How this calculator works
Adult frame size is estimated from the cycling inseam, measured from the floor to a book pressed firmly against the pubic bone, not from trouser size. The traditional multipliers are about 0.66 for road and gravel frames, 0.63 for trekking and hybrid frames, and 0.57 for mountain frames. The saddle height target uses the common LeMond method of inseam times 0.883, measured from the centre of the bottom bracket to the top of the saddle. Standover gives the highest top tube height that still leaves clearance when you stand over the frame: about 3 cm for a road bike and 8 cm for a mountain bike.
Formula
Frame size (cm) = inseam (cm) x multiplier, where the multiplier is 0.66 for road, 0.63 for trekking or hybrid and 0.57 for mountain. Saddle height = inseam x 0.883. Standover target = inseam - clearance, with 3 cm clearance on the road and 8 cm off it.
Worked example
A rider 175 cm tall with an 81 cm cycling inseam: a road frame is 81 x 0.66 = 53.5 cm, a trekking frame is 51.0 cm, and a mountain frame is 46.2 cm. The saddle height target is 81 x 0.883 = 71.5 cm. For a road bike the standover target is 78 cm, so a frame with a 76 cm top tube leaves about 5 cm of clearance.
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Frequently asked questions
Why is the mountain frame number so much smaller?
Mountain bikes have sloping top tubes and shorter seat tubes than road bikes, so the same rider needs a smaller seat tube number. The reach and standover matter more than the nominal size, which is why brands often use S to XL.
Can I use my trouser inseam?
No. Trouser inseam is measured to the ankle and varies with the rise of the trousers. Measure barefoot against a wall with a book pulled up firmly, as if you were sitting on a saddle.
Is the result exact?
No. Geometry, crank length, saddle shape, flexibility and riding style all matter. Use the result to narrow the choices and then compare the actual frame dimensions, or have a bike fitter check the fit.
How do child bike sizes work?
Childrens bikes are usually sized by wheel diameter rather than frame size. The ranges here are a rough starting point; the child should be able to stand over the frame with both feet flat and reach the brakes comfortably.