Does the rated max tyre width already include clearance?

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.

Why does rim width affect tyre size?

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.)

Can I fit 28mm tyres on my road bike?

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.

What about mudguard clearance?

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.

Do tyre brands measure differently?

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.

BikeCalcs

Tyre Clearance Calculator — Will It Fit?

Check if a tyre size fits your frame and fork with adequate clearance.

Frame Max Tyre (at chainstays)

Manufacturer-stated max tyre width at the chainstays, in mm (from the bike spec sheet). Already includes the safety gap.

Fork Max Tyre

Manufacturer-stated max tyre width the fork clears, in mm — often the tighter of the two.

Desired Tyre Width

Labelled (nominal) width of the tyre you want to fit, in mm, e.g. 25, 28, 32.

Internal Rim WidthInner bead-to-bead rim width in mm (typ. 19–21mm modern road, 23–25mm gravel). Wider rims make the tyre measure wider.

MudguardsFitted mudguard type — subtracts the space it needs above and around the tyre (race blades 2mm, clip-ons 3mm, full guards 6mm).

How We Calculate This

Clearance Calculation

Rated max tyre = Min(Frame max, Fork max) — the figure already includes the manufacturer’s safety gap (≈4mm each side per ISO 4210 / Cannondale practice), so no further margin is subtracted.
Usable max tyre width = Rated max − Mudguard space − any Extra buffer you set
Mounted width = Labelled width + 0.4mm per 1mm of rim above the 19mm reference rim (ETRTO/Continental road figure; 0 growth at 19mm)
Headroom = Usable max − Mounted width. It fits when this is ≥ 0.

Mudguard space: race blades ≈2mm, clip-ons ≈3mm, full guards ≈6mm.

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Always verify results and consult a professional bike fitter where appropriate.