What an ICO file is for
ICO is the icon format used by Windows and by the favicon of a website, the small picture that appears in the browser tab, in bookmarks and in the history list. Unlike a PNG, a single ICO file can hold the same picture at several sizes, so the system can choose a crisp 16-pixel version for a tab and a larger one for a desktop shortcut.
People convert PNG to ICO for a handful of everyday jobs: adding a favicon to a WordPress, Shopify or hand-coded site, customizing the icon of a folder or a desktop shortcut in Windows, giving a small program or script its own icon, or preparing assets for a desktop app built with tools that expect an .ico file.
Several sizes in one file
ConvertirDocs builds the icon with several sizes inside, from 16 up to 256 pixels. Small sizes are used in tabs, lists and the taskbar, and the larger ones in desktop shortcuts and file explorer views with big icons. You don't have to resize anything yourself or assemble the sizes with a separate program.
The icon is square and keeps a transparent background. If your PNG is wider or taller than it is high, it's fitted inside the square, so it's worth cropping it to a square first with “Crop image” if you want it to fill the whole icon.
Tips for an icon that looks sharp
Start from the largest, cleanest PNG you have, at least 256 pixels on each side if possible. A small PNG has to be enlarged for the bigger icon sizes, and enlarged pixels look blurry.
Keep the design simple. At 16 pixels, fine text and thin lines disappear, so a bold symbol, a single letter or a simplified version of your logo works much better than the full logo with a slogan. A transparent background lets the icon blend with light and dark browser themes. If your logo sits on a white background, use “Remove white background” first.
Made on your device
The conversion happens in your browser, so your logo is never uploaded or stored on a server. There's no account and no limit, and you can convert several PNGs at once and download the icons in a ZIP. To use the favicon, upload the .ico to your site's settings or place it at the root of your website as favicon.ico, depending on your platform.