Video
Video Bitrate, Explained: What It Does to Size and Quality
Video bitrate is how many bits the file spends on each second of video, usually in megabits per second (Mbps). File size is roughly bitrate times duration. Halve the bitrate and you halve the file, at the cost of detail.
Most Mac tools now let you set CRF instead, a quality target that picks the bitrate for you. Below: what each does, what to choose, and a real test.
What is video bitrate?
A video file is a stream of compressed frames. Bitrate is the budget for that stream. A 10 Mbps video uses 10 megabits, about 1.25 megabytes, for every second. A ten-minute clip at that rate is about 750 MB before audio.
The formula is simple: size in MB = Mbps × seconds ÷ 8. Use it to predict a file before you export.
Bitrate is not the same as resolution or frame rate. A 4K clip and a 720p clip can share a bitrate. The 4K one will just look softer, because the same bits cover four times as many pixels. The audio equivalent is covered in audio bitrate explained.
What is CRF, and why not just set a bitrate?
CRF (constant rate factor) tells the encoder how much quality to keep, not how many bits to spend. Busy scenes get more bits. Still scenes get fewer. Lower CRF means higher quality and a bigger file. Higher CRF means a smaller file.
A fixed bitrate wastes bits on easy scenes and starves hard ones. CRF avoids that, so it is the better default for files you keep or share. Fixed bitrate still matters when a platform or a streaming server demands a ceiling.
CRF numbers are not portable between codecs. A typical H.264 CRF of 23 does not look like H.265 at 23. Smol’s defaults reflect that: its “High” label is CRF 24 and “Web” is CRF 28 on H.265. More on codecs in H.265 vs H.264.
How much does bitrate change the file size?
We encoded one synthetic 6-second 1080p clip (a generated fractal, no audio) to H.264 at four CRF values. Synthetic footage is harder to compress than a talking head and easier than grain-heavy film, so treat the sizes as a pattern, not a prediction for your clips.
| CRF (H.264) | File size | Average bitrate |
|---|---|---|
| 18 | 12.8 MB | about 17 Mbps |
| 23 | 7.0 MB | about 9.3 Mbps |
| 28 | 3.0 MB | about 4.0 Mbps |
| 33 | 1.0 MB | about 1.4 Mbps |
Each step of five CRF points cut the file by roughly half or more. That is the rule of thumb: about six CRF points halves the size. It held here.
What bitrate should I use?
Start from the destination, not from a number you saw on a forum.
- Archive or edit later: low CRF, or keep the original. Do not re-compress a master.
- Send to a client or colleague: a moderate CRF, original resolution.
- Social, chat or email: a higher CRF and a lower resolution. The platform will re-encode anyway.
- Website background: high CRF, 720p, no audio. See compressing video for a website.
Check what your file uses now with ffprobe, which ships with ffmpeg:
ffprobe -v error -show_entries format=bit_rate,duration -of default=nw=1 clip.mp4If the bitrate it reports is far higher than the table above for your resolution, there is room to shrink. Our compress video on Mac guide covers the full workflow.
How do I lower video bitrate on a Mac?
You re-encode. There is no way to just trim bits out of an existing file. The three levers, in order of effect: raise CRF, drop the resolution, switch H.264 to H.265. Removing the audio track helps on small files.
In QuickTime Player, File > Export As offers 4K, 1080p, 720p and 480p, but no CRF or bitrate control. Free ffmpeg gives full control from Terminal (this uses its default H.264 encoder):
ffmpeg -i in.mov -crf 26 -c:a aac -b:a 128k out.mp4Re-encoding always costs some quality, so keep the original until you have checked the result.
When is Smol the wrong tool for this?
If you only need to understand bitrate, or shrink one clip, QuickTime Player’s Export As or a one-line ffmpeg command is enough. Smol is not needed to learn any of the above.
Smol earns its $29 (once, one Mac) when you do this repeatedly. You pick H.265 or H.264, a quality label or an exact CRF from 0 to 63, a resolution, and drop in a whole folder. It runs on your Mac and nothing is uploaded. See pricing, or the best video compressor for Mac roundup.
How we checked this
Encoded a generated 6 s 1080p30 clip to H.264 at CRF 18, 23, 28 and 33 (medium preset, no audio) with ffmpeg on a Mac and read sizes and bitrates with ffprobe. Facts last checked . We make Smol, so treat our claims about it with the same care as any vendor’s, and see our editorial policy.
Video bitrate: common questions
What is a good bitrate for 1080p video?
It depends on content and codec, so no single number is right. In our test, a busy 1080p30 clip at H.264 CRF 23 averaged about 9 Mbps, and CRF 28 about 4 Mbps. Screen recordings and talking heads need far less. Pick a CRF, then check the result by eye.
Does higher bitrate always mean better quality?
No. Past a point the extra bits are wasted, because the encoder already preserved what the eye can see. Quality also depends on the codec, so H.265 at a lower bitrate can match H.264 at a higher one. Never raise bitrate above the source: you cannot add detail that is not there.
Is CRF better than bitrate?
For files you keep, share or upload, yes. CRF spends bits where the picture needs them. A fixed bitrate is better when a platform enforces a limit, or when you must predict file size exactly. Many tools let you use CRF and still cap the maximum bitrate.
How do I calculate file size from bitrate?
Multiply megabits per second by the duration in seconds, then divide by 8 to get megabytes. A 5 Mbps video of 120 seconds is about 75 MB. Add the audio bitrate, usually 96 to 192 kbps, which is small next to the video stream.
Does lowering bitrate reduce resolution?
No. Resolution stays the same unless you change it. A lower bitrate spreads fewer bits across the same pixels, so you see blockiness and smeared detail instead. If quality drops too far, lower the resolution as well and keep the bitrate, which often looks cleaner.
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