Convert a video to 9:16 or 1:1

How Video Aspect Ratio Converter 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.

Why black bars look like an amateur upload

A 16:9 screen recording dropped into a 9:16 Reel leaves the platform two choices: crop the sides, or letterbox it with black. Both look wrong — the first cuts off half the interface, the second shrinks the video to a strip in the middle of the screen.

The third option is to fill the empty space with a blurred, darkened, enlarged copy of the video itself. It carries the colours of the frame, so the eye reads it as background rather than as a mistake, and the original shot stays completely visible on top.

The exact filter chain

Background layer: scale up until the frame covers 1080×1920, crop the overflow, blur with a Gaussian of sigma 28, and darken slightly so the foreground separates. Foreground layer: scale down until the whole frame fits inside 1080×1920. Composite: overlay the foreground centred on the background.

The crop mode is the short version — scale up until it covers, crop, done. It keeps every pixel of the frame area at full quality and loses the edges. For a talking-head clip that is usually the better look; for a screen recording it destroys the interface.

Re-encoding, and the quality it costs

Changing the frame means decoding every frame and encoding it again, which is lossy even at high quality. The encoder here runs libx264 at CRF 22 with the veryfast preset: visually close to the source for social delivery, and fast enough to finish a two-minute clip in a couple of minutes on a laptop.

Audio is copied through AAC at 128 kbps. Because the encode happens in WebAssembly inside the tab, there is no upload, no watermark, no export limit and no queue — the same trade the paid editors make, minus the subscription.

The 31 MB that buys all of it

The video engine is a full build of ffmpeg compiled to WebAssembly. It is about 31 MB and downloads on the first conversion, then stays in the browser cache for the session. That is the honest cost of doing this locally instead of renting a server: one heavy download instead of a monthly fee.

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