BMP to PNG

Why is a BMP so big, and how much smaller is BMP to PNG?

A BMP is big because it usually stores every pixel uncompressed. Converted to PNG, PixVibe's test screenshot came out 98% smaller and the test photo 21% smaller. The Library of Congress description of BMP (fdd000189, checked 15 November 2026) notes that “In practice most BMP images are uncompressed”. A 1600 × 1000 picture at 24 bits per pixel therefore takes 3 bytes for each of its 1.6 million pixels, about 4.8 MB (1 MB = 1,000,000 bytes), no matter what it shows.

PNG compresses without loss, so the saving depends on the content. On Fast in Chrome, our screenshot went from 4.8 MB to 98.4 KB, a 256-color diagram from 1.6 MB to 208.4 KB, and a transparent logo from 2.56 MB to 67.3 KB. A photo is different: grain and gradients leave little to compress, and ours shrank only 21%, to 3.77 MB. Compact brought it to 2.59 MB with the same pixels; “Reduce colors” at 256 colors reached 1.04 MB, but that setting is lossy. If a photo has to be really small, a JPG made with the JPG converter is the usual answer.

Will a transparent BMP stay transparent as PNG?

In PixVibe's tests in Chrome (November 2026), a 32-bit BMP that uses a V5 header (BITMAPV5HEADER) with an alpha mask kept its transparency as a PNG, while one written with the basic 40-byte header (BITMAPINFOHEADER) came out opaque, often with black where it had been transparent. Microsoft's documentation of BITMAPV5HEADER (checked 15 November 2026) describes an alpha mask that “specifies the alpha component of each pixel”, a header field that tells programs which byte holds transparency. For this test we drew a separate 400 × 400 logo (not the 800 × 800 one in the table) with a clear background, soft edges and a 50%-transparent ring, saved it three ways and converted each with PixVibe's BMP to PNG tool.

V5 header with an alpha mask: the PNG kept the transparency, and every alpha value matched.

Basic 40-byte header, alpha written into the fourth byte of each pixel (some programs and image libraries still save it this way, including Pillow 12.2, the Python imaging library (the version we tested)): that byte was ignored and the PNG came out fully opaque. The clear areas turned black in our file, because they show whatever color was stored there. The preview in the tool already shows this before you convert.

Basic 40-byte header, fourth byte all zeros: the PNG was opaque and showed the stored colors exactly; it did not turn invisible.

If a logo arrives with a black box around it, export it again from the program that made it, straight to PNG. In the case that kept transparency (the V5 header with an alpha mask), 1,219 of the 29,056 partly transparent pixels changed by 1 on the 0–255 scale in a single color channel. The tool draws the image on a browser canvas, and under the HTML standard a canvas stores semi-transparent colors premultiplied by their alpha, a conversion the standard describes as lossy for colors that are not fully opaque. Canvas rounding changed one color value of some of those pixels by 1; fully opaque and fully clear pixels were exact.

Which unusual BMP files can be converted?

In PixVibe's tests in Chrome (November 2026), the BMP to PNG tool converted most unusual BMPs: RLE8 and RLE4 compressed files, a top-down bitmap (stored with a negative height), 1-bit black and white, 4-bit, 16-bit (565 and 555), and files with the old OS/2 and the V4 headers. All came out the right way up, and the palette and 24-bit ones matched the original pixel for pixel. A 16-bit file shows the colors it actually holds: our red of 220, 30, 30, saved as 16-bit 565, came back as 222, 28, 25, because 16 bits hold fewer shades.

Two kinds of BMP did not convert correctly in these tests. BMPs that wrap a JPEG or PNG inside (compression types BI_JPEG and BI_PNG) stopped with “Couldn't read this image — it may be corrupted or not a real image file.” A file cut off halfway converted without any warning, and the missing half became transparent, so look at the preview when a BMP may be damaged. The tool relies on the browser's own BMP decoder; we tested Chrome only, and other browsers may handle these files differently.