Convert PNG to ICO

1. Choose a PNG file or drop it onto the area above. A square image of at least 256 × 256 pixels gives the best result.
2. The ICO file is created at once with all six sizes. Untick sizes you do not need.
3. Optional: in the advanced settings choose how non square images are fitted.
4. Check the preview of the sizes and choose “Download ICO”.

An ICO file holds several images in different sizes so that Windows can pick the right one for each display. Sizes up to 128 pixels are stored as 32 bit bitmaps with an alpha channel and an additional transparency mask, the 256 × 256 size as PNG, as Windows itself has done since Vista. The directory of the file follows the ICO format.

The finished file is read again with a separate checker: directory, position and length of every image, bitmap headers and the PNG structure of the 256 pixel size. Only then is the download offered.

Transparency of the PNG is kept in every size. Icons are square. An image with another aspect ratio is never stretched: by default it is fitted completely and the free margins stay transparent. Alternatively it fills the square and overhanging edges are cropped.

If the PNG is smaller than a chosen size it is enlarged for that size, and the tool warns about possible blur. The PNG may be up to 50 MB and 50 megapixels, on smartphones and devices with little memory 16.8 megapixels. Large reductions are calculated in several steps so that small icons do not look jagged.