ICO is a Windows icon, capped at 256 pixels a side by the file structure itself. PNG is lossless compression with a full 8-bit alpha channel. ImageMagick sits between the two, and this page says exactly what happens to an ICO file on the way to becoming a PNG one.
What runs when an ICO file becomes PNG
ImageMagick 6.9.12, on a machine we rent and watch. Your ICO file is uploaded once, ImageMagick runs once, the PNG comes back, and neither file is kept. ICO to PNG 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 PNG
The pixel dimensions of your ICO file. The PNG comes out the size the ICO went in, because nothing on this path resizes anything.
Transparency. ICO and PNG both carry a real alpha channel, so a cut-out sent as ICO is still a cut-out as PNG.
Every pixel value. ICO and PNG are both lossless, so the PNG file adds no compression artefacts that the ICO did not already have.
What ICO to PNG costs you
Nothing measurable, which is rare enough to say plainly. ICO and PNG carry the same kind of content with the same fidelity, so this pair changes the container and not the picture. What does change is the size, and often by a lot.
If a model is going to read the PNG
A PNG file is pixels, and pixels are expensive for a model to read. Handed this PNG, 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 PNG, measured rather than promised
Running ICO to PNG against the real engine with a real file, ImageMagick wrote 810 bytes of PNG in under a tenth of a second, 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 PNG 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 PNG 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 PNG 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 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 PNG, and what they measured
Among the published routes into PNG, ICO is the number 11 largest output of the 13 measured. The witness files differ, so this ranks the probe run and not your document.
HEIF to PNG: 1,403 bytes in under a tenth of a second.
EPS to PNG: 1,303 bytes in 0.1 seconds.
HEIC to PNG: 1,403 bytes in under a tenth of a second.
JPG to PNG: 805 bytes in under a tenth of a second.
WEBP to PNG: 4,003 bytes in under a tenth of a second.
What we will not pretend about ICO to PNG
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 PNG 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 ICO anywhere on this path. Converting a picture to PNG 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 PNG, 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 PNG: what people ask
What actually converts my ICO file to PNG?
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 PNG comes back, and neither file is kept afterwards.
How long does ICO to PNG take?
On the file the probe used, ImageMagick took under a tenth of a second and wrote 810 bytes of PNG. 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 PNG?
The one to know about first: Nothing measurable, which is rare enough to say plainly. ICO and PNG carry the same kind of content with the same fidelity, so this pair changes the container and not the picture. What does change is the size, and often by a lot.
Does the PNG come out the same size as the ICO?
The same pixel size, yes: nothing on this path resizes anything, so the PNG is as wide and as tall as the ICO was. The number of bytes is a different question and usually moves a long way, which is normally the reason for doing this at all.
Is ICO to PNG free?
There is a free allowance every month, and one ICO to PNG 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 PNG.