How can one ICO file hold several icon sizes?

One ICO can carry a tiny icon for a menu and a larger version for other spaces. Here's what's inside—and why checking the biggest preview isn't enough.

FluffyBench · 7 min read · Reviewed October 15, 2026

What does one ICO file hold?

An ICO is like an envelope of ready-made prints of the same icon. Each picture has its own pixel size, so an app can pick the version that fits its space.

One source, separate pictures, one ICO

Selected sizes are drawn from the same source, like making several prints from one original. How we reviewed these results

An ICO isn't one big picture with a list of suggested display sizes. The smaller pictures are already saved inside it. That matters when you're checking an icon: a clear large preview doesn't tell you how the separate menu-sized picture looks.

What do the stored icon sizes look like?

Here's a generated star-and-circle icon from one recorded ICO. Compare the largest stored picture with the tiny versions: the star points and white center have fewer pixels to work with.

Star-and-circle icon stored at 256 by 256 pixels. — The small pictures above use their own pixel dimensions on the page; the enlarged views below expose their pixel shapes.

At the tiny size, look for recognition: can you still tell what the icon is? The enlargement helps you see how an edge was drawn. The display-size picture tells you whether the design reads without zooming in. Those are different checks, and you need the small view for the second one.

Why can a smaller icon picture take more file space?

In the recorded ICO, the 128 px picture takes about 5 times the space of the 256 px picture. The smaller one stores an uncompressed bitmap—a grid of pixels—while the larger one uses PNG compression.

Star-and-circle icon stored at 128 by 128 pixels. — Both pictures are displayed at the same width here. Byte counts cover each stored picture's data, not the whole ICO.

So pixel dimensions and file size answer different questions. Dimensions tell you how much room a picture has for detail; bytes tell you how much storage its saved form uses. Don't use either number as a substitute for looking at the icon.

Doing it in FluffyBench: create and check your ICO

FluffyBench's ICO converter bundles the sizes you choose into one file. Use the size list requested by the site or app receiving it. If you're preparing website icons, the favicon size guide explains which files different placements ask for.

Create your ICO from an image you can still edit. Keep that source for changes if the tiny version needs simpler shapes.

Step 1. Choose your source image, confirm ICO under To, and open the ICO settings.
1. Choose images opens your device’s file picker.
2. To is already set to ICO on this page.
3. The ICO gear button opens the size choices.
Step 2. Match Included sizes to your destination’s request; the checked default set is an example.
1. Each checked size adds a separate picture to one ICO.
Step 3. Press the ICO gear button again to return to your image.
1. The ICO gear button closes the size panel.
Step 4. Press Convert 1 to ICO.
1. Convert makes one ICO with the checked sizes.
Step 5. Check the output readout, then use the Download arrow.
1. Dimensions describe the largest picture; bytes cover the whole ICO.
2. The Download arrow saves the ICO to your device.

FluffyBench's Image extractor opens the downloaded ICO's separate sizes. It gives you PNGs to inspect without changing the ICO you'll hand off.

Step 1. Choose the ICO you just downloaded.
1. Choose files opens the file picker for your ICO.
Step 2. Leave Take on Everything inside so you can inspect every stored size.
1. Everything inside extracts each stored picture as a PNG.
Step 3. Press Extract.
1. Extract opens the pictures stored in your ICO.
Step 4. Find the smallest size in the results and download its PNG for a display-size check.
1. 16 by 16 identifies the smallest picture in this example.
2. Download saves that picture as a separate PNG.
Step 5. If you need the complete set of PNGs, save them together.
1. Download all (ZIP) saves the extracted PNGs together.

Here's the smallest PNG extracted from the walkthrough's ICO, beside its source reference. Check your own smallest picture without zooming in: can you still recognize the design?

Need to extract the stored sizes from an ICO?

Image extractor

What should you check before handing off an ICO?

Open your ICO in the receiving app or site to check that it accepts the file and the icon looks right in place. A design that reads well in a large page image still needs to work in its tiny placement.

Illustration of an icon in a browser tab

Illustrated placement: the tab gives the icon much less room than the page image.

How we reviewed

Reviewed October 15, 2026

The recorded picture capture was made on 2026-10-15 with the production converter in Chromium 153.0.8010.12 on Windows, using AMD Ryzen 9 5950X (16 processing cores). It used a 512 px by 512 px Synthetic geometric icon with a star, circle, and rounded-square background. Input, selected sizes, downloaded ICO and directory data are recorded in the measurement record under figures/toolCapture. The original source PNG isn't retained among the page assets; the diagram uses the largest extracted picture as a reference.

The UI illustrations aren't timing benchmarks. The historical timing runs were not repeated.

The recorded ICO contains 8 entries: 16 px, 24 px, 32 px, 48 px, 64 px, 96 px, 128 px, 256 px. Its total is 153,516 bytes. The smaller pictures use DIB bitmaps with 32 bits per pixel; the largest uses PNG. Byte counts come from that ICO's directory, not the extracted PNGs.

The default sizes are 16 px, 32 px, 48 px, 256 px; the available sizes are 16 px, 24 px, 32 px, 48 px, 64 px, 96 px, 128 px, 256 px. The historical capture selected every size.

The historical upload and timing records lack a run date, browser versions, operating system, hardware and source byte lengths. Timings are desktop medians with all sizes selected; they describe the recorded sources and environment.

Chromium · 4,000 × 3,000 · before: 331 ms.

Chromium · 4,000 × 3,000 · after: 156 ms.

Chromium · 6,000 × 4,000 · before: 538 ms.

Chromium · 6,000 × 4,000 · after: 255 ms.

Firefox · 4,000 × 3,000 · before: 382 ms.

Firefox · 4,000 × 3,000 · after: 190 ms.

Firefox · 6,000 × 4,000 · before: 594 ms.

Firefox · 6,000 × 4,000 · after: 284 ms.

Each timing is the median of 3 runs from Convert to a completed file, including decoding, drawing and encoding each picture, and assembling the ICO. The earlier path copied the full source onto a drawing surface for every size; the updated path copied it once.

Chromium: 8 calls before, 1 call after; the upload step took 200.5 ms before and 28.7 ms after.

Firefox: 8 calls before, 1 call after; the upload step took 334 ms before and 38 ms after.

Upload counts cover putImageData calls larger than 256 px by 256 px on the larger source. They measure copies onto a drawing surface, not browser memory or total conversion time.

Chromium whole-file bytes matched in 20 cases of 20 cases. Firefox decoded picture pixels matched in 20 cases of 20 cases. Both comparisons covered ICO and CUR.

A whole-file check compares the downloaded bytes; a decoded-picture check compares pixels after opening. Files can hold the same pictures while using different bytes to store them.

Walkthroughs were captured locally on October 23, 2026. The ICO converter's illustrated input is the retained 256 px PNG; the extractor's input is its default-size ICO download. The placement diagram is drawn for illustration. All assets stay local during capture.