How to Convert BMP to WebP in Your Browser

How BMP becomes WebP

1. The browser reads the BMP's pixel array more or less directly, since BMP typically stores each pixel's color values with little to no compression — there's no meaningful decompression step.
2. Any alpha channel present in a 32-bit BMP is read alongside the color data; most BMP files, though, are 24-bit with no transparency information at all.
3. The raw pixel grid is drawn onto an in-memory canvas.
4. The browser's WebP encoder compresses that canvas data at your chosen quality, applying the prediction and entropy coding BMP never had in the first place.

What changes when you convert

The size difference is usually the headline. Because BMP stores pixel data with little to no compression, its file size scales directly with pixel dimensions — a large uncompressed BMP can easily run tens of megabytes for an image a compressed format would store in a fraction of that. WebP exploits redundancy in the image data the way BMP never attempts to, so converting to WebP typically produces one of the larger size reductions you'll see among the conversions on this site, for visually equivalent content.

Transparency is the exception rather than the rule. Most BMP files are 24-bit and simply have no alpha channel to carry forward, so the WebP output ends up fully opaque. When the source is a 32-bit BMP with real alpha data, the browser decodes that channel, and it can be preserved in the WebP encode when your browser's encoder supports alpha writes.

When WebP is the right choice — and when it isn't

WebP is close to a strict improvement here for anything headed to the web: BMP was never designed for compressed delivery, and a WebP with broad current browser and software support removes the size penalty a raw bitmap imposes, without giving up meaningful quality. Keep the BMP if you need to preserve every raw pixel value for print or archival purposes, or the file has to stay compatible with older Windows software that specifically expects an uncompressed bitmap — those aren't cases WebP was built to serve.

For most practical purposes — attaching an image to a page, sharing it online, or just reclaiming storage — converting away from BMP is one of the more clear-cut format decisions available, since the source offers no compression at all to begin with.

Limitations and privacy

Because most BMP files carry no alpha channel, there's nothing this conversion can add — if the source has no transparency, the WebP output won't either. Very large uncompressed BMP files can also take noticeably longer to decode in the browser than an equivalent compressed source would.

Conversion happens locally in your browser; the BMP is read, canvas-rendered, and re-encoded as WebP without being uploaded anywhere.