Enter your inseam to see what the published crank length rules suggest, and which of them rest on data. Then pick a length and see what it changes: your saddle height, the room at the top of the stroke and how hard your gears feel.
Measured barefoot, floor to crotch. Crank lengths are always in mm.
Rounded to the lengths Shimano and SRAM sell for the road.
In the study most rules point back to, Martin and Spirduso tested 16 trained cyclists on cranks from 120 to 220 mm. Maximal power changed by less than 4% across that whole range, from 1149 W on the longest cranks to 1194 W on 145 mm. Their optimum worked out at 20% of leg length, but that proportion explained only about a fifth of the differences between riders.
At steady efforts the picture is similar. Comparing 145, 170 and 195 mm cranks, McDaniel and colleagues found that the energy cost of pedalling followed how fast the foot moved, not the crank length itself. Changing a rider's usual length by 5 mm either way made no measurable difference to efficiency. One study in beginners did find better economy on 145 mm than on 175 mm cranks.
What crank length clearly changes is how far your hip and knee bend: longer cranks close them more at the top of the stroke. That is why shorter cranks interest riders in low aero positions, although no study has yet shown that they make a time trial faster.
Only one rule comes from an experiment: 20% of leg length, from Martin and Spirduso. The calculator applies it to your inseam, and it explained only a small part of the differences in power between riders.
Kirby Palm's 21.6% of inseam starts from the idea that 170 mm is right for a 31-inch inseam and scales from there. Palm himself calls it not a generally accepted rule.
Lennard Zinn, a frame builder and technical writer, suggests 20% to 21.5% of inner leg length, measured barefoot from the floor to the pubic bone as on this page. It comes from his experience, not from a study.
The last rule is a straight line that Machinehead Software fitted to a size table in The Racing Bike Book: 1.25 times your inseam in centimetres, plus 65 mm.
The calculator shows them side by side and does not average them. An average of rules that have no data behind them would look more precise than it is.
Shimano's Ultegra and 105 road cranksets come in 160, 165, 170, 172.5 and 175 mm, and Dura-Ace adds 167.5 and 177.5 mm. SRAM's road cranksets run from 160 to 175 mm, and SRAM also lists 150 and 155 mm for Red. The calculator rounds each rule to the nearest of these.
The rules disagree by 10 mm or more, and the only one based on data explains a small part of the differences between riders. Treat the result as a range.
If a hip or knee feels cramped or sore at the top of the stroke, that is worth a fitter's look before you buy a new crankset.
The lengths it rounds to are road cranksets. Mountain bike and track choices follow other constraints, such as pedal strikes and gearing.
Road cranksets are sold mostly between 160 and 175 mm, and research suggests the exact choice barely changes your power. Use the rules here as a range, then decide on comfort: if your hip or knee feels cramped at the top of the stroke, go shorter.
Not for power: across 120 to 220 mm, maximal power changed less than 4% in the lab. They give your hip and knee more room at the top of the stroke, and they make the same gear feel slightly harder.
Yes, by the difference: 5 mm shorter cranks, 5 mm higher saddle, measured from the bottom bracket. The distance to the pedal at the bottom of the stroke stays the same.
At the same cadence a shorter crank moves your foot more slowly around a smaller circle: at 90 rpm, about 1.56 m/s on 165 mm cranks against 1.63 m/s on 172.5 mm.
The difference is 2.5 mm, about 1.5% of the crank. Studies that changed crank length by 5 mm found no difference in efficiency, so choose the one that feels better at the top of the stroke.
