ICO (Windows Icon) Format — Complete Guide

What Is ICO?

ICO is a file format for icons used by the Microsoft Windows operating system and Windows applications. An ICO file is a container — not a single image, but a collection of images stored at multiple sizes and colour depths, all in one file. When Windows renders an icon (on the desktop, in the taskbar, in folder views, or in application titlebars), it selects the best-matching image from the ICO container based on the display context and screen DPI.

The format dates to Windows 1.0 (1985) and has evolved across Windows versions. Modern ICO files support PNG-compressed images at up to 256 × 256 pixels and RGBA colour with full 8-bit alpha transparency.

ICO Container Structure

ICONDIR Header

Every ICO file begins with a 6-byte ICONDIR (Icon Directory) header:

Reserved     : 2 bytes (always 0)
Type         : 2 bytes (1 = icon, 2 = cursor)
Count        : 2 bytes (number of images in the container)

Following the header, there is one ICONDIRENTRY (16 bytes) per image:

Width        : 1 byte  (0 = 256)
Height       : 1 byte  (0 = 256)
ColorCount   : 1 byte  (0 for 32-bit)
Reserved     : 1 byte  (always 0)
Planes       : 2 bytes (colour planes; always 1)
BitCount     : 2 bytes (bits per pixel)
BytesInRes   : 4 bytes (size of image data in bytes)
ImageOffset  : 4 bytes (offset from file start to image data)

Each image in the ICO is either a DIB (Device-Independent Bitmap) or, since Windows Vista, a PNG stream.

Standard Sizes and When Windows Uses Each

Windows applications are expected to provide icons at specific sizes. Display contexts dictate which is selected:

Size	Use case
16 × 16	Taskbar, window titlebar, small folder view, browser favicon
24 × 24	Toolbar icons (some applications)
32 × 32	Standard desktop icon (96 DPI / 100 % scaling)
48 × 48	Large icons in File Explorer
64 × 64	Extra-large icons; high-DPI desktops
128 × 128	High-DPI large icons
256 × 256	Windows Vista+ "extra-large" view; HiDPI / 200 % scaling

A well-prepared ICO file for a modern Windows application typically contains 7–9 sizes covering 16, 24, 32, 48, 64, 128, and 256 px, ensuring crisp rendering at all DPI scaling levels.

Colour Depths

32-bit RGBA (Modern Standard)

32-bit ICO images use 8 bits per channel (RGBA): full 24-bit colour plus an 8-bit alpha channel. This allows smooth anti-aliasing, soft shadows, and semi-transparent edges — essential for icons that look professional against any desktop wallpaper.

8-bit Indexed (256 Colours)

Used for backward compatibility with very old Windows versions. The image stores pixel values as indices into a 256-colour palette. Transparency in 8-bit ICO is binary (1-bit XOR mask) — pixels are either fully opaque or fully transparent with no anti-aliasing.

4-bit (16 Colours) and 1-bit (Monochrome)

Legacy formats rarely used today. Windows still renders them in high-contrast accessibility modes. Modern ICO generators do not include them unless explicitly required.

PNG Compression Inside ICO (Windows Vista+)

Since Windows Vista, images within ICO files may be stored as complete, valid PNG streams instead of raw DIB data. PNG-compressed ICO entries:

Dramatically reduce file size for large images (256 × 256 PNG ≈ 10–30 KB vs. 300 KB as raw 32-bit bitmap)
Retain full RGBA colour and alpha transparency
Are fully supported by Windows Vista, 7, 8, 10, and 11 natively
Are also supported by all major browsers as favicon.ico files

The practical recommendation is to store the 256 × 256 layer (and optionally 128 × 128) as PNG, and smaller layers (16–64 px) as raw 32-bit DIB for widest compatibility.

Favicon Use in Web Development

The browser favicon is a critical web-development use case for ICO files. A favicon.ico placed at the server root is automatically requested by all browsers:

<!-- Basic -->
<link rel="icon" href="/favicon.ico" sizes="any">

<!-- Modern complement (SVG) -->
<link rel="icon" href="/favicon.svg" type="image/svg+xml">

<!-- Apple Touch Icon -->
<link rel="apple-touch-icon" href="/apple-touch-icon.png">

Why Use ICO for Favicons Instead of PNG?

Multi-size container — one ICO file serves 16 × 16 (address bar), 32 × 32 (browser tab), and larger sizes; PNG requires separate files per size
Universal browser support — ICO has been supported since Netscape 5.0 (1999) and works in every browser without declaration
Graceful degradation — older systems always find a compatible size within the ICO container

Creating ICO Files

From PNG Sources

Best practice: design the icon at 256 × 256 px in a vector tool (Illustrator, Figma, Inkscape), export to PNG at each required size, then compile into an ICO. Dedicated tools and online converters (KaijuConverter) automate this compilation.

Design Considerations for Small Sizes

At 16 × 16 px, complex logos become indistinguishable. Professional icon design involves:

Simplification — remove fine details; use bold shapes
Pixel hinting — align critical edges to pixel grid to avoid blurry sub-pixel rendering
Separate artwork per size — the 16 px icon is often a radically simplified version of the 256 px icon, not just a downscale

Anti-aliasing and Alpha Transparency

32-bit ICO images support smooth anti-aliased edges against a transparent background. This allows icons to blend naturally over any colour — desktop wallpapers, browser tabs, dark-mode UIs — without visible jagged edges or a fixed background colour halo.

Animated Icons (ANI Format)

Windows also supports ANI (Animated Cursor) files for animated icons used as cursors. ANI is a distinct format based on RIFF (like WAV/AVI), containing multiple ICO-like frames with per-frame timing. Standard desktop application icons use static ICO; animated icons are reserved for cursor themes.

Converting Images to ICO

PNG → ICO

The most common conversion: PNG images at multiple sizes are compiled into a single ICO container. Transparency from the PNG alpha channel is preserved in the ICO's 32-bit RGBA layers.

SVG → ICO

SVG source allows lossless rasterisation at any target size. Converting SVG to ICO involves rasterising at 16, 32, 48, 64, 128, and 256 px, then packing into ICO. This preserves maximum sharpness at every size.

JPEG → ICO

JPEG has no alpha channel, producing icons with solid (non-transparent) backgrounds. Generally unsuitable for desktop icons; acceptable only for simple solid-background logos.

Advantages and Limitations

Aspect	Detail
Multi-size container	Yes — one file for all DPI levels
Alpha transparency	Full 8-bit (32-bit RGBA layers)
Animation	No (use ANI for animated cursors)
Colour accuracy	Full RGB; not HDR
File size	5–100 KB for a complete modern set
Platform	Windows-native; browsers support favicon.ico universally
macOS equivalent	ICNS (Apple Icon Image)

ICO remains the definitive icon format for Windows software and web favicons, providing a versatile multi-size container trusted across four decades of computing.

ICO Format: Windows Icon Files Explained — Sizes, Layers & Conversion