How video file size works
A video's size depends on two things: its bitrate (how much data each second takes) and its length. Resolution and frame rate only matter through the bitrate. Double the bitrate and the file doubles; double the length and it doubles too. The formula is simple: size in bytes = (video bitrate + audio bitrate) × seconds ÷ 8, because there are 8 bits in a byte.
Bitrates are written in megabits per second (Mbps), and file sizes in megabytes (MB). The lowercase b and uppercase B are easy to confuse: 8 Mbps is 1 MB per second, so a minute at 8 Mbps is about 60 MB.
Choosing a bitrate
- For uploading (YouTube, Vimeo, social): follow the platform's recommendation. They re-encode everything, so going much higher only makes the upload slower.
- For a size limit (email, chat apps, festival portals): use "Fit a size" and encode with two-pass or a capped bitrate so the file can't overshoot.
- For editing and masters: use an intra-frame codec such as ProRes or DNxHR. Files are 10 to 20 times bigger than H.264, but they decode smoothly and survive re-encoding.
- Codec matters: H.265 (HEVC) and AV1 look about as good as H.264 at roughly half to two-thirds of the bitrate, if the place you're sending the file supports them.
Why the real file is a little different
Most encoders use variable bitrate: simple shots get less data, busy ones more, and the average lands near your setting rather than on it. Some apps also show sizes in binary units (1 MiB = 1,048,576 bytes), which look about 5% smaller than the decimal megabytes used here.
Where to go next
The course lesson Export and delivery covers master files, platform versions and archiving. Check the shape of the export with the aspect ratio calculator, and the length with the timecode calculator.