Crank Length Calculator — What the Evidence Actually Supports

The Honest Headline

Crank length changes your maximum power by less than you have been led to believe, and the evidence for a precise personal optimum is thin. This page gives you the only published ratios that exist, tells you how much of the variance they explain, and then spends most of its output on the thing that genuinely changes when you fit different cranks: your saddle height.

If you came here hoping for a number that will make you faster, the honest answer is in the next section.

What the Study Found

There is essentially one study people are citing. Martin and Spirduso, "Determinants of maximal cycling power: crank length, pedaling rate and pedal speed", *European Journal of Applied Physiology* 2001;84:413–418. Sixteen trained cyclists on maximal inertial-load ergometry at crank lengths of 120, 145, 170, 195 and 220 mm.

Maximum power ran from 1,149 W (SD 20) at 220 mm to 1,194 W (SD 21) at 145 mm.

That is a 45 W spread — under 4% — across a 100 mm range of crank length. 145 and 170 mm beat 120 and 220 mm significantly (P < 0.05), and nothing in the paper separates 145 from 170. Manufacturers sell cranks in 2.5 mm steps; this study could not resolve 25 mm steps between its two best lengths.

The ratios everyone quotes come from the same paper:

• Crank length as a fraction of leg length**: optimum 20%, explaining **20.5% of the variance in maximum power.
• Crank length as a fraction of tibia length**: optimum 41%, explaining **21.1%.

Read those percentages again. Around a fifth of the variation in maximal power is attributable to the crank-to-limb ratio. Four fifths is something else.

And the authors' own conclusion, which almost never survives into the calculator pages that cite them: "use of the standard 170-mm length cranks should not substantially compromise maximum power in most adults."

A later study went further, finding crank length had no significant effect on relative joint-specific power once pedalling rate and pedal speed were accounted for — which points at cadence as the thing actually doing the work.

What Does Change: Cadence and Pedal Speed

The clearest finding in the paper is not about power at all. As cranks got longer:

• Optimal pedalling rate fell from 136 rpm at 120 mm to 110 rpm at 220 mm.
• Optimal pedal speed rose from 1.71 to 2.53 m/s.

Both are consequences of geometry — a longer crank means a bigger circle, so the same pedal speed needs fewer revolutions — and this page computes pedal speed for your current and recommended cranks so you can see the size of the effect at your cadence.

One warning about that 136 rpm. It is a maximal power optimum, measured on an ergometer at all-out effort. It is not a suggestion to ride at 136 rpm, and misreading it as one is the main way this number gets abused.

The Number That Actually Matters

Change crank length and your fit changes, immediately and by exactly the amount you changed.

At the bottom of the pedal stroke, the pedal sits one crank length below the bottom bracket. Shorten the crank by 5 mm and the pedal is 5 mm higher at that point, so your leg no longer extends as far:

Saddle change = old crank − new crank

Shorter cranks: raise the saddle by the difference. Longer cranks: lower it. This is geometry, not inference, and it is the one number on this page with no uncertainty attached.

What this does not fully capture: the top of the stroke. A shorter crank also brings the pedal down at top dead centre, which opens the hip and knee angle at the most closed point of the circle. That is not a number you can read off a formula, and it is the real reason to consider shorter cranks — if your hip closes up in an aggressive position, or your knee flexion is uncomfortable at the top, a shorter crank buys you range of motion that no amount of saddle adjustment will.

Which makes crank length a fit question, not a power question. The power argument, on the evidence above, barely exists.

How to Measure Your Inseam

The input this page wants is not your trouser size.

Barefoot, back against a wall, feet about 15 cm apart. Pull a hardback book up firmly between your legs, spine horizontal, until it is snug against your pubic bone — firmly is the operative word, and it will be uncomfortable. Have someone measure from the floor to the top edge of the book.

If you also want the tibia figure: seated with the knee at 90°, measure from the floor to the centre of the knee joint on the outside of the leg.

What to Actually Do

A short decision list, in order of how much the evidence supports it:

1. If your current cranks are inside the suggested range, change nothing. There is no finding here that justifies a purchase. 2. If you have a fit problem at the top of the stroke — hip impingement in the drops, discomfort in an aggressive time-trial position, difficulty getting low — shorter cranks are a real tool, and this is the strongest case for changing them. Shorter is what almost everyone moves towards when they move at all. 3. If you are unusually short or tall, the standard 170/172.5 mm sizing is less likely to suit you, because it was never scaled to you in the first place. The 20% figure is a reasonable starting point. 4. If you change them, move the saddle. Every time. The number is above. 5. Do not expect watts. The entire 120–220 mm range moved maximum power by under 4% in trained cyclists, and a 5 mm change is inside the measurement noise.

Where This Sits Against the Other Tools Here

If you are chasing watts, the gains are elsewhere by an order of magnitude. The CdA calculator covers aerodynamics, where a position change worth 0.01 m² is worth about 10 W at time-trial speed — more than crank length can offer across its entire plausible range. The gear ratio calculator covers the drivetrain side of cadence, which is the variable the crank-length research actually implicates.