How to calculate video file size from bitrate and duration
Updated 2026-06-21
Video file size comes down to a single relationship between three numbers: bitrate, duration and size. Once you know the formula you can predict a file's size before you export it, or work backwards to hit a size target.
The formula
File size (megabytes) = total bitrate (Mbps) × duration (seconds) ÷ 8
You divide by 8 because a byte is 8 bits — bitrate is measured in bits per second, but file size is measured in bytes. "Total bitrate" means your video bitrate plus the audio bitrate.
A worked example
A 10-minute clip at a total bitrate of 10 Mbps:
- Duration in seconds: 10 × 60 = 600 s
- Size: 10 × 600 ÷ 8 = 750 MB
Bump the bitrate to 20 Mbps and the file doubles to 1.5 GB. Halve the duration and it halves. The relationship is perfectly linear, which is why a calculator makes short work of it.
Solving for the other two
The same formula rearranges to answer the other common questions:
- Bitrate from a size target: bitrate (Mbps) = size (MB) × 8 ÷ duration (s). Useful when a platform caps your upload and you need to know the highest bitrate that still fits.
- Duration that fits a size: duration (s) = size (MB) × 8 ÷ bitrate (Mbps).
Do it instantly
Rather than reach for a calculator app, the Video File Size & Bitrate Calculator solves all three directions for you — pick what to solve for, type the other two values, and read the answer. You can also drop in a real video file and it reads the actual duration, resolution and average bitrate locally (nothing is uploaded), then shows MB-per-minute and GB-per-hour rates and whether the file fits common upload caps like email, Discord and TikTok.