The Core Formula
Video file size is determined by total bitrate multiplied by running time. Total bitrate is the sum of video and audio streams.
Formula: Size = ((Video Bitrate + Audio Bitrate) × Duration in Seconds ÷ 8) × (1 + Container Overhead).
Bits vs Bytes
Bitrates are usually quoted in Megabits per second (Mbps). Storage is measured in Megabytes (MB) or Gigabytes (GB). There are 8 bits in one byte.