What Is an ICO File? The Complete Favicon Format Guide

That tiny image in your browser tab? It almost certainly came from an ICO file. The ICO format has been the standard for Windows desktop icons since 1985 and for website favicons since the early 2000s. It is one of the oldest image formats still in active daily use — and it works differently from every other format you know.

Unlike a PNG or JPEG that contains a single image at one resolution, an ICO file is a container. A single .ico file can hold multiple versions of the same icon at different sizes and color depths. The operating system or browser picks whichever version fits the context — a 16x16 version for a browser tab, a 48x48 version for Windows Explorer, a 256x256 version for a high-DPI desktop shortcut. One file, every size covered.

ICO Technical Specifications

The 256x256 layer deserves a note. Starting with Windows Vista, Microsoft allowed the largest icon size to be stored as an embedded PNG instead of raw bitmap data. This keeps file sizes manageable — a 256x256 uncompressed bitmap at 32-bit color is 256 KB on its own, while the PNG-compressed version is typically 10-30 KB.

Common ICO Sizes and When They Are Used

Not every icon size is equally important. Here is what each standard size is for:

For a website favicon, the minimum is 16x16 and 32x32. Including 48x48 covers Windows pinned sites. A favicon file with these three sizes is roughly 5-15 KB — small enough that load time is negligible.

For a Windows application icon, include all sizes from 16x16 through 256x256. The OS switches between them depending on the display context: a tiny 16x16 in the taskbar overflow, a full 256x256 in the desktop icon preview. For detailed browser and device requirements, see our favicon size guide.

How ICO Files Work

An ICO file is structured as a container with a header followed by individual image entries:

1. File header — declares the file type (icon or cursor) and the number of images inside
2. Image directory — one entry per image, listing its width, height, color depth, and byte offset within the file
3. Image data — the actual pixel data for each size, stored as either raw BMP bitmaps or PNG-compressed blocks

When Windows needs to display an icon, it reads the directory, finds the image entry closest to the required display size, and renders that specific image. A 16x16 toolbar icon and a 256x256 desktop preview both come from the same .ico file — the OS just reads different sections of it.

Browsers do the same thing with favicons. When you set a .ico favicon on your website, the browser downloads the file once and pulls out the size it needs — typically 16x16 for the tab and 32x32 for bookmark displays.

This container approach is what makes ICO uniquely practical for icons. With separate PNG files you would need multiple <link> tags, multiple HTTP requests, and the browser would still download sizes it doesn't use. ICO handles it in a single file.

ICO vs PNG for Favicons

Modern browsers accept PNG files as favicons, which raises a fair question: do you still need ICO?

The practical recommendation: Use both. Place a favicon.ico in your site root for automatic detection by all browsers and crawlers, and add a PNG <link> tag for modern browsers that benefit from it. This covers 100% of browsers with zero drawback.

If you only want one format, ICO is the safer choice. Placing favicon.ico at your domain root requires zero HTML — browsers check for it automatically. That convention has survived three decades of web development for good reason.

For more on choosing between image formats for the web, see JPG vs PNG and What Is PNG?.

How to Create ICO Files

You cannot save an ICO file from most standard image editors. Photoshop, GIMP, and Figma export PNG, JPEG, and SVG — but not ICO. You need a conversion tool.

The quickest path:

1. Start with a square PNG at 256x256 or larger. A transparent background is ideal for icons. If you need to prepare your source image first, see our guide on how to reduce image size for tips on optimization.
2. Convert to ICO using Pixotter's converter. Drop your PNG, select ICO as the output, and download the result. The conversion runs entirely in your browser — your image never leaves your device.

For step-by-step instructions with platform-specific tips, see How to Convert PNG to ICO. Starting from a JPEG instead? See How to Convert JPG to ICO.

Design tips for clean icons:

• Work at 256x256 and let the converter scale down — designing at 16x16 and scaling up produces blurry results
• Use simple shapes with strong contrast — fine detail disappears at small sizes
• Test your favicon at 16x16 before exporting — if it's not recognizable at that size, simplify the design
• Keep transparency clean — avoid semi-transparent edges that might create halos on dark or light browser chrome

How to Use ICO for Website Favicons

Setting up a favicon takes two steps: place the file and add the HTML reference.

Step 1: Place the File

Put favicon.ico in your website's root directory (where index.html lives). Browsers automatically check for /favicon.ico at your domain root — no HTML tag required for this default path.

Step 2: Add the HTML Tag (Recommended)

While /favicon.ico auto-detection works, an explicit <link> tag gives you more control:

<link rel="icon" type="image/x-icon" href="/favicon.ico">

To also serve a PNG favicon for modern browsers:

<link rel="icon" type="image/x-icon" href="/favicon.ico">
<link rel="icon" type="image/png" sizes="32x32" href="/favicon-32x32.png">
<link rel="icon" type="image/png" sizes="16x16" href="/favicon-16x16.png">

The browser uses the last matching <link> it supports. Modern browsers will pick the PNG; older browsers fall back to the ICO.

For Apple touch icons (used when someone saves your site to their iPhone home screen), add a separate 180x180 PNG:

<link rel="apple-touch-icon" sizes="180x180" href="/apple-touch-icon.png">

For the full breakdown of every favicon size and where it appears, see our favicon size guide.

ICO vs Other Icon Formats

ICO is not the only icon format. Here is how it compares to the alternatives:

ICO vs SVG: SVG favicons are the newest option — a single vector file that renders at any size without pixelation. Chrome, Firefox, and Safari all support SVG favicons via <link rel="icon" type="image/svg+xml">. The catch: SVG favicons require the HTML <link> tag (no auto-detection) and complex SVG files can render slowly at tiny sizes. For simple logos, SVG is elegant. For photographic or complex icons, ICO with pre-rendered rasters at each size produces crisper results.

ICO vs ICNS: ICNS is Apple's equivalent of ICO — a container format that bundles multiple icon resolutions for macOS and iOS. If you are building a Mac application, you need ICNS. For everything else (Windows applications, websites, cross-platform tools), ICO is the standard.

Frequently Asked Questions

What does ICO stand for?

ICO stands for "icon." The format was created by Microsoft for Windows 1.0 in 1985 to store desktop icons. The .ico extension is a truncation of the word "icon" — file extensions were limited to three characters in early Windows and DOS.

Can I open an ICO file in a regular image viewer?

Most modern image viewers display ICO files, but they typically show only the largest embedded size. To see all sizes contained in an ICO file, you need a dedicated icon editor or a tool like ImageMagick. On Windows, right-clicking an ICO file and selecting "Preview" shows the default size.

Is ICO the same as a favicon?

Not exactly. A favicon is the concept — the small icon associated with a website. ICO is one file format used to deliver favicons. You can also use PNG or SVG as favicons in modern browsers. Historically, "favicon" and "ICO" were synonymous because .ico was the only format browsers recognized for this purpose.

Do I need an ICO file for my website in 2026?

Strictly speaking, no — modern browsers accept PNG and SVG favicons. Practically, yes. A favicon.ico file in your site root is auto-detected by every browser, bot, and RSS reader without requiring any HTML. Some CMS platforms and older tools still look specifically for favicon.ico. Including one costs almost nothing (5-15 KB) and eliminates a class of compatibility issues entirely.

How many sizes should I include in an ICO file?

For a website favicon, three sizes cover nearly all use cases: 16x16, 32x32, and 48x48. For a Windows application icon, include six sizes: 16x16, 24x24, 32x32, 48x48, 64x64, and 256x256. More sizes mean a larger file but better rendering across display contexts.

Can ICO files contain animation?

No. The ICO format does not support animation. Animated cursors in Windows use the separate .ani (animated cursor) format, which is a RIFF container holding multiple ICO frames with timing data. If you need an animated icon for a website, use an animated GIF, APNG, or CSS animation instead.

What is the maximum resolution for an ICO file?

Each image inside an ICO file can be up to 256x256 pixels. This limit was established with Windows Vista. While some tools can technically create ICO files with larger dimensions, Windows and browsers will not render them correctly. For icons larger than 256x256, use PNG or SVG.

Are ICO files larger than PNG files?

For a single image, yes — an ICO containing one 32x32 icon is slightly larger than the equivalent 32x32 PNG because of the container overhead. For multi-resolution icons, the comparison flips: one ICO file with three sizes is more efficient than serving three separate PNG files (fewer HTTP requests, shared header data). A typical multi-size favicon ICO is 5-15 KB total.

Need to convert images to ICO format? Pixotter's converter handles PNG-to-ICO and JPG-to-ICO conversions directly in your browser. Drop your image, pick ICO, and download — no signup, no upload to any server. You can also compress your images before converting to keep file sizes minimal.