Guide
From bitmap to WebP: the biggest size cut for old graphics
Converting a BMP to WebP usually produces the largest saving of any image conversion, because it goes from no compression at all to one of the most efficient formats browsers support. A 6 MB bitmap screenshot can end up around 150 KB, and a 36 MB bitmap photo below 1 MB, while looking practically the same.
What to expect by type of picture
Photographic content compresses the most: savings of 95 percent or more are normal. Screenshots and interface captures compress very well too, although lossy WebP can soften the edges of small text slightly; if that matters, choose PNG in the same tool. Simple drawings with a few flat colors are tiny in both WebP and PNG. Looking at the size column after converting a few typical files tells you more than any rule.
Quality setting
The default of 90 percent keeps edges clean and is a safe choice for interface graphics. For photos, 75 to 85 percent is usually indistinguishable at normal viewing size and noticeably smaller. You can convert the same set twice at different settings and compare before deciding.
Transparency
Ordinary 24-bit BMPs have no transparency. Some 32-bit bitmaps, especially from icon editors and games, include an alpha channel; WebP keeps it, so cut-out shapes stay cut out on a website. If a 32-bit BMP shows as completely transparent elsewhere, its alpha channel may simply be empty, which a few older programs write by mistake.
Using the files on the web
All current browsers display WebP. Upload the converted images to your CMS, or reference them in HTML like any other image. When you also need to serve very old browsers, keep a JPG or PNG copy and use the picture element to offer both. For the smallest possible files, Image to AVIF goes further still, at the cost of slower encoding.
Encoding in Safari
Safari can display WebP but cannot create it from a canvas, and silently writes PNG data instead. The tool checks for this and shows a notice instead of producing a mislabeled file. On a Mac, run the conversion in Chrome, Edge or Firefox, or pick PNG output, which every browser can write.
A legacy clean-up example
Imagine a product manual written years ago with 200 BMP screenshots that makes the online help slow. Drop them all, convert to WebP at 85 percent with the longest side limited to 1600 pixels, download the .zip and replace the files. Page weight falls dramatically while every screenshot stays readable. For a print version of the same manual, convert the originals to PNG instead with BMP to JPG, which also offers PNG output.
Privacy note
The bitmaps are decoded and encoded on your own device. Internal screenshots, lab images and unreleased designs never leave it, and the conversion works offline once the page has loaded.
Comparing WebP with the other options
For the same bitmap, PNG gives the exact original pixels but larger files, JPG gives small files without transparency, and WebP sits in between with small files and transparency. A quick way to decide is to convert a typical screenshot three times, once with each format, and compare the sizes and the sharpness of small text at full zoom. Most people end up using WebP for photos and illustrations on websites, PNG for diagrams that must stay pixel-perfect, and JPG only when an older system insists on it.
Checked against the current version of the BMP to WebP on .