BMP to AVIF converter

BMP stores an image by giving every single pixel its own place in the file. Microsoft defined it as a device-independent bitmap so that the same picture could travel between machines and programs, and the format has carried that job ever since.[1]

Microsoft describes the layout plainly: a header with the file size and the offset where the picture starts, then a second header with the dimensions and the number of bits per pixel, and finally the raw array of points.[2] The flavour you meet most often uses 24 bits per pixel, one byte each for red, green and blue. That layout keeps every pixel value intact, so the file is heavy. A program written decades ago still reads its contents.

AVIF grew out of a completely different idea: it is the compression used by AV1 video, applied to a single frame. The Alliance for Open Media gathered the rules in the AV1 Image File Format specification, whose version 1.2.0 is dated 16 October 2025.[3] The format holds 8 to 12 bits per colour channel, a wide colour gamut, an alpha channel and image sequences, which is how it does animation. It writes the picture either lossily or losslessly, depending on the setting.

Drag your BMP files onto the Add files area, or click it and pick the images from your drive. The Set AVIF quality slider accepts values from 60 to 95 per cent and sits at 80 by default. Then press the Convert button.

You can download the finished images one at a time with the Download button on each queue row, or all at once with Download all. Clear all empties the queue, and the Remove label on a row takes a single file out of it.

In the queue on the right, every file shows its weight before and after conversion together with the difference in per cent. The same summary for the whole batch sits under the buttons, on the Saved line.

The writing itself is done by libavif, the encoder from the Alliance for Open Media project, compiled to WebAssembly and run on your own machine. Browsers read AVIF without any add-on, and writing it from a canvas is still beyond them: on 4 September 2026 we checked Chrome 152, Edge 152 and Firefox 153, and each of them handed back a PNG for that request. The MDN documentation states this behaviour outright — ask for a format the browser does not know and you get a PNG with no warning at all.[4]

Our own encoder writes AVIF whatever the browser can do, and the image stays on your device.

Under the converter sits a format switch reading BMP to AVIF. Clicking either of the two formats opens a list, and picking from it takes you to the converter page for that pair. Every pair in this group is also gathered on the Format conversion page.

You can work out the weight of a BMP file in advance, and the result has nothing to do with what the picture shows. Every pixel takes three bytes, and every row of the image is padded with zeros up to a full four bytes; the Microsoft documentation gives the formula for it.[5] On top of that come 54 bytes of header.

Let us walk through a photo measuring 3000 by 2000 pixels. One row holds 3000 pixels at three bytes each, which is 9,000 bytes. At this width nothing needs padding. There are 2000 such rows, so the pixels alone take 18,000,000 bytes. With the header the file weighs 18,000,054 bytes, which is 17.2 MB in the notation the tool uses in its summary.

AVIF approaches the same picture from another side. The encoder predicts what the next patch of the image looks like from the ones beside it and stores only the difference, and in lossy mode it drops detail that the eye will not catch in ordinary viewing. The MDN guide puts it at around half the weight of a JPEG at comparable quality, and against WebP the median saving runs 50 per cent to 30.[6] Against a BMP, which packs nothing at all, the gap is far wider than that. When the file also has to open in older browsers, the BMP to WebP converter prepares the same image.

Our 3000 by 2000 pixel photo came out of the converter at 603.6 KB with the slider left at its default. A Saved line then appears under the buttons, carrying the number of megabytes and the percentage the file shed.