An ICO file is the Windows icon image format. It stores one or more small images, each at a different size and color depth, inside a single file so Windows can pick the best version for program icons, shortcuts, file type icons and website favicons.

Why does one ICO file contain multiple images?

An icon needs to look sharp at many sizes, such as 16x16 in the taskbar and 256x256 on the desktop. Instead of scaling one image, the ICO format bundles several ready-made sizes in one file and Windows shows whichever fits the context best.

What sizes can an ICO file hold?

Common icon sizes are 16x16, 24x24, 32x32, 48x48, 64x64, 128x128 and 256x256 pixels. Each entry in an ICO can use a different color depth, up to 32-bit with an alpha channel for smooth transparency.

What is an ICO File? - Windows Icon Format Explained

📋 On This Page

What is ICO? Quick Facts One File, Many Sizes Inside an ICO File ICO vs PNG Sizes & Color Depths The Favicon How to Open How to Create ICO vs CUR Limitations FAQ

📖 What is an ICO File?

An ICO file is the image format Windows uses for icons. Unlike an ordinary picture, a single ICO is a small container that holds several versions of the same icon, each at a different pixel size and color depth.

That design solves a real problem. The same icon may appear as a tiny 16x16 image in a crowded taskbar and as a large 256x256 image on the desktop. Rather than stretch one picture and lose sharpness, the ICO bundles purpose-built sizes together and Windows simply shows whichever one fits the spot best.

You will find ICO files behind program icons, desktop shortcuts, file type icons and the small favicons that appear on browser tabs. The format was introduced by Microsoft in the earliest versions of Windows and is still in daily use today.

⚡ Quick Facts

Full Name	Windows Icon (Icon image file)
Extension	.ico
Developer	Microsoft
MIME Type	image/vnd.microsoft.icon (also image/x-icon)
Holds	Multiple images in one file
Common Sizes	16, 24, 32, 48, 64, 128, 256 pixels
Max Size Per Image	256 x 256 pixels
Color Depths	1, 4, 8, 24 and 32-bit
Image Encoding	BMP (DIB) or PNG (Windows Vista and later)
Related Format	CUR (Windows cursor)

🧩 One File, Many Sizes

The defining feature of the ICO format is that one file carries many images. Each is a complete, ready-made icon at its own resolution. Here is the same icon stored at four common sizes inside a single ICO:

16x16

32x32

48x48

256x256

💡 Why it matters: Windows reads the ICO and picks the size that matches the context, the small one for a list view and the large one for a desktop icon. Because each size is drawn on purpose, the icon looks crisp instead of blurry at every scale. The checkerboard above represents the transparent areas an icon can have.

🔬 Inside an ICO File

An ICO is laid out as a short directory followed by the image data it points to. The structure is simple by design:

Icon Directory Header

A few bytes that mark the file as an icon and record how many images it contains.

Directory Entries

One entry per image, listing its width, height, color count, bit depth, data size and where the image sits in the file.

Image Data

The actual pictures, stored either as a BMP style bitmap or, since Windows Vista, as compressed PNG data for the larger sizes.

🫧 Transparency: Early icons paired each image with a 1-bit mask that switched pixels fully on or off. Modern 32-bit icons carry a full 8-bit alpha channel instead, so edges can fade smoothly and blend with any background behind them.

⚖️ ICO vs PNG

People often ask whether they should just use a PNG. The difference comes down to one being a single picture and the other being a container built for icons:

Feature	ICO	PNG
Holds multiple sizes	Yes, many in one file	No, one image only
Built for	Windows icons and favicons	General web and app images
Transparency	1-bit mask or 32-bit alpha	8-bit alpha channel
Compression	Raw bitmap, or PNG for large sizes	Always compressed
Used as an app icon	Yes, the native choice	Not directly on Windows

✅ Good to know: The two formats work together. A modern ICO can store its 256x256 image as PNG data inside the container, which keeps large, detailed icons small on disk.

📐 Sizes & Color Depths

Each image inside an ICO has its own size and color depth, so you can mix a simple low-color version for tiny sizes with a rich full-color version for large ones.

🎯

Sizes

16, 24, 32, 48, 64, 128 and 256 pixels square. The directory entry stores width and height in one byte each, so 256 is the maximum, written as a zero.

🎨

Color Depths

1-bit (2 colors), 4-bit (16), 8-bit (256), 24-bit (true color) and 32-bit (true color plus alpha) are all valid within the same file.

🫧

Alpha Channel

The 32-bit depth adds an 8-bit alpha channel, giving 256 levels of transparency for smooth, anti-aliased icon edges.

🌐 The Favicon

The most familiar ICO on the web is the favicon, the little icon shown on a browser tab, in bookmarks and next to a saved shortcut. Browsers look for a file named favicon.ico at the root of a website.

Because a favicon appears at several sizes across tabs, history and desktop shortcuts, a single favicon.ico usually bundles 16x16, 32x32 and 48x48 versions. That is the multi-size strength of the ICO format put to everyday use.

⚠️ Note: Modern sites also use PNG and SVG icons through link tags, but a classic favicon.ico at the site root remains the most widely supported fallback across browsers.

📂 How to Open an ICO File

An ICO opens in many tools, though most show only one size. To inspect every image packed inside, use a dedicated viewer:

✅ See Every Size (Recommended)

Open Univik ICO Viewer in your browser.
Drag and drop the .ico file, or browse to it.
View each embedded image with its size and color depth.
Export any single size to PNG when you need just one.

🧰 Other Ways to Open

Web browser: drag the file into Chrome, Edge or Firefox.
Image editors: GIMP and IrfanView open ICO files directly.
Windows Explorer: shows the icon as a thumbnail.
Windows Photos: opens one size, with limited support.

🛠️ How to Create an ICO File

There are two common ways to end up with an ICO file, depending on where the artwork starts:

🎨 From Your Own Image

To turn a PNG or other picture into an ICO, such as a favicon, use an icon editor or a favicon generator:

Start from a square, high-resolution image so every size stays sharp.
Add the sizes you want. For a favicon, include 16x16, 32x32 and 48x48.
Export them together as a single .ico file.

🧰 From Icons Inside a File

To pull icons out of an ICO, DLL or EXE file and save them out, use the Univik ICO Converter:

Load an ICO, DLL or EXE file that contains icons.
Preview and select the icons you need.
Export them to ICO, PNG or BMP.

💡 Tip: Design at the largest size first. Very small sizes such as 16x16 often look better with simpler shapes and fewer fine details.

🖱️ ICO vs CUR

The Windows cursor format, CUR, is almost the same file as an ICO. Both share the directory structure and image layout, with two differences:

🖼️

ICO

Marked as an icon. Holds static images for program and file icons.

🖱️

CUR

Marked as a cursor and adds hotspot coordinates that tell Windows the exact click point.

⚠️ ICO Limitations

🚫 What ICO Is Not For

Photos: capped at 256x256, so it suits icons, not pictures.
Animation: ICO is static. Use ANI or GIF for motion.
Cross-platform icons: macOS uses ICNS, not ICO.
Large artwork: bitmap sizes add up without PNG encoding.

✅ What ICO Is Great At

Crisp icons: a tailored image for every size.
Transparency: smooth edges with a 32-bit alpha channel.
Favicons: the broadly supported site icon fallback.
Windows integration: the native icon format.

❓ Frequently Asked Questions

An ICO file is the Windows icon image format. It stores one or more small images, each at a different size and color depth, in a single file so Windows can pick the best version for program icons, shortcuts, file type icons and favicons.

An icon needs to look sharp at many sizes, such as 16x16 in the taskbar and 256x256 on the desktop. Instead of scaling one image, the ICO bundles several ready-made sizes and Windows shows whichever fits the context best.

You can open an ICO file in most web browsers, in image editors like GIMP or IrfanView, or in a dedicated tool such as Univik ICO Viewer, which shows every embedded size and color depth. Windows Explorer also displays an ICO as a thumbnail.

A PNG is a single image. An ICO is a container that can hold many images at different sizes and depths in one file, which is what Windows needs for icons. A modern ICO can even store its large sizes as PNG data inside the container.

Common icon sizes are 16x16, 24x24, 32x32, 48x48, 64x64, 128x128 and 256x256 pixels. Each entry can use a different color depth, up to 32-bit with an alpha channel for smooth transparency.

Traditionally yes. A favicon is the small site icon browsers request as favicon.ico from a website root. A single favicon.ico usually holds 16x16, 32x32 and 48x48 versions so it looks crisp in tabs, bookmarks and shortcuts.

To turn your own image into an ICO, such as a favicon, use an icon editor or favicon generator and export 16x16, 32x32 and 48x48 into one file. To pull icons out of an existing ICO, DLL or EXE and save them as ICO, PNG or BMP, use a tool like the Univik ICO Converter.

Yes. Older ICO images used a 1-bit mask for on or off transparency. Modern 32-bit ICO images include a full 8-bit alpha channel, which gives smooth, anti-aliased edges that blend with any background.

🛠️ Related Tools

ICO File Viewer

Open an ICO and view every embedded size and color depth.

View ICO Files →

ICO Converter

Convert PNG and JPG images into multi-size ICO files.

Convert to ICO →

📝 Summary: Key Points About ICO Files

ICO is the Windows icon image format
Holds many sizes in one file
Sizes from 16x16 up to 256x256
Supports 1, 4, 8, 24 and 32-bit depths
32-bit alpha gives smooth transparency

Large sizes can be stored as PNG inside
Powers app icons and favicons
Nearly identical to the CUR cursor format
Open and inspect with an ICO viewer
Create one with an ICO converter

Type:	Icon image container
Developer:	Microsoft
Opens with:	Browsers, viewers
Binary:	Yes (structured)