What Is ICO Format? Favicon & Icon Files Explained

ICO is a container format used almost exclusively for application icons and browser favicons on Windows and the web. Its defining characteristic is that a single .ico file can store multiple versions of the same icon at different sizes, allowing the operating system or browser to pick the most appropriate resolution for each context.

What Sizes Are Included

jpg.now's ICO converter generates multiple resolutions from a single source image:

16 × 16 px - Browser tab favicon, Windows Explorer small icon
32 × 32 px - Windows taskbar
48 × 48 px - Windows desktop icon
64 × 64 px - High-DPI taskbar
128 × 128 px - Application installer thumbnails
256 × 256 px - Windows Vista+ large icons, stored as compressed PNG within the ICO container

How Browsers Choose an Icon Size

When a browser loads a favicon it selects the size closest to its required display size accounting for screen pixel density. On a 2× Retina display, a 32 px favicon slot uses the 64 px variant. Because the ICO container embeds all sizes, the browser always picks the sharpest match automatically.

Best Practices for Source Images

Use a square image- Non-square inputs are cropped or padded, which may not match your design intent.
At least 512 × 512 px source resolution gives the downscaling algorithm enough pixel data to produce crisp small icons.
Simple, bold designs with high contrast read well at 16 px; intricate illustrations become illegible.
If your logo has a transparent background, convert a PNG (not JPG) source to preserve that transparency in the ICO file.