A universal guide to choosing a wheel size:
The frame size of a bicycle is the distance between the centerline and the end of the seat tube of the frame. Size is usually given in inches (1 inch = 2.54 cm) or centimetres. Sizes may vary slightly according to the make of the bike or the frame design.
For each model, the picture shows the geometry of the bike, the seat tube length is the S/T dimension.
Measurements:
- stand barefoot against the wall
- measure the distance from the ground to the crotch
- multiply the result by 0.57 for mountain bikes, 0.63 for trekking bikes and 0.66 for road bikes
- if you need a measurement in inches, divide the result by 2.54. This value determines the frame size that should suit you.
The calculation is approximate, it also depends on the geometry of the bike, where the seat may be more or less upright, riding style, etc.
The general recommendation is that the frame size of a men's mountain bike should be chosen so that the rider standsstanding straddling the bike has the top (horizontal) frame tube about 5 cm from the crotch - but this is really a very approximate figure.