Bike Frame Size Calculator

How it is calculated

The frame size is a fixed fraction of the rider's inseam, with a different ratio for road and mountain bikes. The mountain bike figure is also converted to inches.

road frame (cm) = inseam × 0.70

MTB frame (in) = inseam × 0.66 ÷ 2.54

The inseam is in centimetres, measured from the crotch to the floor along the inside of the leg, barefoot. Road frames are given in centimetres and mountain bike frames in inches, as they are usually listed by manufacturers. Both numbers are shown to 1 decimal. Inseams from 50 to 110 cm are accepted; others give no result. These are rule-of-thumb ratios for a starting point, not a full fit: reach, torso length, flexibility and each brand's geometry also matter.

Common mistakes

• Entering the rider's height instead of the inseam. A height such as 178 is outside the accepted 50 to 110 cm range and gives no result.
• Entering the inseam in inches (32). The field expects centimetres, so 32 in must be typed as about 81.
• Measuring the inseam with shoes on or along the outside of the leg. Measure barefoot, along the inside, with a book pressed up to the crotch.
• Comparing the road figure in centimetres with the mountain bike figure in inches. They are different units, so 57.4 and 21.3 describe sizes of similar scale.
• Treating the result as a guaranteed fit. Frame sizes are labelled differently by brands, and a rider with long arms or a short torso may need the next size.