BMP to PNG converter

BMP writes an image point by point, in the shape it reaches the screen in. The format was built for OS/2, later moved into Windows and stayed there for good.[1] In its most common variant a single point takes 24 bits, one byte each for red, green and blue. That notation can name 16.7 million shades, and every pixel gets its own place in the file along with its full colour value.

PNG describes the same image differently. Before anything reaches the file, the format puts the image through two stages. First it predicts the value of every pixel from its neighbours and stores only the difference against that prediction. Then it packs the result with Deflate compression. Both stages are reversible, so the finished file gives back exactly the image that went into it. The format was designed in 1995, after Unisys and CompuServe raised patent claims over the LZW compression used in GIF.[2]

The description in force today is the third edition of the specification, which W3C published as a Recommendation on 24 June 2025.[3] It lists five PNG image types and bit depths from 1 to 16 bits per channel. On top of that it adds animation built from frames and a chunk named eXIf, in which the file carries photo metadata.

Drag your BMP files onto the Add files field, or click that field and pick the images from your disk. You can put many files into the queue at once. Then press the Convert button.

You download finished images one at a time with the Download button next to a queue row, or all together with the Download all button. The Clear all button empties the whole queue, and the Remove label next to a row takes a single file out of it.

In the queue on the right, every file shows its size before and after conversion together with the difference in per cent. Our 1200 by 630 pixel screenshot wrote Before: 2.16 MB · After: 184.9 KB (92% less) there. The same summary for the whole batch sits under the buttons, in the Saved line.

The write itself is done by the browser with its own PNG encoder. PNG is the format a browser canvas returns by default, and the quality setting works only with lossy formats such as JPEG and WebP.[4] Conversion to PNG therefore runs on a single setting and gives you the full image every time.

The file stays on your device from the first click to the end of the job. Under the converter sits a format switch reading BMP to PNG. Clicking either of the two formats opens a list, and picking from it takes you to the converter page for that pair. We have also gathered every pair from this group on the Format conversion subcategory page.

The size of a BMP file can be worked out in advance, and the result does not depend on what the picture shows. Every pixel takes three bytes, and every row of the image is padded with zeros up to a full four bytes; Microsoft documentation gives a ready formula for this.[5] On top of that come 54 bytes of header.

Let us follow that through on a photo measuring 3000 by 2000 pixels. One row holds 3000 pixels at three bytes each, which is 9,000 bytes. At this width nothing needs padding, because 9,000 divides by four. There are 2,000 rows, so the pixels alone take 18,000,000 bytes. Together with the header the file weighs 18,000,054 bytes, which is 17.2 MB in the notation the tool's summary uses.

PNG takes a different route and saves space in two steps. The first step is filtering. For each row the encoder picks one of five prediction methods and stores the pixel's difference against its neighbour instead of the value itself.[3] On a smooth surface those differences sit close to zero. The second step is Deflate compression, described in RFC 1951, which walks that sequence and writes every repetition as a single reference.[6]

That is where the spread of results in our measurement comes from. An image with large flat areas leaves long runs of zeros after filtering and slims down heavily: our 800 by 600 pixel graphic went from 1.37 MB to 40.2 KB. A camera photo holds a different value in almost every pixel, so there are few repetitions there. The 3000 by 2000 pixel file went from 17.2 MB to 14.2 MB, and a result like that is normal for photography. When a photo needs to lose much more weight, the BMP to JPG converter will prepare it.