A quick estimate helps avoid repeated exports that exceed a destination limit. The calculation is approximate because audio, metadata, variable bitrate behavior and container overhead add data beyond a simple video-bitrate calculation.
The quick calculation
Multiply total bitrate in megabits per second by duration in seconds, then divide by eight to estimate megabytes using decimal units. Add audio bitrate and a small overhead allowance if the bitrate you used covers video only.
Convert the target backwards
If a one-minute clip must fit under 20 MB, 20 MB × 8 ÷ 60 seconds gives about 2.67 megabits per second total. Leave headroom for audio and variable peaks rather than aiming exactly at the limit.
Why exports differ
Variable bitrate encoders spend more bits on complex scenes and fewer on static scenes. Actual output size can therefore move above or below a rough estimate.
A practical checklist
- Find the maximum file size and clip duration.
- Reserve some space below the limit.
- Calculate a rough total bitrate budget.
- Export a short sample and measure the actual output.
Common mistakes to avoid
- Confusing megabits (Mb) with megabytes (MB): eight bits make one byte.
- Planning right up to a hard upload limit.
- Ignoring attached audio or container overhead.
Questions people ask
Is this estimate exact?
No. It is a planning estimate. Variable bitrate and encoder behavior make final size differ.
Why is my file larger than expected?
The selected bitrate may be video-only, the footage may be complex, or the container and audio may add data.