What is CRF and what value should I use?

CRF (Constant Rate Factor) controls quality vs. file size. Lower CRF = higher quality, larger file. Higher CRF = lower quality, smaller file. For H.264: CRF 18 is visually lossless, CRF 23 is the default (good quality), CRF 28 is a good balance for sharing, CRF 35+ is noticeable quality loss but very small files.

Which input formats are supported?

ffmpeg.wasm accepts most common video containers: MP4, WebM, MOV, AVI, MKV, and more.

What codec does the compressor use?

The compressor uses H.264 (libx264) with the ultrafast preset. This prioritises speed over maximum compression — the output will be slightly larger than a slow-preset encode at the same CRF, but processes much faster in the browser.

How much will the file size be reduced?

Results vary by content. A CRF of 28 typically reduces file size by 30–60% compared to a high-bitrate original. Videos with lots of movement compress less than static scenes.

Is my video uploaded during compression?

No. ffmpeg.wasm runs entirely inside your browser. Your video is processed locally and never sent to any server.

How long does compression take?

Compression re-encodes every frame. Expect 2–10 minutes per minute of 1080p video on a modern laptop using the ultrafast preset.

Will the audio quality be affected?

Audio is re-encoded to AAC at 128 kbps by default, which is transparent quality for most content. The CRF slider only affects video quality.

What is the difference between compressing and trimming to reduce file size?

Trimming removes unwanted footage to reduce duration — no quality loss on the remaining content. Compression reduces the bitrate of the entire video — some quality loss occurs. For the best result, trim first and then compress.

Video Compressor

Reduce video file size with an adjustable CRF quality slider — entirely in your browser, no upload.

100% client-side · no upload

Video file (MP4, WebM, MOV…)

Load a video file to begin.

How to compress a video in your browser

Select your video file. MP4 and WebM are best supported.
Adjust the CRF slider. CRF 28 is a good starting point for sharing. Go higher (30–35) for a smaller file, lower (18–23) for better quality.
Click Compress video and wait. Compression re-encodes every frame — larger files take longer.
Download the compressed MP4.

CRF guide

CRF 18–20: Visually lossless. Use for archiving or further editing.
CRF 23–25: High quality. Good for streaming services and video platforms.
CRF 28–30: Good balance of quality and file size. Ideal for email and messaging.
CRF 33–40: Noticeably lower quality but very small file. Use for previews only.

Related tools: Video Trimmer · Video Converter · Video Muter · Video Speed Changer

よくある質問

What is CRF and what value should I use?: CRF (Constant Rate Factor) controls quality vs. file size. Lower CRF = higher quality, larger file. Higher CRF = lower quality, smaller file. For H.264: CRF 18 is visually lossless, CRF 23 is the default (good quality), CRF 28 is a good balance for sharing, CRF 35+ is noticeable quality loss but very small files.
Which input formats are supported?: ffmpeg.wasm accepts most common video containers: MP4, WebM, MOV, AVI, MKV, and more.
What codec does the compressor use?: The compressor uses H.264 (libx264) with the ultrafast preset. This prioritises speed over maximum compression — the output will be slightly larger than a slow-preset encode at the same CRF, but processes much faster in the browser.
How much will the file size be reduced?: Results vary by content. A CRF of 28 typically reduces file size by 30–60% compared to a high-bitrate original. Videos with lots of movement compress less than static scenes.
Is my video uploaded during compression?: No. ffmpeg.wasm runs entirely inside your browser. Your video is processed locally and never sent to any server.
How long does compression take?: Compression re-encodes every frame. Expect 2–10 minutes per minute of 1080p video on a modern laptop using the ultrafast preset.
Will the audio quality be affected?: Audio is re-encoded to AAC at 128 kbps by default, which is transparent quality for most content. The CRF slider only affects video quality.
What is the difference between compressing and trimming to reduce file size?: Trimming removes unwanted footage to reduce duration — no quality loss on the remaining content. Compression reduces the bitrate of the entire video — some quality loss occurs. For the best result, trim first and then compress.

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