ICO vs WMF
A detailed comparison of ICO Icon and Windows Metafile — file size, quality, compatibility, and which format to choose for your workflow.
ICO Icon
Raster & Vector ImagesICO 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.
About ICO filesWindows Metafile
Raster & Vector ImagesWMF (Windows Metafile) is a legacy Windows vector graphics format.
About WMF filesStrengths Comparison
ICO 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.
WMF Strengths
- Native Windows vector format.
- Efficient for line-art and diagrammatic images.
- Preserved in Office clipboard workflows.
Limitations
ICO 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.
WMF Limitations
- Windows-only — poor Mac and Linux support.
- 2005 security vulnerability eroded trust.
- 16-bit coordinates in original WMF limit canvas size.
- Largely superseded by SVG for modern vector exchange.
- Not a web format.
Technical Specifications
| Specification | ICO | WMF |
|---|---|---|
| MIME type | image/vnd.microsoft.icon | image/wmf |
| 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+) | — |
| Extension | — | .wmf |
| Container | — | Sequence of GDI drawing records |
| Successors | — | .emf (1993), .emf+ / .emfx (2000) |
| Related spec | — | [MS-WMF] (Microsoft Open Specifications) |
Typical File Sizes
ICO
- 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
WMF
- Simple clipart 5-50 KB
- Detailed vector drawing 50-500 KB
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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.