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Video compression here works through a CRF (Constant Rate Factor) slider, not a fixed percentage or a target file size box. Lower CRF values keep more visual detail and produce a larger file; higher values shrink the file more aggressively at the cost of some sharpness, especially in fast motion or fine texture. The slider is capped between 18 and 36, which covers the practical range from near-source quality down to a heavily compressed clip that still looks acceptable on a phone screen.

The encoding preset (ultrafast, veryfast, or medium) changes how efficiently the compressor spends its time, not the visual quality directly. Medium takes longer to process but generally produces a smaller file at the same CRF than ultrafast does, because it spends more effort finding efficient ways to encode each frame. If you are compressing a short clip and have a moment to wait, medium is the better default; ultrafast is there for quick previews or very large files where speed matters more than squeezing out the last few percent.

Audio is kept by default, re-encoded to AAC at 192kbps, which is high enough that most people will not notice a difference from the source track. This matters because a common mistake with browser-based compressors is assuming they silently strip audio to save space — this one does not. If you specifically want a silent, smaller file, use the dedicated Mute Video tool instead, since it removes the audio track entirely rather than just compressing it.

For predictable results, start around CRF 22–24 on the medium preset, check the downloaded file, and adjust from there. If it is still too large, move the slider higher in a few-point steps rather than jumping straight to the maximum, since the difference between CRF 24 and CRF 36 is large and it is easy to over-compress a clip that only needed a small trim in size.