PNG to BMP Converter

This tool converts PNG images to BMP (bitmap) files entirely in your browser. People need BMP for one reason above all: a program that only reads bitmaps. Older Windows software, Office templates, label and point-of-sale printers, laboratory and industrial instruments, embroidery and CNC software, and a long tail of internal tools accept a .bmp and refuse anything newer. If that is your situation, this page produces the file those programs expect.

BMP is the oldest common image format and the simplest. It stores raw pixels — three bytes for every pixel, one each for red, green and blue — behind a small header that says how big the image is. There is no compression at all, which is why a BMP is usually much larger than the PNG it came from; the trade is that software written decades ago can still read it.

The part that surprises people is that no browser can *create* a BMP. The canvas API encodes exactly three formats — PNG, JPEG (JPG) and WebP — and BMP is not among them, in any browser. So this tool writes the file itself, byte by byte, following the layout Microsoft documents: a 14-byte file header, a 40-byte information header, then the pixel rows. It produces an uncompressed 24-bit BMP, the variant every viewer, printer and old Windows program opens.

Because a 24-bit BMP has no alpha channel, it cannot store transparency. A see-through PNG — a logo, a product cut-out, a signature — is composited onto a solid colour before it is written: white unless you pick another. Semi-transparent edge pixels blend smoothly onto that colour, exactly as they would if you flattened the image in an editor. The tool tells you when a file may contain transparency and shows the colour control, so nothing lands on white by surprise.

What is kept: the pixels themselves, copied exactly at their original dimensions, with no resampling, no quality setting and no lossy compression. What is not: transparency (flattened onto the background), anything above 8 bits per channel (a 48-bit PNG is reduced to 24-bit colour), metadata such as EXIF, XMP and embedded colour profiles, and animation — an animated PNG (APNG) keeps only its first frame. The file also grows, usually by several times: an uncompressed 24-bit pixel simply takes three bytes, where PNG's deflate compression can take well under one for flat graphics.

Everything happens on your device. Up to 50 PNG files per batch, each up to 50 MB, and images larger than 100 megapixels or 16,384 pixels on a side are refused, because a browser cannot safely hold them. You can confirm nothing is transmitted in the Network tab of your browser's developer tools: no request carries your image.

1. The file's first eight bytes are checked for the PNG signature (89 50 4E 47 0D 0A 1A 0A). A file that is really a JPG, WebP or GIF with a .png name is caught here and named, because the bytes decide — never the extension. 2. The IHDR chunk, which the PNG format requires to come first, gives the width, the height and the colour type. The dimensions let the exact output size be reported before anything is decoded; the colour type (4 or 6 have an alpha channel) plus any tRNS chunk tell the tool whether transparency is even possible. 3. The flatten colour is validated as hex (#fff or #ffffff) and defaults to #ffffff. 4. The browser decodes the PNG. A canvas of the image's exact size is filled with the flatten colour and the PNG is drawn on top, so transparent pixels become that colour and semi-transparent edges blend into it. The pixels are read back with getImageData as 8-bit sRGB. 5. The pixels are encoded as an uncompressed 24-bit BMP. No browser can do this, so the bytes are written here: • BITMAPFILEHEADER, 14 bytes: 42 4D ('BM'), the file size, two reserved words of 0, and the offset of the pixel data (54). • BITMAPINFOHEADER, 40 bytes: header size 40, width, height (positive, meaning the rows are stored bottom-up), planes 1, 24 bits per pixel, compression BI_RGB (0, uncompressed), image size, 2835 pixels per metre, and 0 for colours used and colours important. • Pixel rows, bottom-up: the bottom image row is written first, three bytes per pixel in blue-green-red order, and each row is padded with zero bytes up to a multiple of four. 6. The result is named after the input with a .bmp extension, and the size before and after is shown for each file. File size = 54 + ((width × 3) rounded up to a multiple of four) × height.