Crank Length Calculator
Your knees ache at the top of every pedal stroke, or your hips rock on the saddle no matter how you set the height, and the cranks that came with the bike have never been questioned. Crank length is the cheapest fit variable to check and the one most riders never measure.
This crank length calculator takes your inseam in millimetres and your riding style, applies the standard inseam formula, adjusts for how you ride, and rounds the result to a crank size you can actually buy, with a sensible range either side of it.
A starting point for a bike fit, not a prescription. Flexibility, femur length, and existing knee or hip issues can move the right answer a size either way, and a test ride beats any formula.
Quick Answer
Key takeaways
- Inseam does the sizing, not height. Enter inseam in millimetres: centimetres entered by mistake produce a result pinned at the 160 mm floor, which is your warning sign.
- Riding style moves the answer by one size at most: mountain biking subtracts 2.5 mm, track adds 2.5 mm, touring subtracts 1 mm, triathlon adds 1 mm.
- Results are rounded to real 2.5 mm crank sizes and held inside 160 to 180 mm, the band mainstream makers actually stock.
- Research on maximal power found a wide neutral zone, so comfort, knee angle, and ground clearance should break any tie inside your range.
- Quick Answer
- How to measure your inseam and use this calculator
- The crank length formula this tool actually runs
- Worked example: the same legs on five different bikes
- Longer is not more powerful, and other things the research settled
- What competing crank calculators get wrong
- Frequently Asked Questions
- Add This Calculator to Your Website
- References
How to measure your inseam and use this calculator
Measure first, in millimetres. Stand barefoot with your back against a wall, feet about 20 cm apart, and press a hardcover book up between your legs until it sits level, like a saddle. Have someone mark the top edge of the book on the wall, then measure from the floor to the mark. Take it twice and use the larger reading; a 5 mm measuring error moves the raw result by about 1 mm, which rarely changes the rounded size.
Measure inseam in mm. 80 cm is entered as 800. The field is labelled in millimetres because the 0.216 factor expects them.
Pick your riding style. This applies the style adjustment after the base formula, before rounding.
Press Calculate. The result is rounded to the nearest 2.5 mm and held between 160 and 180 mm, with a range one size either side.
Reset your saddle if you swap. Changing crank length by 5 mm changes leg extension by 5 mm, so move the saddle by the same amount in the opposite direction and retest.
Height deliberately plays no part in the sizing here. Two riders who are both 175 cm tall can carry inseams of 760 mm and 840 mm, and this calculator gives them 165.0 mm and a clamped 180 mm respectively, which is exactly why inseam, not height, belongs in a sizing formula. While you are working through a full fit, the chain length calculator covers the drivetrain side of a crankset swap, since a new crankset is the natural moment to size a fresh chain.
The crank length formula this tool actually runs
One multiplication, one small adjustment, then rounding to a size that exists. Nothing else is hidden in the code.
The 0.216 factor is the long-standing proportional rule: crank length at about 21.6% of inseam. It sits close to the research finding by Martin and Spirduso that maximal power peaked near a crank length of 20% of leg length, published in the European Journal of Applied Physiology study of crank length and maximal cycling power. Treat the formula as a well-tested starting range. It cannot see your femur-to-tibia proportions, your flexibility, or a dodgy knee, and those are legitimate reasons to land one size off the number it returns.
| Inseam (mm) | Raw result (x 0.216) | Road recommendation | Note |
|---|---|---|---|
| 700 | 151.2 mm | 160.0 mm | Held at floor |
| 760 | 164.2 mm | 165.0 mm | Standard size |
| 800 | 172.8 mm | 172.5 mm | Worked example |
| 840 | 181.4 mm | 180.0 mm | Held at ceiling |
Worked example: the same legs on five different bikes
The calculator is pre-filled with an 800 mm inseam and road style. Every figure below is what it returns, checked against the tool code.
| Riding style | Adjustment | Recommended | Suitable range |
|---|---|---|---|
| Road cycling | 0 mm | 172.5 mm | 170.0 – 175.0 mm |
| Mountain biking | −2.5 mm | 170.0 mm | 167.5 – 172.5 mm |
| Track cycling | +2.5 mm | 175.0 mm | 172.5 – 177.5 mm |
| Touring | −1 mm | 172.5 mm | 170.0 – 175.0 mm |
| Triathlon | +1 mm | 175.0 mm | 172.5 – 177.5 mm |
The mountain adjustment is about the trail, not leverage: a 170 mm crank raises the pedals at the bottom of the stroke, which means fewer pedal strikes on rocks and roots. The track adjustment runs the other way in this tool, favouring leverage for standing starts, although plenty of track riders choose short cranks for cadence, which is a reminder that the style row is a nudge, not a law. If your result and your current cranks differ by 2.5 mm, most riders cannot feel it; a 5 mm or larger gap is where comfort changes usually show up.
Longer is not more powerful, and other things the research settled
Maximum power barely moves across a wide band of crank lengths. In the Martin and Spirduso experiments, trained cyclists produced their best maximal power on 145 mm cranks and nearly the same on 170 mm, with output only falling away at the 120 mm and 220 mm extremes. A later review of sprint cycling research, Maximal muscular power: lessons from sprint cycling, reaches the same practical conclusion: within the normal commercial range, crank length is a fit and comfort decision far more than a power decision.
What does change with length is joint motion. Longer cranks close the hip and flex the knee further at the top of the stroke, which is why riders with knee pain or a low, aerodynamic position often feel better one or two sizes shorter, spinning a slightly higher cadence for the same speed. Shorter cranks also lift the pedal at its lowest point, the ground-clearance benefit the mountain setting builds in. Once the fit is settled, the riding itself is the point, and the bicycle calorie calculator can put numbers on what those rides burn.
What competing crank calculators get wrong
Five rival tools were checked for this page. The recurring problems are unit traps and silent assumptions, not exotic math.
Problem 1: the centimetre trap. Several tools label the field loosely or accept centimetres while running a millimetre formula, and at least one popular guide works an example as 80 x 0.216 and calls the answer 172.8 mm, skipping the unit conversion entirely. Here the field says millimetres, the tooltip shows the book test, and the floor warning tells you when an entry looks like centimetres.
Problem 2: results you cannot buy. Tools that return 172.8 mm leave you to guess the stocked size. Cranks ship in 2.5 mm steps, as the Shimano Ultegra crankset specification shows with its 165, 170, 172.5, and 175 mm options, so this calculator rounds to a purchasable size and gives the range around it.
Problem 3: height charts presented as sizing. Height-only tables put every 175 cm rider on the same crank, ignoring that inseam varies by 8 cm or more at one height. This tool asks for the measurement that actually reaches the pedals.
One honest limit, stated plainly: the 160 to 180 mm band excludes the very short cranks some specialists now fit (150 to 165 mm) and the very long cranks tall riders sometimes order, and no formula replaces a fitter when pain is involved. Checking your contact points as a set also helps, and the bike tyre pressure calculator handles the other end of the bike.
Frequently Asked Questions
Crank length is the distance from the centre of the bottom bracket axle to the centre of the pedal axle, measured in millimetres and usually stamped on the back of the crank arm. Both arms are the same length on a standard crankset. Common adult sizes run from 165 mm to 175 mm, in 2.5 mm steps.
Because the 0.216 factor is calibrated for millimetres: 800 mm x 0.216 = 172.8 mm. If you enter 80 (centimetres) the raw result is 17.3 mm and the tool holds it at the 160 mm floor, which looks wrong because it is. Multiply centimetres by 10 first, or inches by 25.4.
Those are the floor and ceiling of this tool. A raw result below 160 mm (an inseam under about 741 mm) returns 160 mm, and a raw result above 180 mm (an inseam over about 833 mm) returns 180 mm. Specialist makers sell cranks outside the band, and riders at either extreme benefit most from an in-person fit.
The evidence says barely, within normal sizes. Martin and Spirduso (2001) found maximal power was highest on 145 mm cranks, close behind on 170 mm, and only clearly lower at 120 mm and 220 mm extremes. Choose length for knee comfort, hip clearance, and ground clearance; power follows fit, not leverage.
Yes, by the same amount, in the opposite direction. Moving from 175 mm to 170 mm cranks raises the pedal 5 mm at the bottom of the stroke, so raise the saddle about 5 mm to keep the same leg extension, then fine-tune over a few rides. Forget this step and a good crank change can feel like a bad one.
Only as a rough guide. The 160 mm floor means any inseam under about 741 mm returns the same 160 mm answer, while many children and short riders genuinely need 140 to 155 mm cranks, which children bike specialists stock. For a young rider, use the raw maths (inseam x 0.216) and shop the specialist sizes.
Add This Calculator to Your Website
References
- Martin, J.C. and Spirduso, W.W., “Determinants of maximal cycling power: crank length, pedalling rate and pedal speed,” European Journal of Applied Physiology, 2001: optimal crank length near 20% of leg length, with maximal power broadly flat from 145 to 195 mm. ResearchGate: Determinants of maximal cycling power. Accessed October 2026.
- SpringerOpen review, “Maximal muscular power: lessons from sprint cycling,” Sports Medicine – Open, 2021: synthesis of crank length, pedalling rate, and pedal speed research. Springer: Maximal muscular power, lessons from sprint cycling. Accessed October 2026.
- US National Library of Medicine (PMC), full text of “Maximal muscular power: lessons from sprint cycling,” including the Martin and Barratt crank length findings. PMC: Maximal muscular power, lessons from sprint cycling. Accessed October 2026.
- Shimano, Ultegra FC-08 crankset product specification: stocked crank arm lengths of 160, 165, 170, 172.5, and 175 mm in 2.5 and 5 mm steps. Shimano: Ultegra crankset specification. Accessed October 2026.
- Barratt, P.R. et al., “Effect of crank length on joint-specific power during maximal cycling,” Medicine and Science in Sports and Exercise, 2011: joint power shares barely change once pedalling rate and pedal speed are accounted for. Ovid: Effect of crank length on joint-specific power. Accessed October 2026.
- PEZ Cycling News toolbox, “Crank Length: Does Size Really Matter?”: practitioner summary of the Martin research and the 0.2 x leg length sizing rule. PEZ Cycling News: Crank length, does size really matter?. Accessed October 2026.