PNG is lossless compression with a full 8-bit alpha channel. 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 PNG file on the way to becoming an ICO one.
What runs when a PNG file becomes ICO
ImageMagick 6.9.12, on a machine we rent and watch. Your PNG file is uploaded once, ImageMagick runs once, the ICO comes back, and neither file is kept. PNG 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 PNG into the ICO
Transparency. PNG and ICO both carry a real alpha channel, so a cut-out sent as PNG is still a cut-out as ICO.
Every pixel value. PNG and ICO are both lossless, so the ICO file adds no compression artefacts that the PNG did not already have.
What PNG to ICO costs you
Any PNG 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 PNG is mostly words, the useful move is to pull the text out of it, not to change which picture format holds it.
PNG to ICO, measured rather than promised
Running PNG 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 PNG file on one day and not an average, which is why the number is given together with the file that produced it.
The PNG 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 PNG file, sent somewhere else
Other outputs the probe measured out of a PNG 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 PNG witness file.
PNG to PDF: 2,536 bytes in under a tenth of a second, verified by a format signature.
PNG to JPG: 1,568 bytes in under a tenth of a second, verified by a format signature.
PNG to WEBP: 370 bytes in under a tenth of a second, verified by a format signature.
PNG to GIF: 3,512 bytes in 0.1 seconds, verified by a format signature.
PNG to TIFF: 1,466 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, PNG is the third largest output of the 10 measured. The witness files differ, so this ranks the probe run and not your document.
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.
HEIC to ICO: 99,422 bytes in under a tenth of a second.
What we will not pretend about PNG to ICO
A PNG file over 25 MB is refused before the upload finishes rather than after it, so you do not wait for a rejection.
A PNG 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 PNG 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 PNG to ICO, not a marketing number.
One PNG file at a time, chosen in the browser. There is nothing else to set up and nothing else on offer.
PNG to ICO: what people ask
What actually converts my PNG 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 PNG file is uploaded once, ImageMagick runs once, the ICO comes back, and neither file is kept afterwards.
How long does PNG 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 PNG file, not an average and not a promise about yours: a larger PNG takes longer, and past 30 seconds the run is stopped.
What do I lose going from PNG to ICO?
The one to know about first: Any PNG 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.
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 PNG 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 PNG makes a 32 pixel icon. Send a square one if you can.
Is PNG to ICO free?
There is a free allowance every month, and one PNG 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.