JPG to BMP Converter

This tool converts JPG (JPEG) images to BMP (bitmap) files in your browser. It is for anyone whose old Windows program, printer driver, embedder or assignment asks for a .bmp, or who is handing an image to software that will not open a JPG.

Two things are worth knowing before you convert, so they come first. The BMP this tool writes is uncompressed 24-bit, so it is usually several times larger than the JPG — for a 12-megapixel phone photo, roughly 35 MB against a few MB for the JPG. And a BMP holds no more quality than the JPG had: the picture is an exact copy of the JPG as it decoded, but the detail JPEG compression discarded is gone, and nothing here brings it back.

There is a reason this tool is built differently from the JPG-to-PNG converter next door. Every browser can decode a BMP, but none can encode one: the canvas API can only write PNG, JPEG and WebP. So the .bmp is assembled byte by byte here, in the page, with the same encoder the rest of the site uses — a 14-byte file header, a 40-byte bitmap information header, then the pixel rows stored bottom-up in blue-green-red order with each row padded to a multiple of four bytes. That is the plainest, most widely readable kind of BMP: the variant every viewer, printer and old Windows program opens.

Because a JPG has no transparency, there is no background colour to choose, and 24-bit BMP has no alpha channel to store one in — every pixel comes out fully opaque, exactly as the JPG was. For the same reason there is no quality slider: BMP does not compress, so there is no quality setting to make. Your browser decodes the JPG with createImageBitmap, which applies the orientation tag a phone or camera stored, so a photo that looked sideways elsewhere is saved upright; the rest of the EXIF data (date, camera, location) and any embedded colour profile are not copied.

The results list shows each file's size before and after, and states the increase rather than hiding it. Everything happens on your device: the images are read into browser memory, converted and handed back as downloads, never uploaded. You can confirm in your browser's developer tools (Network tab) that no request carries your file. Convert up to 50 images per batch, each up to 50 MB.

1. Each file's first bytes are checked for the JPEG signature FF D8 FF. .jpg, .jpeg, .jpe and .jfif files all pass, because they are the same format; any other signature is refused with a message naming the real format. The file name is never trusted, so a JPEG named .txt still converts. 2. The browser decodes the JPEG with createImageBitmap, applying the EXIF orientation tag so the image is upright. 3. The decoded pixels are drawn onto a canvas of the same width and height and read back as RGBA (ImageData). JPEG is opaque, so every alpha byte is 255 — nothing is composited onto a background, and there is no background control to set. 4. Browsers cannot encode a BMP, so the file is written by hand in three parts: BITMAPFILEHEADER (14 bytes): bfType "BM" (0x4D42), bfSize (the size of the whole file), two zero reserved words, and bfOffBits = 54 — where the pixels start. BITMAPINFOHEADER (40 bytes): biSize 40, biWidth, biHeight positive (which means the rows are stored bottom-up), biPlanes 1, biBitCount 24, biCompression 0 (BI_RGB — uncompressed), biSizeImage, 2835 pixels per metre (≈72 dpi), and zero palette fields. The pixel array: image rows stored bottom-up, each pixel blue-green-red, each row padded with zero bytes to a multiple of four. 5. The file keeps its original name with the extension changed to .bmp, and the size before and after is shown.