Same pixels, smaller file
A BMP usually stores its pixels uncompressed. PNG stores the same pixels with lossless compression: the data is packed more efficiently, and when the file is opened again every pixel comes back exactly as it was. Converting BMP to PNG therefore keeps the image pixel for pixel while, in most cases, cutting the file size considerably.
How much depends on the content. Screenshots, diagrams, interface graphics and drawings with large areas of flat colour compress very well, so their PNGs are often a small fraction of the BMP. Photos and scans of textured material compress less, because lossless compression has to record every speck of grain, but they still usually come out smaller than an uncompressed BMP.
Where BMP files come from, and why PNG fits better
BMP is the bitmap format that has shipped with Windows for decades. It still appears in exports from older Windows software, scanners, lab and industrial equipment, legacy business systems and old game assets. Its simplicity is its only real advantage today: it is bulky, and many websites, content management systems and forms do not accept it.
PNG keeps everything that makes BMP useful for exact work, and adds more:
- Lossless storage: the right choice for screenshots, technical drawings, charts and pixel art, where every pixel matters.
- Wide acceptance: PNG opens in practically every browser, editor, office application and upload form.
- Transparency: PNG has a full alpha channel, so you can cut out parts of the image later in an editor.
- Safe re-editing: you can edit and re-save a PNG as often as you like with no loss.
Details of the conversion
- Bit depth and colour: browsers work with 8 bits per channel and output is in sRGB, which suits typical BMP files.
- Transparency: most BMPs are fully opaque, and so is the resulting PNG. If a BMP does contain transparent pixels, the PNG keeps them.
- Size control: Settings → Size can scale by a percentage or fit within a maximum width and height, keeping the aspect ratio and never enlarging.
- Limits: each file can be up to 50 MB and 100 megapixels on a desktop computer (50 megapixels on iPhone, iPad and devices that report low memory).
- Batches: up to 10 BMPs per batch, processed one at a time and downloadable as a ZIP.
PNG or JPG for your BMP?
PNG is the right target when the image must stay exact: screenshots, interface mock-ups, diagrams, line art, scanned text you will process further, or anything you plan to edit. Of the two, it is also the only one that can carry transparency.
For photographs and photo-like scans that you simply want to share, a JPG is usually much smaller again than the PNG, with a difference that is hard to see at a good quality setting. In that case use BMP to JPG. And if the images are heading for a website, a PNG can be made lighter still with PNG to WebP.
BMP, PNG and JPG compared
| Feature | BMP | PNG | JPG |
|---|---|---|---|
| Compression | Usually none | Lossless | Lossy |
| File size | Very large | Smaller; varies with content | Smallest for photos |
| Exact pixels | Yes | Yes | No |
| Transparency | Rare | Full alpha channel | No |
| Accepted on the web | Rarely | Almost everywhere | Almost everywhere |
| Best for | Legacy software | Screenshots, graphics, editing | Photos for sharing |