Crop a video

How Crop Video works

The file is read with a FileReader, rewritten in memory and offered as a download. This is why large PDFs are limited by your device's RAM rather than by an upload size limit. A 200 MB scan never crosses the network, so the slowest part is your own disk.

That is the whole architecture: static files, one script, no upload endpoint to leak or log. Close the tab and the work is gone.

Platform frames are fixed

Instagram's feed shows 4:5, Reels and TikTok show 9:16, YouTube shows 16:9, and a square 1:1 still works everywhere as a thumbnail. Upload something else and the platform crops for you — usually through the middle of a face.

Cropping here means you decide which part of the frame survives. The filter removes pixels from the sides: nothing is stretched, nothing is squeezed, and the remaining picture keeps its original sharpness.

The maths, briefly

For a vertical frame the height stays and the width shrinks: crop=ih*9/16:ih turns a 1920×1080 clip into 608×1080. For a wide frame the width stays: crop=iw:iw*9/16 turns 1080×1920 into 1080×608.

The x position controls which part of the width survives — left, centre or right. The y position is always centred, because a video crop that cuts the top of someone's head is rarely what anyone wants.

What re-encoding costs

A crop changes the frame size, so the video has to be decoded and re-encoded. At CRF 23 with the veryfast preset the loss is invisible on a phone screen, and the file usually gets smaller because the frame has fewer pixels.

If you need the crop to be lossless, crop in a tool that supports it; a compressed video cannot be re-framed without recompression.

Good to know

• Works offline once loaded, including on flights and in locked-down networks.
• No file size limit beyond your device's own memory.
• Free to use, with no account and no usage counter.
• Results are produced locally, so they are not logged or shared with anyone.

Questions