If you want an estimate of how big your tyre will be on a given rim, here’s a handy calculator.

The Complete Circular Arc Calculator

The measurements I use are the bead to bead dimension of the tyre when it is flattened out.

Deduct the 2x height of the rim walls from this measurement, and that gives you the length of the arc.

For the rim measurement use the distance between the beadhooks, that’s the width of the arc.

On a fat tyre the calculation comes pretty close to a measured casing with at 10psi. Certainly close enough to get an idea if it will fit in a given space.

With some of the new ultrafat tyres that should give me a 6″ tyre on my 132mm rim. 🙂

(I’ve already posted this on another thread, but seeing as this is something folk will search for when they need it, this will make it easier to find)