Convert BMP to PNG

Why BMP files are so big

A typical BMP spends three bytes on every pixel, one each for red, green and blue, and stores them one after another without packing them. The math is simple and unforgiving: a 1920 × 1080 screenshot is about 2 million pixels, so roughly 6 MB as a BMP, whatever is in it.

PNG stores the same pixels but packs them, using the same kind of compression as a ZIP file, and gets them back exactly when the picture opens. How much it saves depends on the picture:

• Screenshots, diagrams and drawings with large flat areas shrink dramatically, often to a small fraction of the BMP.
• Photos and scans shrink less, because their pixels vary so much, but still noticeably.

Either way, not a single pixel changes. Open the PNG beside the BMP and they look identical, because they are.

Where BMPs still turn up

BMP is Windows' own bitmap format and has been part of Windows for decades. You still meet it in:

• older Windows programs that save or export only BMP,
• some scanning, capture and measuring software,
• folders of pictures kept from years ago,
• Paint, which still offers BMP as a choice when you save.

Most websites, email services and phones handle PNG far better than BMP, and some refuse BMP outright. Each conversion also gives disk space back once you delete the BMP.

The one reason to hang on to a BMP is a program or device that reads nothing else, such as an old tool that loads BMP images for a logo or a splash screen. Octoolo opens BMP but does not write it, so keep the original for that use and convert a copy for everything else.

Other ways on Windows

Paint can convert a BMP as well: open it there and save it as a PNG picture. That is fine for a single picture you are already looking at. Octoolo's Convert tool adds a side-by-side view of the BMP and the PNG with both file sizes and the percent saved, so you see the gain before you keep the file, and it sits next to the Photo Editor's other tools, such as crop and resize.

Both routes give the same kind of lossless PNG; the difference is what you see along the way. If you want a smaller file still and can accept a little loss, a photo or scan in BMP makes a much smaller JPG: see BMP to JPG.

Keeping old pictures for good

PNG is a sound format for an archive. It is lossless, it has been a web standard since the 1990s, and practically every program on every system opens it, so a PNG you save today will still open years from now.

A careful way to clean up a folder of BMPs: convert one, open the PNG to check it, and only then delete the BMP yourself. Octoolo never deletes or overwrites the file you opened, so nothing is lost if you change your mind. The PNG keeps the BMP's width and height exactly.

Old pictures sometimes deserve more than a new format. Before exporting, the Photo Editor can straighten a crooked scan, crop off a white border, or liven up faded colors with Image › Auto tone and color, which sets the light and color from the picture itself. Export those changes as a PNG too, and the BMP stays untouched as the original.

One more word on archives: a new format does not protect against a failed disk. Keep important pictures in two places, such as an external drive and a cloud folder.