Video bitrate explained

Bitrate is the amount of data a video uses per second, usually written in kilobits or megabits per second — 5 Mbps, 8000 kbps. It is the most direct predictor of file size there is.

Bitrate and file size

The arithmetic is simple. File size in megabytes is roughly bitrate in megabits per second × duration in seconds ÷ 8. A ten-minute clip at 5 Mbps is about 375MB. That relationship is close to exact, which is why bitrate is what streaming services and upload limits are specified in.

Why there is no single correct bitrate

Bitrate only means something relative to what it has to describe. Five megabits is generous for a 720p podcast recording and thin for 4K sport. Three things set the demand:

• Resolution — four times the pixels needs roughly four times the bits for the same visual quality.
• Frame rate — 60fps has twice as many frames to describe as 30fps.
• Complexity — grain, rain, confetti, fast pans and crowds are expensive. A locked-off interview shot is cheap.

Constant bitrate, variable bitrate, and constant quality

There are three ways to aim. Constant bitrate spends the same amount every second regardless of need — wasteful on easy scenes, starved on hard ones, and mostly used where the transport requires a predictable rate. Variable bitrate lets the rate move around a target average, which is better. Constant quality abandons the target entirely: you say how much loss is acceptable and the encoder spends whatever each moment needs.

Dexta uses constant quality. There is no bitrate field, because a bitrate target on an unknown source is a guess — the same 5 Mbps that looks pristine on one clip looks blocky on another. Setting a quality level and letting the bitrate float gives a consistent result across different footage. The number behind that setting is the CRF.

Mistakes worth avoiding

• Raising the bitrate of an already-compressed file. It cannot restore detail that was discarded; it only makes the file bigger.
• Comparing bitrates across codecs. VP9 or H.265 at 3 Mbps looks considerably better than H.264 at 3 Mbps — the number is only comparable within the same codec.
• Ignoring audio. It is small next to video but not free: 128 kbps of AAC is about 1MB per minute.