Convert BMP to PNG

How to convert BMP to PNG

1. Choose one or more BMP images — drag them in or use the file picker.
2. Download the PNG — a single file, or a zip if you converted several at once.

An uncompressed bitmap becomes a far smaller PNG with every colour value intact

A bitmap and a PNG hold the very same thing — an exact, lossless record of every pixel — but the bitmap stores it raw, writing each row out in full with nothing compressed, while PNG keeps that identical set of colour values and packs them down. That is what makes this the one conversion on the tool with no cost on either side: most of the bytes fall away, and not a single colour value is traded to lose them.

Every other source this tool accepts reached PNG already squeezed: a GIF arrived LZW-packed, a JPEG, WebP or AVIF arrived lossily compressed with quality already spent. A bitmap is the only one that hands over a raster nothing has yet touched, so PNG is the first stage to compress it at all — and the redundancy a screenshot is full of, flat toolbars and repeated scanlines, is all still there to find. Because both formats are lossless, finding it costs no colour value; the shrink is pure.

So bitmap-to-PNG is the pairing with nothing to weigh up. Point the tool at anything else here and there is a catch to accept — a lossy JPEG or WebP output, or a source whose colours were already thinned before it arrived. Send it a bitmap and the lossless PNG is simply smaller and concedes nothing back; a flat screenshot often lands under a tenth of the original size. The check takes seconds — the two images match pixel for pixel while the file listing shows one a fraction of the other.

A 32-bit bitmap’s fourth byte is undefined; a PNG’s alpha channel is not

At 32 bits per pixel a bitmap carries a fourth byte alongside the blue, green and red — and the standard BITMAPINFOHEADER never records what that byte is for. Some encoders write an alpha value into it, some write zeros, and much software treats it as dead padding and reads straight past. Nothing in the pixel data marks which was meant, so the same four bytes are opacity to one program and a wasted byte to the next.

Later revisions tried to pin it down, unevenly. BI_BITFIELDS lets a header declare where the red, green and blue bits sit but says nothing about alpha; only BI_ALPHABITFIELDS and the dedicated alpha channel of the BITMAPV4 and V5 headers name an alpha mask outright — and declaring one is optional, while honouring it on read is far from universal. So one 32-bit .bmp can open transparent in an editor and opaque in the viewer beside it, from the very same bytes.

That is the guessing game the PNG ends. Drawn onto a canvas and re-encoded, a standard bitmap — its fourth byte mere padding — flattens to a solid, opaque raster, the reading almost every file intended for it. PNG then defines its channels outright, colour and alpha alike, rather than leaving a byte’s meaning to convention, so the picture reads the same in every decoder. The only thing dropped is the ambiguity; no pixel a normal bitmap meant to show goes with it.

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