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Guide

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

PC By Pablo Cirre •

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

Recommended Favicon ICO Contents

For a modern website, the favicon.ico should contain:

  • 16 × 16 (32-bit RGBA) — browser address bar, tab strip
  • 32 × 32 (32-bit RGBA) — Windows taskbar pinned site, high-DPI address bar
  • 48 × 48 (32-bit RGBA) — Windows taskbar (large icons)

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.

Related conversions

Frequent conversions across the catalogue:

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Frequently Asked Questions

For a modern Windows application, include at minimum: 16×16, 32×32, 48×48, and 256×256 pixels, all in 32-bit RGBA. Adding 24×24, 64×64, and 128×128 improves rendering at intermediate DPI scaling levels (125%, 150%). Store 256×256 as PNG-compressed inside the ICO to keep file size reasonable.

For a moderno Windows aplicativo, include at mínimo: 16×16, 32×32, 48×48, e 256×256 pixels, all in 32-bit RGBA. Adding 24×24, 64×64, e 128×128 improves rendering at intermediate DPI scaling levels (125%, 150%). Store 256×256 as PNG-compressed inside the ICO to keep tamanho do arquivo reasonable.

For a modern Windows Anwendung, include at minimal: 16×16, 32×32, 48×48, und 256×256 pixels, all in 32-bit RGBA. Adding 24×24, 64×64, und 128×128 improves rendering at intermediate DPI scaling levels (125%, 150%). Store 256×256 as PNG-compressed inside the ICO to keep Dateigröße reasonable.

For a moderno Windows aplicación, include at mínimo: 16×16, 32×32, 48×48, y 256×256 pixels, all in 32-bit RGBA. Adding 24×24, 64×64, y 128×128 improves rendering at intermediate DPI scaling levels (125%, 150%). Store 256×256 as PNG-compressed inside the ICO to keep tamaño de archivo reasonable.

On KaijuConverter every file is processed inside an isolated container, encrypted in transit (TLS 1.3) and at rest, and automatically deleted after 60 minutes with multi-pass overwrite. We never train on, share, or analyze user content. For maximum privacy on extremely sensitive material, prefer offline tools (ImageMagick, FFmpeg, LibreOffice) that you control end-to-end.

Yes, modern browsers support PNG favicons declared with <link rel="icon" type="image/png">. However, a favicon.ico at the server root is automatically requested without any HTML declaration by all browsers and tools (RSS readers, bookmarks, link-sharing previews). Using both is best practice: ICO for universal coverage, PNG for high-resolution.

Sim, moderno browsers support PNG favicons declared com <link rel="icon" type="image/png">. Porém, a favicon.ico at the server root is automatically requested sem any HTML declaration by all browsers e ferramentas (RSS readers, bookmarks, link-sharing previews). usando both is best practice: ICO para universal coverage, PNG para alta-resolution.

Ja, modern browsers support PNG favicons declared mit <link rel="icon" type="image/png">. Jedoch, a favicon.ico at the server root is automatically requested ohne any HTML declaration by all browsers und Werkzeuge (RSS readers, bookmarks, link-sharing previews). Using both is best practice: ICO für universell coverage, PNG für hoch-resolution.

Sí, moderno browsers support PNG favicons declared con <link rel="icon" type="image/png">. Sin embargo, a favicon.ico at the server root is automatically requested sin any HTML declaration by all browsers y herramientas (RSS readers, bookmarks, link-sharing previews). Using both is best practice: ICO para universal coverage, PNG para alta-resolution.

For 95% of use cases, yes — server-side ImageMagick, FFmpeg and LibreOffice produce identical output to the same tools on your laptop. Desktop software wins for: extremely large files (multi-GB), batch jobs of thousands of files, scripted pipelines, or content too sensitive to upload. KaijuConverter caps at 25 MB per file on the free tier (up to 2 GB on paid plans).

The ICO is missing a 256×256 (or 128×128) layer. Windows upscales smaller layers when a larger size is needed, causing blurriness. Regenerate the ICO with a full range of sizes including 256×256. If you also use 150% or 200% DPI scaling, include 96×96 and 192×192.

The ICO is missing a 256×256 (or 128×128) layer. Windows upscales smaller layers when a larger size is needed, causing blurriness. Regenerate the ICO com a full range of sizes including 256×256. If you also usar 150% ou 200% DPI scaling, include 96×96 e 192×192.

The ICO is missing a 256×256 (or 128×128) layer. Windows upscales smaller layers when a larger size is needed, causing blurriness. Regenerate the ICO mit a full range von sizes including 256×256. If you also verwenden 150% oder 200% DPI scaling, include 96×96 und 192×192.

The ICO is missing a 256×256 (or 128×128) layer. Windows upscales smaller layers when a larger size is needed, causing blurriness. Regenerate the ICO con a full range de sizes including 256×256. If you also usar 150% o 200% DPI scaling, include 96×96 y 192×192.

Most format conversions are lossy by design — JPG, MP3, MP4, WebP all discard perceptual data to save bytes. Going through a lossy intermediate compounds the loss. To minimize visible/audible drift: convert from the original master, choose a higher quality setting, and avoid converting back and forth between lossy formats.

The ICO format header uses a 2-byte count field, theoretically supporting up to 65535 images. In practice, ICO files rarely contain more than 10–12 size variants. No practical limit exists for real-world use.

The ICO formato header uses a 2-byte count field, theoretically supporting up to 65535 images. In practice, ICO arquivos rarely contain more than 10–12 size variants. No practical limit exists para real-world use.

The ICO Format header uses a 2-byte count field, theoretically supporting up to 65535 images. In practice, ICO Dateien rarely contain more than 10–12 size variants. No practical limit exists für real-world use.

The ICO formato header uses a 2-byte count field, theoretically supporting up to 65535 images. In practice, ICO archivos rarely contain more than 10–12 size variants. No practical limit exists para real-world use.

Yes — KaijuConverter accepts multiple files in a single drop and returns a ZIP. For very large batches (thousands of files) consider command-line tools or our API: <code>find . -name "*.heic" -exec magick {} {.}.jpg \;</code> or similar one-liners scale to millions of files when run locally.

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