Why a BMP shrinks so much as AVIF
A BMP (the Windows bitmap format from Microsoft) stores the colour of every pixel as-is. Run-length compression exists in the format but almost nothing uses it, so file size depends only on pixel count. A 1920 by 1080 image at 24 bits per pixel is about 6 MB whether it shows a photo or a plain white box.
AVIF encodes the picture with AV1, a video codec (compression method) that drops detail your eye does not notice, so the output is a small fraction of the BMP. The trade is that AVIF is lossy. For a screenshot or a photo going on a web page that's fine. If every pixel must stay exact, say a scanned form or a pixel-art sprite, use BMP to PNG instead; PNG isn't lossy.
How to convert BMP to AVIF
- Drop in up to 5 BMP files. Even a big scanner bitmap fits under the 300 MB limit per file.
- Leave AVIF as the target, or switch one file to PNG if it needs an exact copy.
- Wait for the upload and the encode, then download each .avif.
Conversion runs on our servers, so the upload of a large bitmap is the slowest step.
Where the AVIF will and will not open
Chrome 85, Firefox 93 and Edge 121 or later display AVIF, as do Safari on iPhone from iOS 16 and Safari 16.4 on Mac. Many older desktop programs do not, including some that write BMP. Keep the BMP if you still edit the picture there, or go back with AVIF to BMP. More targets are on the BMP converter page.