# What crank length changes on a bicycle
Crank length is the distance from the centre of the bottom bracket to the centre of the pedal axle. A longer crank makes a larger pedal circle and provides more leverage for the same pedal force. It also asks the knee and hip to close further at the top of the stroke and places the pedal lower at the bottom. A shorter crank does the opposite: it reduces the circle, opens the top of the stroke, and can increase pedal clearance. None of those effects makes one length automatically correct for every rider.# The inseam starting calculation
This calculator begins with a transparent proportional rule: inseam in millimetres multiplied by 0.216. The result is compared with common crank lengths in 2.5 millimetre steps. A small discipline adjustment then moves the starting point shorter for mountain, triathlon, gravel, and city use, or slightly longer for track riding. These adjustments are deliberately modest. They are not measurements of power, flexibility, joint range, or a bicycle frame.| Signal | What it contributes | How to use it |
|---|---|---|
| Inseam | Primary proportional baseline | Measure barefoot with a firm book |
| Height | Leg proportion check | Investigate unusual ratios before buying parts |
| Discipline | Small practical bias | Use mountain for clearance and triathlon for hip room |
| Commercial sizes | Real options you can buy | Compare the recommendation with the nearest sizes |
# Why inseam beats height alone
Total height is a useful sanity check, but it hides body proportions. Two riders can have the same height and different leg lengths, torso lengths, or hip mobility. That is why the input panel uses inseam as the main signal and keeps height as a diagnostic. If the ratio looks unusual, repeat both measurements before interpreting the recommendation. A two centimetre measuring error can move a proportional calculation more than the difference between two neighbouring crank sizes.# Discipline changes the question
Road and gravel
Prioritise a smooth range of motion and fit the crank to the rider rather than chasing leverage. Gravel receives a small clearance bias because uneven terrain changes the cost of pedal strikes.
Mountain and triathlon
Shorter arms can create more ground clearance or more room at the top of an aerodynamic pedal stroke. The bike still needs a full fit after the component change.
City and track
City riding often benefits from an easy, compact circle. Track starts can reward leverage, so the calculator leaves a small longer bias while keeping the result within available sizes.
# Treat the answer as a test window
The highlighted size is a candidate to test, not an instruction to replace a crankset. If you change from 172.5 to 165 millimetres, raise the saddle by roughly the same difference as a first setup step, then check reach, knee comfort, and foot stability. If you change to a longer crank, the bottom of the stroke moves down and the top moves up, which can affect toe overlap, cornering clearance, and hip comfort. Make one change at a time so you can tell what helped.When a calculator is not enough
Persistent pain, a known injury, major left and right differences, restricted hip motion, or a new frame should be assessed with a qualified bike fitter or clinician. This tool does not diagnose a condition, measure joint angles, inspect a bicycle, or guarantee performance.
# A short checklist before buying
Use the estimate well
Repeat the inseam measurement if the height ratio looks unusual.
Compare the recommendation with the sizes actually available for your crankset and bottom bracket.
Recheck saddle height after any crank change.
Choose a shorter test length first when the main problem is a cramped top of stroke or low pedal clearance.
Stop treating the number as a fit verdict if discomfort persists.