SVG is an XML description of shapes, which is why it scales to any size. 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 an SVG file on the way to becoming an ICO one.
What runs when an SVG file becomes ICO
ImageMagick 6.9.12, on a machine we rent and watch. Your SVG file is uploaded once, ImageMagick runs once, the ICO comes back, and neither file is kept. SVG 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 SVG into the ICO
Transparency. SVG and ICO both carry a real alpha channel, so a cut-out sent as SVG is still a cut-out as ICO.
Every pixel value. SVG and ICO are both lossless, so the ICO file adds no compression artefacts that the SVG did not already have.
What SVG to ICO costs you
Scalability. SVG describes shapes, ICO stores a grid of pixels, and the engine rasterises at 150 dots per inch on the way across. That density is a deliberate choice: at the 72 dpi ImageMagick defaults to, an A4 SVG page comes out 595 pixels wide and reads as a failed conversion. Enlarge the ICO afterwards and it blurs, because there is nothing left to redraw from.
Any SVG 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 SVG is mostly words, the useful move is to pull the text out of it, not to change which picture format holds it.
SVG to ICO, measured rather than promised
Running SVG 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 SVG file on one day and not an average, which is why the number is given together with the file that produced it.
The SVG 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 SVG file, sent somewhere else
Other outputs the probe measured out of an SVG 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 SVG witness file.
SVG to PDF: 4,195 bytes in under a tenth of a second, verified by a format signature.
SVG to JPG: 2,483 bytes in under a tenth of a second, verified by a format signature.
SVG to PNG: 1,268 bytes in under a tenth of a second, verified by a format signature.
SVG to WEBP: 608 bytes in under a tenth of a second, verified by a format signature.
SVG to GIF: 1,232 bytes in 0.1 seconds, verified by a format signature.
SVG to TIFF: 2,464 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, SVG is the eighth largest output of the 10 measured. The witness files differ, so this ranks the probe run and not your document.
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.
JPG to ICO: 99,422 bytes in under a tenth of a second.
What we will not pretend about SVG to ICO
An SVG file over 25 MB is refused before the upload finishes rather than after it, so you do not wait for a rejection.
An SVG 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 SVG 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 SVG to ICO, not a marketing number.
One SVG file at a time, chosen in the browser. There is nothing else to set up and nothing else on offer.
SVG to ICO: what people ask
What actually converts my SVG 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 SVG file is uploaded once, ImageMagick runs once, the ICO comes back, and neither file is kept afterwards.
How long does SVG 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 SVG file, not an average and not a promise about yours: a larger SVG takes longer, and past 30 seconds the run is stopped.
What do I lose going from SVG to ICO?
The one to know about first: Scalability. SVG describes shapes, ICO stores a grid of pixels, and the engine rasterises at 150 dots per inch on the way across. That density is a deliberate choice: at the 72 dpi ImageMagick defaults to, an A4 SVG page comes out 595 pixels wide and reads as a failed conversion. Enlarge the ICO afterwards and it blurs, because there is nothing left to redraw from.
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 SVG 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 SVG makes a 32 pixel icon. Send a square one if you can.
Is SVG to ICO free?
There is a free allowance every month, and one SVG 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.