bits/s by 60 seconds and dividing by 8 gives 77,400,000 bytes: 77.4 MB, or approximately 73.814 MiB. This is payload size before a container allowance.
If you enter a hypothetical 3% container allowance, the estimate becomes 79.722 MB. That percentage is an assumption you entered, not a measured overhead for every MP4. Use a real sample when a delivery limit is strict.
Solve a target-size problem
Reverse the calculation to allocate a video bitrate: target payload bits divided by duration, minus the audio allocation. With a 100 MB target, 60 seconds and 320 kbps audio, the zero-overhead video budget is approximately 13.013 Mbps. A smaller target that cannot even hold the selected audio must be rejected.
Why the final file differs
- A VBR target is not necessarily the measured average over the finished program.
- Maximum bitrate and average bitrate are different inputs.
- Extra audio tracks add data; the calculator’s audio input must represent their total.
- Subtitles, metadata and container structure add bytes.
- A quality-based encode may not have a predictable bitrate before it runs.
Use the result correctly
A size budget is not a quality recommendation. Inspect a short export at the chosen settings, measure its size and reopen it. Keep some room below a hard platform limit. Try the inverse bitrate calculator or the file size estimator.
Sources and scope
The worked figures are arithmetic examples, not measured video exports. Rate-control terminology and export behavior depend on the encoder.
