TIFF is lossless, alpha, and several images inside a single file. 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 TIFF file on the way to becoming an ICO one.
What runs when a TIFF file becomes ICO
ImageMagick 6.9.12, on a machine we rent and watch. Your TIFF file is uploaded once, ImageMagick runs once, the ICO comes back, and neither file is kept. TIFF 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 TIFF into the ICO
Transparency. TIFF and ICO both carry a real alpha channel, so a cut-out sent as TIFF is still a cut-out as ICO.
Every pixel value. TIFF and ICO are both lossless, so the ICO file adds no compression artefacts that the TIFF did not already have.
What TIFF to ICO costs you
Every page of the TIFF but the first. A TIFF file can hold several images in one container; an ICO file holds exactly one.
Any TIFF 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 TIFF is mostly words, the useful move is to pull the text out of it, not to change which picture format holds it.
TIFF to ICO, measured rather than promised
Running TIFF 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 TIFF file on one day and not an average, which is why the number is given together with the file that produced it.
The TIFF 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 TIFF file, sent somewhere else
Other outputs the probe measured out of a TIFF 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 TIFF witness file.
TIFF to PDF: 118,721 bytes in under a tenth of a second, verified by a format signature.
TIFF to JPG: 1,568 bytes in under a tenth of a second, verified by a format signature.
TIFF to PNG: 815 bytes in under a tenth of a second, verified by a format signature.
TIFF to WEBP: 370 bytes in under a tenth of a second, verified by a format signature.
TIFF to GIF: 3,512 bytes in 0.1 seconds, verified by a format signature.
Other ways into ICO, and what they measured
Among the published routes into ICO, TIFF is the ninth largest output of the 10 measured. The witness files differ, so this ranks the probe run and not your document.
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.
BMP to ICO: 99,422 bytes in under a tenth of a second.
What we will not pretend about TIFF to ICO
A TIFF file over 25 MB is refused before the upload finishes rather than after it, so you do not wait for a rejection.
A TIFF 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 TIFF 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 TIFF to ICO, not a marketing number.
One TIFF file at a time, chosen in the browser. There is nothing else to set up and nothing else on offer.
TIFF to ICO: what people ask
What actually converts my TIFF 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 TIFF file is uploaded once, ImageMagick runs once, the ICO comes back, and neither file is kept afterwards.
How long does TIFF 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 TIFF file, not an average and not a promise about yours: a larger TIFF takes longer, and past 30 seconds the run is stopped.
What do I lose going from TIFF to ICO?
The one to know about first: Every page of the TIFF but the first. A TIFF file can hold several images in one container; an ICO file holds exactly one.
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 TIFF 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 TIFF makes a 32 pixel icon. Send a square one if you can.
Is TIFF to ICO free?
There is a free allowance every month, and one TIFF 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.