How it's calculated
frame (cm) = inseam (cm) × k
inseam = inside-leg length, k = bike-type factor (road 0.70, mountain 0.66, hybrid 0.685, city 0.61). Result is the seat-tube length; frame in inches = cm ÷ 2.54.
Common questions
Why does inseam matter more than height?
Frame fit depends mostly on leg length for standover clearance and reach, so inseam drives the multiplier for each bike type.
Do road and mountain bikes size the same?
No. They use different inseam multipliers, so the same rider needs a different frame size for a road bike than for a mountain bike.