Bike Frame Size Calculator
Recommended bike frame size from height and inseam for road, gravel, mountain, hybrid and kids’ bikes — in centimetres, inches and S/M/L letters — with stack/reach hints and crank-length suggestion.
How it works
Road frame (cm) ≈ 0.67 × inseam; gravel ≈ 0.66 × inseam; MTB (inches) ≈ 0.226 × inseam; hybrid ≈ 0.685 × inseam; stack ≈ 0.325 × height, reach ≈ 0.215 × height
Frame size charts by height are a blunt instrument because two riders of the same height can have legs differing by ten centimetres, so the classic formulas key off inseam instead: about two thirds of it for a road frame’s seat tube, a quarter of it in inches for a mountain bike. Modern sloping and compact geometry has made seat-tube length nearly meaningless, and the numbers that actually define fit are stack (how high the bars are) and reach (how far away); this calculator gives ballpark values to compare against a manufacturer’s chart. Between sizes, riders with long legs and short torsos size down, and the reverse size up, with stem length and spacers finishing the job. Standover clearance is the final sanity check.
Worked example
178 cm tall with an 84 cm inseam: road frame about 56 cm (M/L), gravel 55 cm, mountain bike 19″ (L); target stack ≈ 578 mm and reach ≈ 383 mm; 172.5 mm cranks.
Frequently asked questions
Should I size up or down between sizes?
For road and gravel, size down if you value handling and can add a longer stem; size up if you want a more upright, stable position. For trail MTB most riders go smaller.
Why do brands’ sizes differ?
A “54” from one brand may have the reach of another’s “56”. Compare stack and reach, not the label, and check standover in the shop.
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This calculator is for educational purposes. Double-check important results before acting on them.