About

The ICO format stores multiple image resolutions within a single file, enabling Windows to select optimal icon sizes for different display contexts - taskbar (16×16), desktop (48×48), or high-DPI tiles (256×256). Shipping a single-resolution icon causes forced scaling, producing blurred edges and aliasing artifacts that degrade professional appearance. This converter constructs a valid ICO binary by embedding PNG-compressed frames for each selected dimension, following Microsoft's modern icon specification. Source BMP images are decoded natively, scaled using high-quality bicubic interpolation, then encoded into the multi-image container structure. Note: input images with aspect ratios deviating from 1:1 will be center-cropped to square before scaling.

Formulas

The ICO file format consists of a header structure followed by concatenated image data. Each entry in the icon directory points to its corresponding image offset within the file.

ICO = ICONDIR + ICONDIRENTRY[n] + ImageData[n]

The ICONDIR header occupies 6 bytes:

ICONDIR = { Reserved: 2B, Type: 2B, Count: 2B }

Each ICONDIRENTRY is 16 bytes, describing one embedded image:

Entry = { W: 1B, H: 1B, Colors: 1B, Reserved: 1B, Planes: 2B, BPP: 2B, Size: 4B, Offset: 4B }

Image offset calculation for the i-th image:

Offset_i = 6 + (16 × n) + i−1∑j=0 Size_j

Where n is total image count, Size_j is byte length of image j. For 256×256 icons, width/height bytes store 0 (special encoding).

Reference Data

Icon Size	Typical Use Case	Windows Context	Recommended Source
16×16	Favicon, title bar	Small icons view, browser tabs	≥64px source
24×24	Toolbar buttons	Windows Explorer toolbar	≥96px source
32×32	Desktop shortcut (standard)	Default desktop icon size	≥128px source
48×48	Desktop shortcut (medium)	Medium icons view	≥192px source
64×64	Large list view	Large icons in Explorer	≥256px source
128×128	Extra large icons	Extra large icons view	≥256px source
256×256	High-DPI, tiles	Windows 10/11 tiles, jumbo view	≥256px source
ICO Format Limits
Max images	255 images per ICO file (practical limit)
Max dimension	256×256 pixels (stored as 0 in header byte)
Color depth	32-bit RGBA (8 bits per channel + alpha)
Compression	PNG for ≥48px (Vista+), raw BMP DIB for legacy
BMP Input Requirements
Bit depth	24-bit or 32-bit recommended
Compression	Uncompressed (BI_RGB) or RLE supported via Canvas
Min size	≥16×16 pixels
Ideal source	512×512 or 1024×1024 for best downscale quality

Frequently Asked Questions

The ICO format uses a single byte for dimensions, limiting values to 0 - 255. A value of 0 is interpreted as 256 pixels. This convention was established in Windows Vista when large icon support was added. The converter handles this encoding automatically.

Include at minimum 16×16, 32×32, and 256×256 for modern Windows compatibility. The operating system selects the closest size and scales if exact match unavailable. Omitting intermediate sizes forces scaling, which degrades quality. For web favicons, 16×16 and 32×32 suffice.

Standard BMP files lack an alpha channel (24-bit). If your source is a 32-bit BMP with embedded alpha (BITMAPV4/V5 header), the browser's Canvas API may decode it correctly. For guaranteed transparency, use PNG sources instead. This converter processes whatever alpha data the browser extracts from the decoded image.

Icons require 1:1 aspect ratio. Non-square images are center-cropped to the largest possible square region before scaling. To preserve your entire image, pad it to square dimensions in an image editor before conversion, using transparent or solid background fill.

ICO is Windows-specific. macOS uses ICNS format (Apple Icon Image), and Linux desktop environments typically use PNG or SVG. For cross-platform applications, generate separate icon formats for each OS. This converter targets Windows executables, shortcuts, and web favicons.

An ICO file contains multiple complete images. Selecting 7 sizes means 7 separate PNG-compressed images stored sequentially. A 256×256 frame alone can exceed 100KB depending on image complexity. Reduce selected sizes or simplify source artwork to decrease file size.