Guide

ICO Format: Complete Guide to Windows Icons and Web Favicons

PC By Pablo Cirre • Updated May 7, 2026

What Is ICO Format?

ICO (Icon) is a file format used for icons in Microsoft Windows and as favicons for websites. Developed by Microsoft and first introduced with Windows 1.0 in 1985, the ICO format has evolved significantly over the decades. Its defining characteristic is the ability to store multiple images of different sizes and color depths within a single file, allowing operating systems and browsers to select the most appropriate version for each display context.

Files use the .ico extension. A related format, CUR (Cursor), uses the same structure but adds hotspot coordinates for cursor positioning.

ICO File Structure

ICO files use a simple binary structure:

ICO File
├── ICONDIR Header (6 bytes)
│   ├── Reserved: 0x0000 (2 bytes)
│   ├── Type: 0x0001 (2 bytes) [1 = ICO, 2 = CUR]
│   └── Image Count: N (2 bytes)
│
├── ICONDIRENTRY[0] (16 bytes per image)
│   ├── Width: 0-255 (1 byte) [0 = 256]
│   ├── Height: 0-255 (1 byte) [0 = 256]
│   ├── Color Count: (1 byte) [0 if > 8-bit]
│   ├── Reserved: 0x00 (1 byte)
│   ├── Planes: (2 bytes)
│   ├── Bit Count: color depth (2 bytes)
│   ├── Size of image data (4 bytes)
│   └── Offset to image data (4 bytes)
│
├── ICONDIRENTRY[1]
│   └── [...]
│
├── Image Data[0]
│   ├── Option A: BMP DIB (BITMAPINFOHEADER + XOR mask + AND mask)
│   └── Option B: PNG (complete PNG file, zlib-compressed, stored verbatim)
│
└── Image Data[N-1]

Image Sizes in ICO Files

Windows uses different icon sizes for different contexts. A comprehensive ICO file for a Windows application should contain all these sizes:

Size	Color depth	Use context
16×16	32-bit RGBA	Taskbar, small toolbar icons
24×24	32-bit RGBA	Toolbar icons in some apps
32×32	32-bit RGBA	Desktop (medium icons), standard toolbar
40×40	32-bit RGBA	Some Windows 10+ contexts
48×48	32-bit RGBA	Desktop (large icons), Windows Explorer
64×64	32-bit RGBA	Desktop (extra large icons)
128×128	32-bit RGBA	Windows 10+ large icon view
256×256	PNG format	Desktop (huge icons), Windows 10/11

For Windows application icons, the minimum recommended set is 16×16, 32×32, 48×48, and 256×256. The 256×256 image must be stored as a PNG (not BMP) per the Vista+ specification.

ICO Image Encoding: BMP vs. PNG

ICO files can store images in two formats:

BMP (DIB) Encoding (Legacy)

Pre-Vista ICO images use BMP Device Independent Bitmap encoding:

BITMAPINFOHEADER (40 bytes): width, height×2 (height is doubled to account for XOR+AND masks), planes, bit count
Optional color table
XOR mask: the actual pixel data (BGRA order for 32-bit)
AND mask: 1-bit transparency mask (0=opaque, 1=transparent)

For 32-bit BMP images in ICO, the alpha channel from the BGRA data is used, and the AND mask is typically all zeros. For older color depths (1-bit, 4-bit, 8-bit), the AND mask controls transparency.

PNG Encoding (Vista and Later)

Windows Vista and later allow 256×256 (and potentially other sizes) to be stored as complete, zlib-compressed PNG files embedded verbatim in the ICO data section. PNG encoding:

Dramatically smaller than equivalent BMP for large images
Supports full 32-bit RGBA (alpha channel)
Any size can theoretically be PNG-encoded, but 256×256 is the primary use case

Favicon: ICO for the Web

The most common use of ICO in 2024 is as a favicon — the small icon displayed in browser tabs, bookmarks, history, and address bars.

Standard Favicon Setup

The minimum favicon is a single favicon.ico file in the website root:

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

However, modern websites should provide multiple formats and sizes for optimal display across all contexts:

<!-- Legacy ICO for old browsers -->
<link rel="icon" href="/favicon.ico" sizes="32x32">

<!-- Modern PNG favicons for current browsers -->
<link rel="icon" type="image/png" sizes="96x96" href="/favicon-96x96.png">
<link rel="icon" type="image/png" sizes="32x32" href="/favicon-32x32.png">
<link rel="icon" type="image/png" sizes="16x16" href="/favicon-16x16.png">

<!-- Apple Touch Icon (iOS home screen bookmark) -->
<link rel="apple-touch-icon" sizes="180x180" href="/apple-touch-icon.png">

<!-- SVG favicon (scalable, supported in Firefox and Chrome) -->
<link rel="icon" type="image/svg+xml" href="/favicon.svg">

<!-- Web App Manifest for PWA -->
<link rel="manifest" href="/site.webmanifest">

Browsers and Favicon Formats

Browser	ICO	PNG	SVG
Chrome	✅	✅	✅ (since v80)
Firefox	✅	✅	✅ (since v41)
Safari	✅	✅	Limited
Edge	✅	✅	✅

SVG favicons are the modern best practice for websites — a single SVG file scales to any size perfectly. Firefox and Chrome use the SVG when available; older browsers fall back to ICO.

Touch Icons

Apple iOS uses "Apple Touch Icon" images when a user adds a website to their home screen. These are regular PNG images at specific sizes:

180×180 px for modern iPhones (Retina displays)
152×152 px for iPad Retina
120×120 px for older iPhones

These are referenced with <link rel="apple-touch-icon"> and should be flat PNG images without rounded corners (iOS rounds them automatically).

Color Depths in ICO

ICO has evolved through several color depth standards:

Color depth	Colors	Use
1-bit	2 (monochrome)	Early Windows icons
4-bit	16	Windows 3.x era
8-bit	256 (palette)	Windows 95/98 era
24-bit	16.7 million	True color without transparency
32-bit	16.7 million + 8-bit alpha	Modern icons — RGBA

Modern ICO files should use 32-bit color depth for all images. The 32-bit format stores full BGRA data, providing smooth transparency for drop shadows and anti-aliased edges — critical for modern high-DPI icons.

Creating ICO Files

From Command Line (ImageMagick)

# Create ICO with multiple sizes from a single PNG
magick convert icon-256.png \
  \( -clone 0 -resize 256x256 \) \
  \( -clone 0 -resize 128x128 \) \
  \( -clone 0 -resize 64x64 \) \
  \( -clone 0 -resize 48x48 \) \
  \( -clone 0 -resize 32x32 \) \
  \( -clone 0 -resize 16x16 \) \
  -delete 0 favicon.ico

# Simpler version (let ImageMagick pick sizes)
magick convert icon.png -define icon:auto-resize=256,128,64,48,32,16 favicon.ico

From GIMP

File → Export As → choose .ico → dialog asks for which sizes to include

Online Tools

RealFaviconGenerator.net — generates complete favicon package with ICO + PNG + manifest
favicon.io — converts PNG/text/emoji to ICO + PNG set
icoconvert.com — simple ICO conversion

ICO vs. SVG for Modern Applications

Use case	ICO	SVG
Windows application icon	✅ Required	❌
Windows taskbar/desktop	✅	❌
Browser favicon (modern)	Works	✅ Better (scales perfectly)
macOS application icon	❌ (use ICNS)	❌
Progressive Web App	✅ + PNG	✅ Best
Email favicon	✅	❌

For Windows applications, ICO is still required by the OS. For web favicons, SVG + ICO fallback is the modern best practice. For macOS, Apple uses the ICNS format (a similar multi-size container for Mac icons).

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Frequently Asked Questions

For a website favicon.ico, the most important sizes are 16×16 and 32×32 pixels — these cover browser tabs and bookmark displays in all browsers. For Windows applications, a comprehensive ICO should include 16×16, 24×24, 32×32, 48×48, 64×64, 128×128, and 256×256 pixels. The 256×256 image in modern ICO files is stored as a compressed PNG for efficiency. Tools like RealFaviconGenerator.net automatically create the correct multi-size ICO plus separate PNG files for Apple Touch Icons and the Web App Manifest. For modern websites, also provide an SVG favicon for browsers that support it — SVG scales perfectly to any size.

For a website favicon.ico, the most important sizes are 16×16 e 32×32 pixels — these cover browser tabs e bookmark displays in all browsers. para Windows aplicativos, a comprehensive ICO should include 16×16, 24×24, 32×32, 48×48, 64×64, 128×128, e 256×256 pixels. The 256×256 image in moderno ICO arquivos is stored como um comprimido PNG para efficiency. ferramentas like RealFaviconGenerator.net automatically create the correct multi-size ICO plus separate PNG arquivos para Apple Touch Icons e the Web App Manifest. para moderno sites, also provide an SVG favicon para browsers that support it — SVG scales perfectly to any size.

For a website favicon.ico, the most important sizes are 16×16 und 32×32 pixels — these cover browser tabs und bookmark displays in all browsers. für Windows Anwendungen, a comprehensive ICO should include 16×16, 24×24, 32×32, 48×48, 64×64, 128×128, und 256×256 pixels. The 256×256 image in modern ICO Dateien is stored als ein komprimiert PNG für efficiency. Werkzeuge like RealFaviconGenerator.net automatically create the correct multi-size ICO plus separate PNG Dateien für Apple Touch Icons und the Web App Manifest. für modern Websites, also provide an SVG favicon für browsers that support it — SVG scales perfectly to any size.

For a website favicon.ico, the most important sizes are 16×16 y 32×32 pixels — these cover browser tabs y bookmark displays in all browsers. para Windows aplicaciones, a comprehensive ICO should include 16×16, 24×24, 32×32, 48×48, 64×64, 128×128, y 256×256 pixels. The 256×256 image in moderno ICO archivos is stored como un comprimido PNG para efficiency. herramientas like RealFaviconGenerator.net automatically create the correct multi-size ICO plus separate PNG archivos para Apple Touch Icons y the Web App Manifest. para moderno sitios web, also provide an SVG favicon para browsers that support it — SVG scales perfectly to any size.

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 (Chrome, Firefox, Edge, Safari) all support PNG favicons. You can use <link rel="icon" type="image/png" sizes="32x32" href="/favicon-32x32.png"> in your HTML. However, providing a favicon.ico file as well (or as the primary icon) ensures compatibility with older browsers, various web crawlers, RSS readers, and some tools that look for favicon.ico at the website root by default. The common best practice is: provide favicon.ico (with 16×16 and 32×32 inside it), PNG files for specific sizes, SVG for modern browsers, and apple-touch-icon.png for iOS.

Sim, moderno browsers (Chrome, Firefox, Edge, Safari) all support PNG favicons. Você pode usar <link rel="icon" type="image/png" sizes="32x32" href="/favicon-32x32.png"> in your HTML. Porém, providing a favicon.ico arquivo as well (or como o primary icon) ensures compatibilidade com older browsers, various web crawlers, RSS readers, e some ferramentas that look para favicon.ico at o site root by default. The common best practice is: provide favicon.ico (with 16×16 e 32×32 inside it), PNG arquivos para specific sizes, SVG para moderno browsers, e apple-touch-icon.png para iOS.

Ja, modern browsers (Chrome, Firefox, Edge, Safari) all support PNG favicons. Sie können verwenden <link rel="icon" type="image/png" sizes="32x32" href="/favicon-32x32.png"> in your HTML. Jedoch, providing a favicon.ico Datei as well (or als das primary icon) ensures Kompatibilität mit older browsers, various web crawlers, RSS readers, und some Werkzeuge that look für favicon.ico at die Website root by default. The common best practice is: provide favicon.ico (with 16×16 und 32×32 inside it), PNG Dateien für specific sizes, SVG für modern browsers, und apple-touch-icon.png für iOS.

Sí, moderno browsers (Chrome, Firefox, Edge, Safari) all support PNG favicons. Puedes usar <link rel="icon" type="image/png" sizes="32x32" href="/favicon-32x32.png"> in your HTML. Sin embargo, providing a favicon.ico archivo as well (or como el primary icon) ensures compatibilidad con older browsers, various web crawlers, RSS readers, y some herramientas that look para favicon.ico at el sitio web root by default. The common best practice is: provide favicon.ico (with 16×16 y 32×32 inside it), PNG archivos para specific sizes, SVG para moderno browsers, y apple-touch-icon.png para iOS.

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).

Several methods work: (1) ImageMagick command line: magick convert icon.png -define icon:auto-resize=256,128,64,48,32,16 favicon.ico — this creates a multi-size ICO from any PNG input. (2) GIMP: open the PNG, go to File → Export As, type favicon.ico, and GIMP presents size options. (3) Online converters: favicon.io, icoconvert.com, and RealFaviconGenerator.net all convert PNG to ICO in the browser with no software needed. (4) For best results, start with a 512×512 or 1024×1024 source PNG with transparency and let the tool downscale to smaller sizes — avoid upscaling from small images.

Several methods work: (1) ImageMagick command line: magick converter icon.png -define icon:auto-resize=256,128,64,48,32,16 favicon.ico — this creates a multi-size ICO de any PNG input. (2) GIMP: abrir the PNG, go to arquivo → Export As, type favicon.ico, e GIMP presents size options. (3) Online converters: favicon.io, icoconvert.com, e RealFaviconGenerator.net all converter PNG to ICO in o navegador com no software needed. (4) para best results, start com a 512×512 ou 1024×1024 source PNG com transparência e let a ferramenta downscale to smaller sizes — avoid upscaling de small images.

Several methods work: (1) ImageMagick command line: magick umwandeln icon.png -define icon:auto-resize=256,128,64,48,32,16 favicon.ico — this creates a multi-size ICO von any PNG input. (2) GIMP: öffnen the PNG, go to Datei → Export As, type favicon.ico, und GIMP presents size options. (3) Online converters: favicon.io, icoconvert.com, und RealFaviconGenerator.net all umwandeln PNG to ICO in der Browser mit no Software needed. (4) für best results, start mit a 512×512 oder 1024×1024 source PNG mit Transparenz und let das Werkzeug downscale to smaller sizes — avoid upscaling von small images.

Several methods work: (1) ImageMagick command line: magick convertir icon.png -define icon:auto-resize=256,128,64,48,32,16 favicon.ico — this creates a multi-size ICO de any PNG input. (2) GIMP: abrir the PNG, go to archivo → Export As, type favicon.ico, y GIMP presents size options. (3) Online converters: favicon.io, icoconvert.com, y RealFaviconGenerator.net all convertir PNG to ICO in el navegador con no software needed. (4) para best results, start con a 512×512 o 1024×1024 source PNG con transparencia y let la herramienta downscale to smaller sizes — avoid upscaling de small images.

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.

Both are multi-size icon container formats, but they are platform-specific. ICO is Microsoft's format for Windows icons and web favicons — it uses a simple directory structure with BMP or PNG image data. ICNS is Apple's format for macOS application icons — it uses a four-character code system ("ic08" for 512×512, "ic09" for 1024×1024, etc.) and stores images as PNG or JPEG 2000. Windows software uses .ico files, macOS software uses .icns files, and web browsers use .ico files (or PNG/SVG for modern favicons). If you're creating icons for cross-platform software, you typically need to generate both ICO (for Windows) and ICNS (for macOS) from the same source artwork.

Both are multi-size icon container formatoos, mas they are plataforma-specific. ICO is Microsoft's formato para Windows icons e web favicons — it uses a simples directory structure com BMP ou PNG image data. ICNS is Apple's formato para macOS aplicativo icons — it uses a four-character code system ("ic08" para 512×512, "ic09" para 1024×1024, etc.) e stores images as PNG ou JPEG 2000. Windows software uses .ico files, macOS software uses .icns files, e web browsers usar .ico arquivos (or PNG/SVG para moderno favicons). If you're creating icons para cross-platform software, you tipicamente precisar generate both ICO (for Windows) e ICNS (for macOS) de the same source artwork.

Both are multi-size icon Container Formate, aber they are Plattform-specific. ICO is Microsoft's Format für Windows icons und web favicons — it uses a einfach directory structure mit BMP oder PNG image data. ICNS is Apple's Format für macOS Anwendung icons — it uses a four-character code system ("ic08" für 512×512, "ic09" für 1024×1024, etc.) und stores images as PNG oder JPEG 2000. Windows Software uses .ico files, macOS Software uses .icns files, und web browsers verwenden .ico Dateien (or PNG/SVG für modern favicons). If you're creating icons für cross-platform Software, you typically need to generate both ICO (for Windows) und ICNS (for macOS) von the same source artwork.

Both are multi-size icon contenedor formatoos, pero they are plataforma-specific. ICO is Microsoft's formato para Windows icons y web favicons — it uses a simple directory structure con BMP o PNG image data. ICNS is Apple's formato para macOS aplicación icons — it uses a four-character code system ("ic08" para 512×512, "ic09" para 1024×1024, etc.) y stores images as PNG o JPEG 2000. Windows software uses .ico files, macOS software uses .icns files, y web browsers usar .ico archivos (or PNG/SVG para moderno favicons). If you're creating icons para cross-platform software, you typically need to generate both ICO (for Windows) y ICNS (for macOS) de the same source artwork.

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.