The formula
File size is bitrate times length. Bitrate is in megabits a second (Mbps), and there are 8 bits in a byte, so:
size in MB = (video Mbps + audio Mbps) × seconds ÷ 8A 10 minute 1080p video at YouTube’s recommended 8 Mbps, with 384 kbps audio, is (8 + 0.384) × 600 ÷ 8 = 629 MB. The calculator shows sizes in decimal megabytes (1 MB = 1,000,000 bytes), the same way the Mac’s Finder does.
Where the presets come from
H.264, YouTube recommended uses YouTube’s recommended upload encoding settings for SDR video: 5 Mbps at 720p, 8 Mbps at 1080p, 16 Mbps at 1440p and 35 to 45 Mbps at 4K, with higher figures (7.5, 12, 24 and 53 to 68 Mbps) at 48 to 60 frames a second. Stereo audio is 384 kbps.
ProRes presets use the target data rates in Apple’s ProRes white paper (April 2017). At 1920×1080 and 29.97 frames a second those are:
| Codec | 1080p, 29.97 fps | 4K UHD, 29.97 fps | Per hour at 1080p |
|---|---|---|---|
| ProRes 422 Proxy | 45 Mbps | 182 Mbps | 20 GB |
| ProRes 422 LT | 102 Mbps | 410 Mbps | 46 GB |
| ProRes 422 | 147 Mbps | 589 Mbps | 66 GB |
| ProRes 422 HQ | 220 Mbps | 884 Mbps | 99 GB |
| ProRes 4444 | 330 Mbps | 1,326 Mbps | 149 GB |
For other sizes and frame rates the calculator scales those numbers by pixel count and frame rate, so treat them as estimates.
Why screen recordings are smaller than you’d think
H.264 and HEVC only store what changes between frames. A screen recording is mostly a still desktop with a moving pointer, so at the same resolution it usually comes out much smaller than camera footage. The “H.264, small” preset (half YouTube’s figure) is a fairer guess for a typical screen recording. A recording full of scrolling, video playback or animation will land nearer the YouTube figure.
ProRes is different: every frame is stored on its own, so its size barely depends on what’s on screen. That’s why a long ProRes screen recording can fill a drive quickly.
Making a video fit a size limit
Type the limit into “Make it fit a size limit”. Gmail’s attachment limit is 25 MB, for example. The calculator gives the highest video bitrate that fits, after the audio. Set that as the target bitrate when you export (in HandBrake, it’s “Avg Bitrate” on the Video tab). Leave a little headroom, because encoders don’t hit a target exactly.
If the bitrate it suggests is very low, you’ll get a sharper result by lowering the resolution too, or by sending a link instead of the file. Share links don’t have an attachment limit.
FAQ
How big is a 1 hour 1080p video?
At YouTube’s recommended 8 Mbps for 1080p, plus 384 kbps stereo audio, one hour is about 3.8 GB. In ProRes 422 it’s about 66 GB. A screen recording in H.264 is often much smaller, because little on screen changes.
How do I calculate video file size from bitrate?
Add the video and audio bitrates in megabits a second, multiply by the length in seconds, and divide by 8 to get megabytes. For example, 8.384 Mbps for 600 seconds is 629 MB.
How many GB is 4K video per minute?
At YouTube’s recommended 45 Mbps for 4K at 24 to 30 fps, about 340 MB a minute (20 GB an hour). In ProRes 422 at 29.97 fps, about 4.4 GB a minute.
Why is my screen recording so much smaller than this?
H.264 and HEVC only store what changes between frames, and a screen recording is mostly a still desktop. Pick the “H.264, small” preset for a closer estimate.