The formula
size = bitrate × duration ÷ 8, converting kilobits to megabytes
Where it comes from
Bitrate is how much data per second the video uses, and multiplied by the duration it gives the size. The only delicate part is the units: bitrate is in kilobits and size in megabytes, and between bits and bytes there is a factor of eight that gets forgotten constantly.
How to work it out by hand
- Add the video bitrate and the audio bitrate
- Multiply by the duration in seconds: that gives kilobits
- Divide by 8 to convert to kilobytes
- Divide by 1,000 to reach megabytes
What is worth knowing
The reverse calculation is the one used most in practice: you have a size limit — a platform that will not take more than 500 MB, a USB stick, a data allowance — and you need to know what bitrate to export at. There it pays to reserve the audio bitrate first, because compressing it saves very little and hurts a lot. For reference, YouTube recommends about 8,000 kbps for 1080p at 30 fps and 45,000 for 4K at 60. And note: with variable bitrate the figure in the exporter is an average, so the real file can land a little above or below.