Tyre Clearance Calculator — Will It Fit?

Yes. Manufacturers quote the largest tyre that fits with their recommended safety gap already built in — Cannondale and most brands derive it from measured tyre width plus about 4mm of space each side (the ISO 4210 practice is 4–6mm per side). So a frame rated for a 32mm tyre is meant to take a tyre that measures up to roughly 32mm; you should not subtract a further margin. This calculator compares your mounted width directly against the rated figure.

Wider rims spread the tyre bead further apart, so the tyre measures wider when mounted. Per ETRTO/Continental, a road tyre grows about 0.4mm for every 1mm the rim is wider than the tyre’s reference rim. Modern road tyres are now sized on a 19mm internal rim, so a 25mm tyre on a 19mm rim measures about 25mm, but on a 25mm rim it measures roughly 25 + 0.4×6 ≈ 27.4mm. (Gravel/CX tyres grow nearer 0.3mm per mm.)

Most modern road frames accept 28mm and many 2018-onward frames take 30–32mm. Check the rated max for BOTH frame (at the chainstays) and fork, since the fork is often the tighter of the two, and remember a 28mm tyre measures wider on a wide rim. Brand also matters — a Continental 28mm often measures ~27mm, a Schwalbe 28mm nearer 29mm.

Mudguards sit in space the bare-frame rating did not allow for, so they reduce the usable max tyre width. Full mudguards (SKS, Crud) need about 6mm above and around the tyre, clip-on guards about 3mm, and race blades about 2mm. This calculator subtracts that space from the rated max.

Yes, significantly. On the same rim a Continental 28mm may measure ~27mm while a Schwalbe 28mm measures ~29mm. The labelled size is nominal, so check manufacturer data or reviews for the actual mounted width before judging clearance — a tyre marked 28 that really measures 30 needs the clearance of a 30.