WebP is lossy or lossless depending on the encoder, with alpha and animation. 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 WEBP file on the way to becoming an ICO one.
What runs when a WEBP file becomes ICO
ImageMagick 6.9.12, on a machine we rent and watch. Your WEBP file is uploaded once, ImageMagick runs once, the ICO comes back, and neither file is kept. WEBP 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 WEBP into the ICO
Transparency. WEBP and ICO both carry a real alpha channel, so a cut-out sent as WEBP is still a cut-out as ICO.
What WEBP to ICO costs you
Every frame of the WEBP but the first. WEBP 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 WEBP 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.
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 WEBP is mostly words, the useful move is to pull the text out of it, not to change which picture format holds it.
WEBP to ICO, measured rather than promised
Running WEBP 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 WEBP file on one day and not an average, which is why the number is given together with the file that produced it.
The WEBP 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 WEBP file, sent somewhere else
Other outputs the probe measured out of a WEBP 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 WEBP witness file.
WEBP to PDF: 94,782 bytes in under a tenth of a second, verified by a format signature.
WEBP to JPG: 2,340 bytes in under a tenth of a second, verified by a format signature.
WEBP to PNG: 4,003 bytes in under a tenth of a second, verified by a format signature.
WEBP to GIF: 4,744 bytes in 0.1 seconds, verified by a format signature.
WEBP to TIFF: 72,264 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, WEBP is the tenth largest output of the 10 measured. The witness files differ, so this ranks the probe run and not your document.
PNG to ICO: 99,422 bytes in under a tenth of a second.
PDF to ICO: 161,630 bytes in 0.1 seconds.
JPG to ICO: 99,422 bytes in under a tenth of a second.
BMP to ICO: 99,422 bytes in under a tenth of a second.
GIF to ICO: 202,814 bytes in under a tenth of a second.
What we will not pretend about WEBP to ICO
A WEBP file over 25 MB is refused before the upload finishes rather than after it, so you do not wait for a rejection.
A WEBP 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 WEBP 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 WEBP to ICO, not a marketing number.
One WEBP file at a time, chosen in the browser. There is nothing else to set up and nothing else on offer.
WEBP to ICO: what people ask
What actually converts my WEBP 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 WEBP file is uploaded once, ImageMagick runs once, the ICO comes back, and neither file is kept afterwards.
How long does WEBP 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 WEBP file, not an average and not a promise about yours: a larger WEBP takes longer, and past 30 seconds the run is stopped.
What do I lose going from WEBP to ICO?
The one to know about first: Every frame of the WEBP but the first. WEBP 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 WEBP 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 WEBP makes a 32 pixel icon. Send a square one if you can.
Is WEBP to ICO free?
There is a free allowance every month, and one WEBP 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.