HEIF is the container HEIC lives in, read here and not written. 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 HEIF file on the way to becoming an ICO one.
What runs when a HEIF file becomes ICO
ImageMagick 6.9.12, on a machine we rent and watch. Your HEIF file is uploaded once, ImageMagick runs once, the ICO comes back, and neither file is kept. HEIF 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 HEIF into the ICO
Transparency. HEIF and ICO both carry a real alpha channel, so a cut-out sent as HEIF is still a cut-out as ICO.
What HEIF to ICO costs you
Every frame of the HEIF but the first. HEIF can hold an animation and ICO cannot, so frame one is what comes back. The alternative would be one numbered ICO file per frame and no single answer to hand you.
Any HEIF 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.
Where HEIF and ICO files come from
HEIF. The container standard; HEIC is the name it goes by when the picture inside is HEVC. Read here and not written, because the write alias is not wired on the engine and pretending otherwise would hand you a file under the wrong name.
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 HEIF is mostly words, the useful move is to pull the text out of it, not to change which picture format holds it.
HEIF to ICO, measured rather than promised
Running HEIF 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 HEIF file on one day and not an average, which is why the number is given together with the file that produced it.
The HEIF 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 HEIF file, sent somewhere else
Other outputs the probe measured out of a HEIF 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 HEIF witness file.
HEIF to PDF: 94,782 bytes in under a tenth of a second, verified by a format signature.
HEIF to JPG: 1,666 bytes in under a tenth of a second, verified by a format signature.
HEIF to PNG: 1,403 bytes in under a tenth of a second, verified by a format signature.
HEIF to WEBP: 416 bytes in under a tenth of a second, verified by a format signature.
HEIF to GIF: 2,918 bytes in 0.1 seconds, verified by a format signature.
HEIF 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, HEIF is the seventh largest output of the 10 measured. The witness files differ, so this ranks the probe run and not your document.
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.
PDF to ICO: 161,630 bytes in 0.1 seconds.
What we will not pretend about HEIF to ICO
A HEIF file over 25 MB is refused before the upload finishes rather than after it, so you do not wait for a rejection.
A HEIF 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.
No text is read out of the HEIF 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 HEIF to ICO, not a marketing number.
One HEIF file at a time, chosen in the browser. There is nothing else to set up and nothing else on offer.
HEIF to ICO: what people ask
What actually converts my HEIF 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 HEIF file is uploaded once, ImageMagick runs once, the ICO comes back, and neither file is kept afterwards.
How long does HEIF 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 HEIF file, not an average and not a promise about yours: a larger HEIF takes longer, and past 30 seconds the run is stopped.
What do I lose going from HEIF to ICO?
The one to know about first: Every frame of the HEIF but the first. HEIF can hold an animation and ICO cannot, so frame one is what comes back. The alternative would be one numbered ICO file per frame and no single answer to hand you.
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 HEIF 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 HEIF makes a 32 pixel icon. Send a square one if you can.
Is HEIF to ICO free?
There is a free allowance every month, and one HEIF 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.