BMP to ICO Converter

Windows still uses the ICO format for program, shortcut and folder icons, and many older tools only accept icons whose small sizes are stored as bitmaps. This converter draws your BMP at each size you tick and writes a real multi-image .ico — not a renamed BMP. Small sizes (48 px and below) are stored as classic 32-bit BMP entries by default, the form Windows XP and old icon editors expect; larger sizes are PNG-compressed. Everything runs in your browser.

How to use this tool

1. Choose one or more .bmp files; each becomes its own .ico.
2. Tick the sizes to include: 16, 32, 48 and 256 cover Explorer, the taskbar and desktop shortcuts.
3. Choose how a non-square picture is made square: padded, cropped or stretched.
4. Keep "Classic BMP entries" on for old Windows versions, then download the icon.

Formula

ICO = 6-byte header (0, type 1, count) + one 16-byte entry per size (width, height — 0 means 256 — 1 plane, 32 bits, byte length, offset) + the images. A classic entry is a 40-byte BITMAPINFOHEADER with the height doubled, bottom-up BGRA rows and a 1-bit transparency (AND) mask.

Each mask row is padded to a multiple of 4 bytes. A pixel is marked transparent in the mask when its alpha is below 128, so old systems that ignore alpha still show a clean outline. Sizes from 64 px up are stored as PNG, which Windows Vista and later read directly.

Example

With only 16 and 32 px ticked as classic entries, the icon is exactly 5,430 bytes: 6 + 2 × 16 = 38 bytes of directory, 40 + 1,024 + 64 = 1,128 bytes for the 16 px image and 40 + 4,096 + 128 = 4,264 bytes for the 32 px one.