CRF (Constant Rate Factor) in FFmpeg: which value to use
· updated · 6 min read
What Constant Rate Factor means in FFmpeg's x264 and x265 encoders, which CRF values are worth using, why the scales differ, and when to use a bitrate instead.
Video compression guides
Every setting in the compressor is a trade between file size, picture quality and encoding time. These guides explain that trade well enough that you can make the choice deliberately instead of dragging a slider and hoping.
The model these guides share
Video compression works by not storing most of what you see. A codec keeps one reference frame, then describes the frames after it as differences from what came before — plus a set of deliberate approximations in the parts of the picture the eye is worst at scrutinising. Every "quality" control you will ever touch is really a control over how aggressive those approximations are allowed to be.
There are three levers, and they are not equal. Bitrate — bits spent per second — dominates file size and is the first thing to change. Resolution decides how many pixels those bits are spread across, so lowering it makes a given bitrate go much further. Encoder effort costs you time rather than quality: a slower preset searches harder for savings and reaches a smaller file at the same visual result.
The fourth thing worth knowing is that none of this is uniform across content. A static screen recording and a handheld shot of rain need wildly different bitrates to look equally good, which is why every number in these guides is a starting point to test from rather than a setting to copy.
Want to apply this immediately? Compress a video in your browser or shrink one to a specific size limit.
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All guides, newest first
Discord File Size Limit (20MB): Every Way Around It
· updated · 7 min read
Discord's free upload limit is 20MB as of September 2026, 50MB with Nitro Basic and 500MB with Nitro. How long a video fits, and every way around it.
H.264 vs H.265: the real file size difference
· 5 min read
H.264 vs H.265 file size compared: how much smaller HEVC really makes a video, why the 50% figure is optimistic, the encode-time cost, and when H.264 wins.
iPhone Video Size per Minute, and Why the Files Are So Big
· updated · 6 min read
How many MB a minute of iPhone video takes at 720p, 1080p and 4K, from Apple's own figures. Why HEVC and ProRes matter, and how to make the files smaller.
How much of your video file is audio?
· 7 min read
Audio costs the same per minute whatever the resolution, so its share grows as the video shrinks. What each format costs in megabytes, and when to strip or downmix.
How to reduce screen recording file size and keep text sharp
· updated · 8 min read
How to reduce screen recording file size without smearing text: why not to downscale, what cutting frame rate really saves, CRF values, and an FFmpeg command.
Why your video won't play: containers, codecs and compatibility
· 7 min read
The difference between a container and a codec, how to read the error you are getting, and why a remux fixes most playback problems without touching quality.
How to trim a video to fit a size limit
· 7 min read
File size is bitrate times duration, and duration is the term nobody treats as a setting. What a second of runtime costs, and where to cut without re-encoding.
ffmpeg.wasm explained: how FFmpeg runs in the browser
· updated · 5 min read
How ffmpeg.wasm runs FFmpeg inside a browser tab: the WebAssembly build, the in-memory filesystem, and why multi-threading needs COOP and COEP headers.
Read this if you want to know how an encoder ends up running inside a browser tab at all — WebAssembly, SharedArrayBuffer, COOP and COEP.
How to compress video for the web: web-optimized MP4
· updated · 6 min read
How to compress video for the web with FFmpeg: codec, CRF and resolution, plus -movflags +faststart for a web-optimized MP4 that starts playing straight away.
Read this when the destination is a web page and you are choosing between H.264, VP9 and a resolution.
How to reduce video file size without losing quality
· updated · 6 min read
How to cut video file size without visible quality loss: CRF values by content type, when to downscale, audio savings, and encoder preset trade-offs.
Read this when you know what bitrate is and want to know how far you can push it on your specific footage.
What is video bitrate, and what happens when you lower it?
· updated · 5 min read
What video bitrate controls, how the encoding bitrate drives file size and quality, what a lower bitrate costs you, CBR vs VBR vs CRF, and how to pick a number.
Read this first if any of the numbers in the compressor are meaningless to you. Everything else on this page assumes it.
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Elsewhere on the site
- Shrink a video to a target file size — the arithmetic for a hard size limit.
- Supported video formats — MP4, MOV, MKV and WebM, and when to remux instead of re-encode.
- How browser-based compression works — the mechanism, and its honest limits.
- Video compression FAQ — the short answers.
- RSS feed — every guide, newest first.