Video compression guide
Video compression: quality, bitrate, and file size
What controls video file size?
Duration and total bitrate are the main drivers of video file size. A two-minute video encoded at twice the bitrate of another two-minute video will usually be close to twice the size. Resolution, frame rate, codec, and visual complexity matter because they affect the bitrate needed to preserve a given level of quality.
Audio also takes space. A 128 kbps audio stream adds roughly 0.96 MB per minute before container overhead. That is small beside a high-bitrate video stream, but it matters when the target is a strict messaging or email limit.
Bitrate connects duration and file size
Average bitrate is the amount of data used each second. A useful planning formula is:
file size (MB) ≈ total bitrate (kbps) × duration (seconds) ÷ 8192For a target-size export, rearrange the formula and subtract the audio bitrate. Real output includes a small amount of container data, so choose a target below the platform limit instead of using every available byte.
Quality-based and bitrate-based encoding
Quality-based encoding asks the encoder to preserve a consistent visual quality and lets file size vary. In H.264 workflows this is often expressed as CRF: lower values preserve more detail and create larger files, while higher values create smaller files.
Average-bitrate encoding is more useful when file size must stay near a fixed target. Quality can vary from scene to scene because the encoder has a limited data budget. Two-pass encoding usually allocates that budget more effectively, but it requires more processing time.
| Goal | Prefer | Tradeoff |
|---|---|---|
| Consistent visual quality | CRF or quality mode | Final size is not fixed |
| Strict upload limit | Average bitrate | Quality varies with content |
| Fast preview | Faster preset | Less compression efficiency |
Resolution and frame rate
Reducing 4K video to 1080p removes three quarters of the pixel count in each frame, which can greatly reduce the bitrate needed for a clean result. Reducing 60 fps footage to 30 fps can also save data, but motion will look different. Do not upscale a smaller source during compression; it creates more pixels without restoring lost detail.
Text, screen recordings, animation, and fast camera movement compress differently. Treat online recommendations as starting points and inspect the actual output at normal viewing size.
A practical compression workflow
- Keep the original file as the master copy.
- Start with a balanced quality setting and the original frame rate.
- Cap unusually large sources at the resolution needed for delivery.
- Review fine detail, motion, audio synchronization, and the final size.
- Only lower quality further when the current file still exceeds the delivery limit.