Bike Fit Calculator
Find your ideal road bike frame size from your inseam length. Use it before buying a new bike as a starting point for the brand’s size chart.
Last updated: September 2026
Formula below · 1 source (Wikipedia) · Updated Sep 2026
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About this calculator
Proper bike fit starts with frame size, most reliably derived from inseam length — the distance from the floor to your crotch. For road bikes the widely used rule is: Frame size (cm) = inseam (cm) × 0.67, which this calculator applies and rounds to 0.1 cm. Mountain bikes are sized smaller for standover clearance and usually in S/M/L or inches, so use the brand’s chart for them. Height and arm span are not inputs here; they refine reach and handlebar height after the primary frame size is set. A longer arm span relative to height suggests you can handle a longer top tube, while poorer flexibility calls for a higher, closer bar position.
How to use
A rider with an inseam of 82 cm gets a base frame size of about 82 × 0.67 ≈ 55 cm — a 'Medium' road frame. Always confirm against the brand's geometry chart for stack and reach.
Frequently asked questions
How do I measure my inseam correctly for a bike fit calculator?
Stand barefoot with your back against a wall on a hard floor. Place a hardcover book between your legs, spine up, and press it firmly into your crotch as you would a bicycle saddle. Have someone measure from the top of the book spine to the floor. Take the measurement two or three times and average the results. Inseam measured this way is typically 1–3 cm longer than trouser inseam, so always use the bike-fit measurement rather than clothing measurements. Accuracy to within 1 cm is sufficient for frame size selection.
What is the difference between road bike and mountain bike frame sizing?
Road bikes are traditionally sized in centimeters based on seat-tube length, and the 0.67 × inseam formula gives a seat tube that places the saddle at the correct height with minimal post exposure. Mountain bikes prioritize standover clearance — the gap between the top tube and your crotch when straddling the bike flat-footed — so frames are built smaller and sized in inches or S/M/L labels. This calculator gives the road (seat-tube) size only, so for a mountain bike use the brand’s size chart. Additionally, MTB geometry uses a slacker head angle and longer wheelbase, which affects reach but not the fundamental frame size derivation.
When should I get a professional bike fitting instead of using an online calculator?
An online calculator is sufficient for selecting the right frame size when buying a new bike, but a professional fitting is worthwhile if you ride more than 5–6 hours per week, experience any knee, back, or neck pain, have switched disciplines (e.g., from MTB to road), or are targeting a specific event. A fitter uses dynamic measurements — pedaling motion, cleat position, and flexibility assessment — that no static formula can capture. Professional fits typically cost £80–£200 and can prevent injuries that would cost far more in lost training time and medical bills.