How to calculate the video bitrate you need

If you know how long your video is and how big the file may be, you can work out the bitrate in a few seconds. This guide gives you the formula, a reference table, and a way to tell whether the result will look good.

In short

The one formula you need

Bitrate is how many bits are spent on each second of video. File size is bitrate times duration, so you can turn it around: bitrate equals size divided by duration. Multiply megabytes by 8 to get megabits.

Example: a 10 MB limit and a 60 second clip give 10 × 8 ÷ 60 = 1.3 Mbps for video and audio together.

File size = bitrate × time

The area of each box is the file size, and both boxes hold the same 10 MB. Halve the length of the clip and every second can use twice the bitrate.

Reference table

Total bitrate in Mbps, before subtracting audio. The compressor also keeps about 3 percent in reserve for the MP4 container, so aim a little lower than these numbers.

  • 10 MB: 30 s → 2.7 Mbps, 60 s → 1.3 Mbps, 120 s → 0.7 Mbps, 300 s → 0.3 Mbps
  • 25 MB: 30 s → 6.7 Mbps, 60 s → 3.3 Mbps, 120 s → 1.7 Mbps, 300 s → 0.7 Mbps
  • 50 MB: 30 s → 13.3 Mbps, 60 s → 6.7 Mbps, 120 s → 3.3 Mbps, 300 s → 1.3 Mbps
Technical details

Take the audio out first

Audio needs far less than video: stereo AAC at 96 kbps is 0.096 Mbps, and 64 kbps mono is 0.064 Mbps. Subtract it from the total and the rest is your video bitrate. For very small budgets, audio can be a noticeable slice, which is why the compressor lowers it when space is tight.

Will it look good?

Divide the video bitrate by width, height and frame rate to get bits per pixel. Above roughly 0.08 the picture usually looks clean, below roughly 0.05 it turns blocky. If your number is too low, change the picture rather than the target.

Bits per pixel: will the picture look clean?

bits per pixel, per frame. The same 2.5 Mbps goes a long way further at 720p and 30 fps. The thresholds (0.05 and 0.08) are rules of thumb, and the compressor uses them to warn you before you start.

If the number is too low

  • Trim the clip. Fewer seconds means a higher bitrate for the rest.
  • Lower the resolution one step.
  • Lower the frame rate from 60 to 30 fps if the content is not fast motion.
  • Accept a larger file if the limit allows it.

Fewer pixels, more bits for each pixel

At the same bitrate, 720p gives each pixel about 2.3 times as many bits as 1080p, and 480p about 5 times as many. That is why stepping down a resolution often looks cleaner than starving a large picture.

Half the frames, twice the data per frame

frames in one second (simplified). Dropping from 60 to 30 fps removes every other frame, so each frame that stays can use twice as many bits. Fast motion loses some smoothness.