What is an .ico file?
A PNG holds one image. A JPG holds one image. An .ico holds as many as you put in it — it is a container, not a picture. Inside sits a small directory listing each image's dimensions and where its bytes begin, then the images themselves, stacked back to back. When Windows or a browser needs an icon, it reads the directory, picks the entry closest to the slot it has to fill, and draws that one.
This is the single fact behind almost every bad favicon on the internet. Hand a site one 256 px image renamed to favicon.ico and the browser has nothing at 16 px to reach for, so it shrinks the big one on the spot, in one step, by a factor of sixteen. Thin strokes vanish. Small text goes to grey mush. The logo looks sharp on the desktop and awful in the tab, and nobody can work out why.
Converting PNG to ICO properly means rendering each size separately from the original, then packing all of them into one file. That is what the ICO Converter does. Drop a PNG or JPG and you get back a real multi size ico with 16, 32 and 48 px inside it, each one built from your source rather than from the size above it.
Most favicon generators want something before they hand over the file. An email address. An account. A "download your icon package" button that turns into a paywall on the last click, a free tier that stamps the output, or a design suite where icon export is one buried feature inside a per-seat subscription you renew forever. The pattern is always the same: the conversion is trivial, the gate is the product. There is no gate here. Drop the image, take the file, close the tab.
How the ICO Converter builds the file
Three things happen, in order, and all of them happen on your machine.
First, the image is decoded and painted into a square canvas at each size you ticked. The downscale is stepped, not single-shot: the image is halved repeatedly until it is within 2× of the target, and only the last step draws to the final size. Going from 512 px straight to 16 px in one draw leaves a smear in every browser. Halving first is the standard fix, and it is the difference between a readable tab icon and a blob.
Second, each square is encoded. Sizes below 128 px go in as a raw 32-bit bitmap, because that is what every version of Windows can read and the bitmaps are tiny anyway. 128 and 256 px go in as complete PNG files, the extension Windows Vista added, because the raw form gets expensive fast. Both paths carry a full 8-bit alpha channel, so soft edges and drop shadows survive either way.
Third, the container is assembled: a 6-byte header, one 16-byte entry per size, then the image data. The byte cost of the small entries is fixed and knowable before anything is drawn.
Directory = 6 bytes + 16 bytes per size. Each bitmap entry = 40 + (px × px × 4) + mask bytes. A 48 px entry costs exactly 9,640 bytes. The standard 16 / 32 / 48 set comes to 15,086 bytes — 14.7 KB — for every image on earth, because the math doesn't care what your logo looks like.
What each icon size is actually for
The ICO Converter offers six sizes, and three of them are ticked the moment you drop a file. Here is what each one buys you and what it costs.
| Size | Stored as | Exact cost | Where it gets drawn |
|---|---|---|---|
| 16 px | Bitmap | 1,128 bytes | Browser tab, bookmarks bar, history list |
| 32 px | Bitmap | 4,264 bytes | Windows taskbar, desktop shortcut, the 16 px slot on a high-density screen |
| 48 px | Bitmap | 9,640 bytes | File Explorer, medium-icon view |
| 64 px | Bitmap | 16,936 bytes | Nothing asks for it specifically — skip unless you have a reason |
| 128 px | PNG | Depends on the image | Large-icon views, older macOS builds |
| 256 px | PNG | Depends on the image | File Explorer, extra-large icons; Windows desktop shortcuts |
The 128 and 256 px rows say "depends" for a real reason. A 256 px bitmap entry would weigh 270,376 bytes — a quarter of a megabyte for one icon — while the same artwork written as PNG usually lands under 20 KB. That gap is exactly why the PNG-in-ICO extension exists, and why the split sits at 128 px rather than somewhere tidier.
The default set of 16, 32 and 48 is not a shortcut. It is what a favicon.ico has held since Windows XP, and it covers the three places the icon is genuinely seen. Tick all six and the file grows to 31.3 KB with two PNG entries inside; tick the default three and it is 14.7 KB flat. Add 256 if the icon will also sit on a Windows desktop at large-icon size. Leave 64 and 128 alone unless something specific is asking for them, because every extra entry makes the download bigger without improving the tab, and the tab is where the icon lives.
Converting a PNG to ICO, start to finish
Take a real file: logo.png, 512 × 512 px, 48,213 bytes on disk. Drop it on the ICO Converter and conversion starts immediately — there is no convert button to hunt for, because there is nothing to decide first.
The line above the checkboxes reads PNG · 512 × 512 px · 47.1 KB. 16, 32 and 48 are already ticked. Because the image is square, the fit control stays hidden — it would do nothing, so it isn't there. Three previews appear, drawn at their true pixel sizes, so the 16 px preview is literally what your tab will show rather than a shrunk-down approximation of it. The download button reads Download logo.ico and the line beside it reads 14.7 KB · 3 sizes.
That 14.7 KB is 54 bytes of directory plus 1,128 + 4,264 + 9,640 bytes of image data. Rename the download to favicon.ico, drop it in your site's root folder, and you are done.
Two variations worth knowing. Tick all six sizes and the same logo produces a 31.3 KB file with 128 and 256 stored as PNG. Push the padding slider to 20% and the ICO Converter warns you that your 16 px icon now has 10 px of artwork left — below about 12 px a tab icon stops being readable, so it says so rather than letting you find out in production. The filename survives whatever you throw at it, too: drop My Logo v2 (final).png and the download is My Logo v2 (final).ico, extension swapped, everything else intact.
Where favicons go wrong
Four things account for most of the bad results people get converting PNG to ICO, and every one of them is visible before you download.
Starting from a JPG. JPG has no alpha channel at all, so there is no transparency to keep. Run a JPG to ICO conversion and whatever background the photo has becomes a solid block behind your icon — a logo on white becomes a white square in the tab. The tool says so when it sees a JPG. The fix is a PNG with a genuinely transparent background, not a PNG exported from a JPG.
A source that isn't square. Every entry in an .ico is square, so a 1,600 × 400 wordmark has to become one somehow. Fitting it in keeps the whole mark visible and leaves the sides transparent, which is right for a logotype. Filling and cropping keeps the artwork large and trims the long edges, which is right for a photo or texture. The choice only appears when your image isn't already square.
Upscaling. Tick 256 px with a 128 px source and you get a note saying the 256 px icon is scaled up and will look soft, with the minimum source size you'd need for a crisp one. No amount of clever resampling invents detail that was never in the file.
The cache. Browsers hang on to favicons harder than almost anything else. After you replace the file, hard-reload and clear the cache, or you will spend an afternoon debugging a conversion that worked fine the first time.
Related image tools
If your source needs work before it becomes an icon, fix it first: the image cropper will square up a rectangular logo exactly where you want the crop, rather than leaving the decision to a fit setting, and the image resizer brings an oversized export down to something sane. If your logo arrived in the wrong format, the image format converter moves between PNG, JPG and WebP — worth doing before conversion, since a PNG source is the only one that can carry transparency into the icon.
For the rest of the site's images, PNG to WebP and the image compressor handle the weight problem, and the profile picture maker covers the other square-image job — avatars sized correctly per platform. All of them run in your browser, same as this one. And 10% of every dollar Microapp earns goes to charity, off the top, audited quarterly.
Frequently asked questions
What size image should I start from?
Square, PNG, at least 256 px on a side. 512 px is a good default — every icon size is rendered down from your original, so more source pixels mean better results at 48 and 256, and there is no penalty for starting big. You can go up to 50 MB, though an icon never needs a source anywhere near that.
Why does the .ico format stop at 256 px?
The directory stores each entry's width and height in a single byte, which tops out at 255. The format works around it by treating 0 as 256, so 256 is the real ceiling. Writing 255 instead is the classic bug behind icons that render one pixel short on Windows — worth knowing if you ever hand-build one of these.
How big should a favicon.ico file be?
The standard three-size file is 14.7 KB and that is fine. Under 30 KB is comfortable even with 256 px included. If yours is several hundred kilobytes, something wrote the large entries as raw bitmaps instead of PNG — a 256 px bitmap entry alone is 270,376 bytes.
Do I need Apple touch icons and PNG favicons as well?
Yes, for a complete setup. The modern arrangement is both: an .ico at the site root as the fallback any browser will guess at, plus PNG or SVG icons declared in your <head> for everything that reads them, and an Apple touch icon for iOS home screens. The .ico is the safety net, not the main event — but it is the one that catches RSS readers, crawlers and old bookmarks.
Can an .ico file be animated?
No. The format holds still images at several sizes and nothing else — there are no frames and no timing information. Animated tab icons are done in JavaScript by swapping the icon link at intervals, which is a different job entirely and one most sites are better off skipping.
Does the ICO Converter work without an internet connection?
Once the page has loaded, yes. Decoding, painting and byte assembly all happen on your device, so the conversion keeps working if your connection drops. Nothing is uploaded, which also means it is safe to use on an unreleased logo or a client's file — an ICO converter with no upload step has nowhere to leak it to.
What happens if I use a photo instead of a logo?
A JPG to ICO conversion of a photograph works fine and looks like nothing at 16 px. A tab icon is 256 pixels total — roughly the resolution of a letter of text. Photographs carry their meaning in detail that simply isn't there at that size. Flat shapes, heavy strokes and two or three colours survive the trip; faces and landscapes don't.