Crank Length Calculator — Optimal Crank Arm Length

Most road bikes ship with 170 mm or 172.5 mm cranks. By the inseam × 2.16 formula used here, that corresponds to an inseam of roughly 78-80 cm. The × 2.16 figure (Lennard Zinn upper bound) tends to recommend a longer crank than many fitters now prefer — there is a strong trend towards shorter cranks, so do not be surprised if a 170-172.5 mm crank feels right even at a slightly longer inseam.

For most riders, the difference between 170 mm and 172.5 mm is negligible. However, very short or tall riders can benefit significantly from correct crank length, especially for comfort and injury prevention.

There is a trend towards shorter cranks, especially for time trials and triathlons, as they allow a more aerodynamic position. Shorter cranks also suit riders with hip flexibility issues.