HEIC is what an iPhone writes unless you tell it not to: HEVC compression inside an ISO container, about half the size of the JPEG it replaced. ICO is a Windows icon, capped at 256 pixels a side by the file structure itself. ImageMagick sits between the two, and this page says exactly what happens to a HEIC file on the way to becoming an ICO one.
What runs when a HEIC file becomes ICO
ImageMagick 6.9.12, on a machine we rent and watch. Your HEIC file is uploaded once, ImageMagick runs once, the ICO comes back, and neither file is kept. HEIC to ICO is one of the 3649 pairs that engine was probed on with a real file, which is why it has a page here and why the pairs the probe could not prove do not.
What survives from the HEIC into the ICO
Transparency. HEIC and ICO both carry a real alpha channel, so a cut-out sent as HEIC is still a cut-out as ICO.
What HEIC to ICO costs you
Every page of the HEIC but the first. A HEIC file can hold several images in one container; an ICO file holds exactly one.
Any HEIC bigger than 256 pixels a side. That is not a setting, it is the ICO file structure: width and height each live in a single byte. The engine scales larger sources down, and only down, so a phone photo becomes a 256 pixel icon.
Nothing further, and that is the point worth making: ICO cannot undo what HEIC already threw away. The compression artefacts are baked into the pixels, ICO stores them faithfully, and the file usually comes out several times larger for exactly the same picture.
If a model is going to read the ICO
An ICO file is pixels, and pixels are expensive for a model to read. Handed this ICO, an assistant spends its budget looking at the picture rather than reading it, and it will paraphrase what it thinks it saw. If your HEIC is mostly words, the useful move is to pull the text out of it, not to change which picture format holds it.
HEIC to ICO, measured rather than promised
Running HEIC to ICO against the real engine with a real file, ImageMagick wrote 99,422 bytes of ICO in under a tenth of a second, machine otherwise idle. That is one HEIC file on one day and not an average, which is why the number is given together with the file that produced it.
The HEIC to ICO verdict was reached by a format signature, meaning at least four bytes unique to the format were found at the right offset. Which check was used matters, because they do not all prove the same thing, and a status code of 200 proves nothing whatsoever about whether the ICO file has anything inside it.
The same HEIC file, sent somewhere else
Other outputs the probe measured out of a HEIC file, so the cost of choosing ICO can be read against something. Sizes do not compare across engines, because each family was probed with its own HEIC witness file.
HEIC to PDF: 94,782 bytes in under a tenth of a second, verified by a format signature.
HEIC to JPG: 1,666 bytes in under a tenth of a second, verified by a format signature.
HEIC to PNG: 1,403 bytes in under a tenth of a second, verified by a format signature.
HEIC to WEBP: 416 bytes in under a tenth of a second, verified by a format signature.
HEIC to GIF: 2,918 bytes in 0.1 seconds, verified by a format signature.
HEIC to TIFF: 90,264 bytes in under a tenth of a second, verified by a format signature.
Other ways into ICO, and what they measured
Among the published routes into ICO, HEIC is the sixth largest output of the 10 measured. The witness files differ, so this ranks the probe run and not your document.
HEIF to ICO: 99,422 bytes in under a tenth of a second.
SVG to ICO: 99,422 bytes in under a tenth of a second.
TIFF to ICO: 99,422 bytes in under a tenth of a second.
WEBP to ICO: 99,422 bytes in under a tenth of a second.
PNG to ICO: 99,422 bytes in under a tenth of a second.
What we will not pretend about HEIC to ICO
A HEIC file over 25 MB is refused before the upload finishes rather than after it, so you do not wait for a rejection.
A HEIC to ICO run that passes 30 seconds is killed, and the ImageMagick process is killed with it. A run left behind would sit on one of the machine's two cores until somebody noticed.
Until the end of August this machine could not read a HEIC at all, and reported the opposite. ImageMagick declared HEIC readable, but libheif carried only its AV1 plugins, which serve AVIF; the HEVC decoder was absent, and a HEIC decoded quietly as something else. The plugin was installed on the 30th, and that is the whole reason this pair exists.
The HEIC to ICO proof used a file genuinely encoded in HEVC, and that detail is not pedantry: ImageMagick refuses to write HEIC and hands you a PNG under that name if you ask it to, so a HEIC forged for the test would have made a false result look like a true one. The ICO it produced was 99,422 bytes, recognised by a format signature.
No text is read out of the HEIC anywhere on this path. Converting a picture to ICO never turns it into words, whatever the picture happens to contain.
ImageMagick was not saturated at sixteen simultaneous conversions: it held 47.5 a second while latency moved from 0.22 s to 0.34 s, and the service memory did not move off 51 MB. That is the measured headroom behind HEIC to ICO, not a marketing number.
One HEIC file at a time, chosen in the browser. There is nothing else to set up and nothing else on offer.
HEIC to ICO: what people ask
What actually converts my HEIC file to ICO?
ImageMagick does it, 6.9.12, on a machine we rent and watch. Not a browser trick and not somebody else service: the HEIC file is uploaded once, ImageMagick runs once, the ICO comes back, and neither file is kept afterwards.
How long does HEIC to ICO take?
On the file the probe used, ImageMagick took under a tenth of a second and wrote 99,422 bytes of ICO. That is one real measurement on one real HEIC file, not an average and not a promise about yours: a larger HEIC takes longer, and past 30 seconds the run is stopped.
What do I lose going from HEIC to ICO?
The one to know about first: Every page of the HEIC but the first. A HEIC file can hold several images in one container; an ICO file holds exactly one.
Can I use the ICO as a favicon?
Yes, and it is the usual reason for this pair. ICO is the Windows icon container, and the size of the HEIC you send decides what you get: anything over 256 pixels a side is scaled down to fit, anything under is left exactly as it is, so a 32 pixel HEIC makes a 32 pixel icon. Send a square one if you can.
Is HEIC to ICO free?
There is a free allowance every month, and one HEIC to ICO conversion costs half a credit against it. When the allowance runs out the tool says so and stops, rather than quietly handing you a worse ICO.