ICO vs PNM — Differences, File Size & When to Use Each

Q: What is an ICO file?

ICO (Icon) is Microsoft&#039;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.

Q: How do I open an ICO file?

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.

Q: How do I convert PNG to ICO?

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.

Q: What size should a favicon.ico be?

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.

Q: ICO vs PNG for web icons: which?

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 &lt;link&gt; tags in HTML.

ICO vs PNM

A detailed comparison of ICO Icon and Portable Anymap — file size, quality, compatibility, and which format to choose for your workflow.

Convert ICO → PNM Convert PNM → ICO

ICO

ICO Icon

Raster & Vector Images

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.

About ICO files

PNM

Portable Anymap

Raster & Vector Images

PNM (Portable Anymap) is a family of simple image formats comprising PBM, PGM, and PPM. These formats store pixel data in straightforward ASCII or binary layouts, making them easy to generate and parse programmatically.

About PNM files

Strengths 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.

PNM Strengths

Stupidly simple — a 50-line parser handles every variant.
ASCII variant is human-readable and diff-able.
Universal Unix tooling support.
40+ years of stability.
Wildcard extension covers three related formats.

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.

PNM Limitations

No compression — files are huge.
No color profile, metadata, or transparency.
Strictly a pipeline intermediate, not a delivery format.

Technical Specifications

Specification	ICO	PNM
MIME type	image/vnd.microsoft.icon	image/x-portable-anymap
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	—	.pnm (umbrella), .pbm, .pgm, .ppm
Variants	—	P1-P6 (ASCII or binary × bitmap/graymap/pixmap)
Toolkit	—	Netpbm
Creator	—	Jef Poskanzer (1988)

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

PNM

512×512 grayscale (binary) ~256 KB
1920×1080 RGB (binary) ~6 MB

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Convert between ICO and PNM online, free, and without installing anything. Encrypted upload, automatic deletion after 2 hours.

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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.