Bike Frame Size Calculator

Mountain bikes need extra standover clearance so you can get off the saddle over rough terrain without hitting the top tube, which is why the calculator both uses a slightly lower multiplier (0.66 vs. 0.665 for road) and subtracts a flat 2 cm on top. The flat offset represents the fixed clearance margin MTB frames build in regardless of your exact inseam, while the multiplier handles the proportional scaling.

Frame size is the seat tube length from bottom bracket to top, standover clearance is how much gap is left between your crotch and the top tube when you straddle the bike flat-footed, and effective top tube is the horizontal distance from the seat tube to the head tube that determines your reach. This calculator reports all three separately because a bike can have the right frame size but a cramped or stretched-out reach, or plenty of standover clearance but the wrong reach for your torso and arms.

They're just two units describing the identical seat tube length — the calculator takes the computed centimeter value and divides by 2.54 to get inches, which is the industry-standard unit for most mountain bike sizing while road bikes are usually sold in centimeters. Neither number is more "correct"; use whichever unit the manufacturer's size chart lists.

The calculator estimates a reach target from your arm length (about 90% of arm length), subtracts the effective top tube length it already computed from your height and arm length, then adds 10 cm as a baseline stem allowance, with a 6 cm floor so the result never suggests an unrealistically short stem. Because both the reach target and top tube figures are rough empirical approximations rather than measurements of your actual torso flexibility and riding position, a professional fitting can refine this further based on how you actually sit on the bike.