What a ICO file actually is
The ICO format is Windows' icon container, from Windows 1.0 in 1985. One file holds a set of images of the same picture at different sizes, typically 16, 32, 48 and 256 pixels, so that the system can pick the right one for a taskbar, a desktop or a file list. Since Windows Vista the 256-pixel entry is usually a PNG embedded inside the ICO, which is why the files are small.
On the web the same format became the favicon: the little picture in a browser tab, saved as favicon.ico at the root of a site since Internet Explorer 5 in 1999. Browsers now accept PNG and SVG favicons too, but every one of them still reads ICO, and it remains the one format that works in every browser and every bookmark list.
So the file is not hard to open. Every browser displays it, and Windows shows it as itself. The two real questions are how to make one from a logo, and how to get a full-size PNG back out of one, and both are conversions the image converter handles in the browser.
How to open a ICO file on each device
Windows
Explorer shows an ICO as its own icon, and Photos and Paint open it, at the largest size it holds. Paint saves ICO too, but only one size. IrfanView and XnView open every size in the file and are free.
macOS
Preview and Quick Look open ICO and let you pick the size in the sidebar. Preview exports the chosen size to PNG.
iPhone and iPad
Files and Safari display an ICO. There is little reason to open one on a phone, but nothing refuses it.
Android
Chrome and Files by Google open ICO. Gallery apps usually ignore it.
In a browser
Chrome, Firefox, Edge and Safari all render ICO, and use it as the favicon of a site. Drag the file into a tab to see it at its largest size. The box above uses exactly that.
In Photoshop, GIMP and other editors
GIMP and Affinity open and save ICO with all sizes. Photoshop needs a free plug-in to read or write it; without one, export a PNG and convert it here. Figma, Sketch and Illustrator export PNG or SVG and leave the ICO step to a converter.
When it will not open, or opens wrong
The editor opens a tiny 16-pixel version, not the big one
The ICO holds several sizes and the editor picked the first entry.
The fix: Open it in GIMP, IrfanView or macOS Preview, all of which list every size and let you choose the largest. Converting to PNG here extracts the largest entry.
Photoshop will not open or save it
Photoshop has never shipped ICO support.
The fix: Install a free ICO plug-in, or export a PNG from Photoshop and convert it to ICO with the image converter.
The favicon does not show in the browser
The file is not at the root of the site, is referenced with the wrong path, or is cached from a previous version.
The fix: Put it at /favicon.ico, add a link tag in the page head, and force-reload; browsers cache favicons stubbornly, so a fresh private window is the honest test.
The icon looks blurry in the taskbar or on the desktop
The file holds only a small size and Windows is scaling it up.
The fix: Rebuild the ICO from a 256-pixel PNG so that every size is present. Start from a square PNG at least 256 pixels wide.
The icon is a square with the background filled in
The source image had no transparency, or was converted from JPG, which has none.
The fix: Make the icon from a PNG with a transparent background. The background remover on kliptools.io produces one from a logo on a solid colour.
A site or an app wants a PNG icon, not an ICO
Modern browsers, iOS home screens, Android and app stores take PNG or SVG.
The fix: Convert to PNG here at the size they ask for. Keep the ICO for /favicon.ico and old browsers.
What to do with it next
Questions people ask
What is an ICO file?
A Windows icon file: the same image stored at several sizes in one container, used for program icons and for website favicons. Every browser and operating system displays it.
How do I open an ICO file?
Double-click it on Windows or a Mac, or drag it into any browser. Preview, Paint, GIMP and IrfanView open it; Photoshop needs a plug-in.
How do I convert a PNG to ICO?
Use the image converter on this site: drop a square PNG, choose ICO, download. Start from at least 256 pixels so the icon stays sharp at every size.
How do I get a PNG out of an ICO?
Convert it with the image converter, which extracts the largest size, or open it in Preview or GIMP and export the size you want.
Why does my favicon not update?
Browsers cache favicons aggressively. Clear the cache or open the site in a private window, and check the file is really at /favicon.ico or linked from the page head.
Can I use a PNG instead of an ICO for a favicon?
Yes, every modern browser accepts a PNG or SVG favicon via a link tag. Keep an ICO at /favicon.ico as well for older browsers and for tools that look there without reading the page.