About ICO Files
ICO Icon
ICO is the icon file format used for favicons and Windows application icons. A single ICO file can contain multiple image sizes and color depths for different display contexts.
Family
Raster & Vector Images
Extension
.ico
MIME Type
image/x-icon, image/vnd.microsoft.icon
Can Use As
49 43 4F 46 00 01 00 00 4B 41 49 4A
ASCII: ICOF....KAIJ
📈 Target Minimum File Size (Guaranteed ≥ KB)
Lossless JPEG Q100 + standard COM segment padding for government & exam portals
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HOW ICO CAME TO BE.
The ICO file was born on November 20, 1985, the day Microsoft shipped Windows 1.0. Every application icon, every folder glyph, every cursor needed a container, and Microsoft designed .ico as a raw-pixel, multi-resolution bundle: one file holds multiple sizes (16×16, 32×32, 48×48, 256×256) and the OS picks the best one for the current context.
For 15 years ICO was a Windows-only curiosity. Then in 1999 Microsoft quietly slipped a line into Internet Explorer 5: if a site hosts /favicon.ico, the browser will display it in the tab and bookmarks. Overnight every webmaster on Earth needed one. That single convention turned .ico into the most widely requested image file type on the internet — every browser, every day, checks every domain for it.
Technical Specifications
- MIME type
- image/vnd.microsoft.icon
- Max resolutions per file
- 65 535 images
- Max single image size
- 256×256 px
- Color depths
- 1, 4, 8, 24, 32 bits per pixel
- Compression
- Uncompressed bitmap or embedded PNG (Vista+)
Typical Sizes & Weights
Classic favicon (16×16 only)
< 2 KB
Multi-size favicon pack (16/32/48/256)
5-15 KB
Full Windows app icon set
20-100 KB
STRENGTHS & LIMITATIONS.
Strengths
- Multi-resolution: one file, many sizes, OS picks the right one.
- Universal favicon support in every browser since IE5.
- Supports transparency (1-bit since 1985, full alpha since XP).
- Tiny file size — an entire favicon pack typically fits in under 15 KB.
- No licensing or patent concerns — fully in the public domain spec-wise.
Limitations
- Cannot compress continuous-tone images efficiently — use PNG or WebP for photos.
- Format is essentially frozen in 1999 — no HDR, no wide gamut, no modern features.
- Maximum image dimension is 256×256 px (inside an ICO container).
- Editing requires specialized tools — most image editors treat it as a curiosity.
CURIOSITIES & TRIVIA.
Every web browser still requests /favicon.ico on every domain, even in 2026 — it is the most-requested image format on the internet by frequency.
A single .ico file can hold up to 65 535 images of different sizes and color depths, chosen by the OS at display time.
Windows XP introduced 32-bit ICOs with alpha transparency — before that, icons used a magenta-keying trick for transparent areas.
macOS had its own .icns format but Safari still looks for favicon.ico first when fetching a site icon.
The Windows "Blue Screen of Death" sad-face emoji (Windows 10+) is literally an .ico file shipped in the kernel.
Specialized Utilities for .ICO Files
Favicon Generator — Multi-Resolution .ICO from PNG, SVG or WEBP
Generate a production-ready multi-resolution favicon.ico containing 16x16, 32x32, 48x48, 64x64, 128x128, and 256x256 alpha layers from any logo.
Resize Image
Resize images to exact dimensions or scale by percentage. Lock aspect ratio to prevent distortion. Supports all common image formats.
Crop Image & Photo Resizer
Crop and resize JPG, PNG, WEBP, and AVIF images to exact pixel dimensions, social media aspect ratios, or custom print DPI online for free.
PNG Compressor — Reduce PNG Size Preserving Transparency
Compress PNG images up to 80% while preserving full alpha-channel transparency, or lock exact file size targets (20 KB, 50 KB, 100 KB).
Compress Image to 20KB
Compress signature scans, stamp images, and ID photos to strictly ≤ 20 KB (20,000 bytes) for strict online exam and government application portals using binary-search JPEG quantization.
CONVERT FROM ICO
CONVERT TO ICO
Common Use Cases
Website favicons, Windows desktop icons, application icons.
Related Formats
Popular comparisons
ICO vs JPG
Differences, file size, and when to choose each format.
ICO vs PNG
Differences, file size, and when to choose each format.
ICO vs WEBP
Differences, file size, and when to choose each format.
ICO vs AVIF
Differences, file size, and when to choose each format.
ICO vs GIF
Differences, file size, and when to choose each format.
ICO vs BMP
Differences, file size, and when to choose each format.
Popular ICO conversions
The most-requested destinations when starting from ICO.
Frequently Asked Questions
ICO (Icon) is Microsoft's 1985 multi-resolution icon format, originally shipped with Windows 1.0. A single .ico file holds multiple sizes (16×16, 32×32, 48×48, 256×256) so the OS can pick the best one for the current display context. Since 1999, every website uses a favicon.ico to show its icon in browser tabs.
On Windows, ICO files open natively in File Explorer and Photos. On macOS, Preview handles basic display. For editing, use GIMP (free), Photoshop with a plugin, or dedicated icon editors like IcoFX.
Use the PNG-to-ICO converter on KaijuConverter — upload a PNG (ideally square, at least 256×256) and download a multi-resolution ICO with all standard favicon sizes embedded.
A complete favicon pack includes 16×16, 32×32, 48×48, and 256×256 px variants all in one ICO file. The total file size is typically 5-15 KB. Browsers automatically pick the right size for tabs, bookmarks, and desktop shortcuts.
ICO for /favicon.ico (every browser requests this URL automatically). PNG for everywhere else — social media profile images, in-page icons, app logos. Modern favicon best practice includes both an .ico at the root and multiple .png sizes referenced via <link> tags in HTML.
Yes. Every browser still requests /favicon.ico on every domain as its first icon fallback. Modern sites typically provide both favicon.ico and higher-quality SVG or PNG icons via <link rel="icon"> tags — browsers pick the best match.
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