ICO is a Windows icon, capped at 256 pixels a side by the file structure itself. 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. ImageMagick sits between the two, and this page says exactly what happens to an ICO file on the way to becoming a HEIC one.
What runs when an ICO file becomes HEIC
ImageMagick 6.9.12, on a machine we rent and watch. Your ICO file is uploaded once, ImageMagick runs once, the HEIC comes back, and neither file is kept. ICO to HEIC 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 ICO into the HEIC
The pixel dimensions of your ICO file. The HEIC comes out the size the ICO went in, because nothing on this path resizes anything.
Transparency. ICO and HEIC both carry a real alpha channel, so a cut-out sent as ICO is still a cut-out as HEIC.
What ICO to HEIC costs you
Exactness. HEIC is a lossy encoding, so the file you get back approximates the ICO you sent. The artefacts appear once and never come out again, which is why converting the HEIC back to ICO afterwards restores nothing.
If a model is going to read the HEIC
A HEIC file is pixels, and pixels are expensive for a model to read. Handed this HEIC, an assistant spends its budget looking at the picture rather than reading it, and it will paraphrase what it thinks it saw. If your ICO is mostly words, the useful move is to pull the text out of it, not to change which picture format holds it.
ICO to HEIC, measured rather than promised
Running ICO to HEIC against the real engine with a real file, ImageMagick wrote 807 bytes of HEIC in 0.1 seconds, machine otherwise idle. That is one ICO file on one day and not an average, which is why the number is given together with the file that produced it.
The ICO to HEIC 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 HEIC file has anything inside it.
The same ICO file, sent somewhere else
Other outputs the probe measured out of an ICO file, so the cost of choosing HEIC can be read against something. Sizes do not compare across engines, because each family was probed with its own ICO witness file.
ICO to PDF: 95,344 bytes in under a tenth of a second, verified by a format signature.
ICO to JPG: 1,568 bytes in under a tenth of a second, verified by a format signature.
ICO to PNG: 810 bytes in under a tenth of a second, verified by a format signature.
ICO to WEBP: 370 bytes in under a tenth of a second, verified by a format signature.
ICO to GIF: 3,799 bytes in 0.1 seconds, verified by a format signature.
ICO to TIFF: 96,278 bytes in under a tenth of a second, verified by a format signature.
Other ways into HEIC, and what they measured
Among the published routes into HEIC, ICO is the fourth largest output of the 10 measured. The witness files differ, so this ranks the probe run and not your document.
SVG to HEIC: 1,009 bytes in 0.1 seconds.
TIFF to HEIC: 481 bytes in under a tenth of a second.
WEBP to HEIC: 579 bytes in under a tenth of a second.
PNG to HEIC: 481 bytes in under a tenth of a second.
PDF to HEIC: 982 bytes in 0.1 seconds.
What we will not pretend about ICO to HEIC
An ICO file over 25 MB is refused before the upload finishes rather than after it, so you do not wait for a rejection.
An ICO to HEIC 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 ICO to HEIC 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 HEIC it produced was 807 bytes, recognised by a format signature.
No text is read out of the ICO anywhere on this path. Converting a picture to HEIC 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 ICO to HEIC, not a marketing number.
One ICO file at a time, chosen in the browser. There is nothing else to set up and nothing else on offer.
ICO to HEIC: what people ask
What actually converts my ICO file to HEIC?
ImageMagick does it, 6.9.12, on a machine we rent and watch. Not a browser trick and not somebody else service: the ICO file is uploaded once, ImageMagick runs once, the HEIC comes back, and neither file is kept afterwards.
How long does ICO to HEIC take?
On the file the probe used, ImageMagick took 0.1 seconds and wrote 807 bytes of HEIC. That is one real measurement on one real ICO file, not an average and not a promise about yours: a larger ICO takes longer, and past 30 seconds the run is stopped.
What do I lose going from ICO to HEIC?
The one to know about first: Exactness. HEIC is a lossy encoding, so the file you get back approximates the ICO you sent. The artefacts appear once and never come out again, which is why converting the HEIC back to ICO afterwards restores nothing.
Will the HEIC look worse than the ICO?
Slightly, and permanently. HEIC is a lossy encoding: it discards detail on the way out and the file never gets it back, so converting the HEIC to ICO again restores nothing. Keep the ICO if it is your only copy of the picture.
Is ICO to HEIC free?
There is a free allowance every month, and one ICO to HEIC 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 HEIC.