Convert BMP to JPG

How to convert BMP to JPG

1. Choose one or more BMP images — drag them in or use the file picker.
2. Set the JPG quality if you want a smaller file or sharper output.
3. Download the JPG — a single file, or a zip if you converted several at once.

A 1920×1080 bitmap is 6.2 MB whether it shows a crowd or a blank wall

A 24-bit Windows bitmap spends three bytes on every pixel, one each for blue, green and red, and adds a 14-byte file header and a 40-byte information header in front of them. At 1920×1080 that is 5,760 bytes for a row of pixels and 6,220,800 for the picture, 6,220,854 with the headers — a shade over 6 MB. A photograph of a crowded street and a blank white rectangle of the same dimensions both weigh exactly that.

Each row is also padded up to a multiple of four bytes, which is invisible at 1920 pixels because 5,760 already divides by four, but a 1023-pixel-wide bitmap needs 3,069 bytes a row and is rounded to 3,072. The format does define run-length compression, BI_RLE4 and BI_RLE8, but it is available only to 4-bit and 8-bit palettised images. For 24-bit colour there is no compressed variant at all, so nothing in the picture can alter the total.

That is why the saving here dwarfs anything else on this site: the JPEG encoder is not refining a previous encoder’s work, it is compressing pixels that were never compressed at all. A 6 MB bitmap of a photograph typically lands in the low hundreds of kilobytes, better than ninety per cent gone. The total scales with the pixel count, so a 4000×3000 scan starts near 36 MB and shrinks in the same proportion.

Nobody sets out to make a BMP; some program saved one by default

Pressing Print Screen drops a device-independent bitmap onto the clipboard, and an editor that pastes and saves it writes exactly that structure to disk; Paint made BMP its default for years, so pasted screen grabs are a common way to end up holding one. Whatever filled the screen, the file is the same fixed size — a 2560×1440 capture is 11 megabytes of raw pixels whether it froze a video or an empty desktop, because the grab copied three bytes per pixel with nothing compressing them.

Outside consumer software the format is chosen on purpose, and for output worth handling carefully. Microscopes, medical imagers and factory inspection cameras often dump a bitmap because a raw block of pixels needs no compression library and cannot be accused of altering a measurement. Encoding that frame to JPEG puts back exactly the lossy step the instrument was written to avoid: the 8×8 transform and chroma subsampling quietly nudge the numbers. The JPEG is a copy to circulate; the bitmap stays the record.

The same bulk that makes a bitmap awkward to email is why it stays worth keeping. Those megabytes are every pixel written out in the open at three bytes each — no codec sits between the file and the original samples, so nothing has to be undone to read them back. The JPEG built here is the small copy for sending or posting; the bitmap is the exact record, which matters when the picture will be reopened and edited rather than merely viewed.

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