In reply to The Potato:
Unless you know the specific tyre it's a bit of a gamble unless you have plenty of frame clearance because different makes and models measure up differently in reality. I have continental tyres that are 2 mm wider and taller off the rim than some Hutchinson tyres that are both supposedly 25s.
If you have previous experience or knowledge about how the tyres size up on a more regular rim then you could just do the maths and add 2 mm to the circumference and work back from there.