Crank arm length is measured from the center of the bottom bracket axle to the center of the pedal spindle hole, and the right length for you depends primarily on your leg length and riding style. Most bikes ship with 170mm, 172.5mm, or 175mm cranks based on frame size alone, which means many riders end up on a default length that may not actually suit their body. Choosing well comes down to a few measurable factors and knowing what signs to watch for.
How to Measure Your Current Cranks
Before changing anything, check what you already have. Look for a stamping on the inside or back of your crank arm, usually near the pedal hole. Shimano, SRAM, and Campagnolo all print the length in millimeters (e.g., “172.5” or “FC-R7000 170”). If the marking has worn off, measure from the exact center of the bottom bracket bolt to the center of the pedal spindle hole using a ruler or tape measure. That distance, in millimeters, is your crank length.
Using Height and Inseam to Pick a Length
The simplest starting point is the rule that crank length should be roughly 9.7% of your height. A rider who is 5’10” (178 cm) lands at 172.5mm, which happens to be the most common stock size. But height alone assumes average proportions, and legs vary. A more precise approach uses inseam length, or better yet, inseam plus what bike fitters call “greater trochanter height,” the distance from the bony knob at the top of your thigh bone to the floor.
Here’s a sizing reference that accounts for both measurements:
- Up to 165 cm tall / inseam under 77 cm: 162.5mm
- 165–170 cm / inseam 77–79 cm: 165mm
- 170–173 cm / inseam 79–81 cm: 167.5mm
- 173–179 cm / inseam 81–83.5 cm: 170mm
- 179–185.5 cm / inseam 83.5–86 cm: 172.5mm
- 185.5–192 cm / inseam 86–89.5 cm: 175mm
- Above 192 cm / inseam above 90 cm: 177.5mm
These ranges assume average leg-to-torso ratios. If you have proportionally long legs for your height, you may fit one size up. Proportionally short legs, one size down. Measuring your inseam (stand barefoot against a wall, place a book snug into your crotch, and mark the wall at the top of the book) gives you a better starting number than height alone.
What the Research Says About Power and Efficiency
Riders often assume longer cranks produce more power because of greater leverage. The reality is less dramatic. A 2025 study comparing 165mm, 170mm, and 175mm cranks in high-level amateur road cyclists found no significant differences in sprint power, cycling efficiency, heart rate, or perceived effort across those three lengths. Small adjustments in crank length, in other words, don’t meaningfully change how much power you produce or how much energy you burn at a given pace.
That finding is consistent across most of the literature: within the standard 165–175mm range, performance differences are minimal for trained riders at submaximal intensities. There is some evidence that longer cranks can boost peak power for professional cyclists under specific conditions, and separate evidence that novice cyclists actually improve their economy on shorter cranks, possibly because shorter cranks allow slower muscle contraction speeds and more extended joint positions. But for most recreational and competitive riders choosing between stock sizes, raw power output shouldn’t be the deciding factor. Comfort, joint health, and riding position matter more.
How Crank Length Affects Your Joints
Every millimeter of crank length changes how far your knee and hip bend during each pedal revolution. Research from Cal Poly measured this directly: riders on 150mm cranks had a knee range of motion of 67 degrees per stroke, while riders on 180mm cranks moved through 75 degrees. Hip range of motion jumped from 45 degrees to 51 degrees over that same span. That might sound small, but consider that you repeat the pedal stroke roughly 5,000 times per hour at a moderate cadence. Even a few extra degrees of flexion, repeated thousands of times, can stress tissues that are already near their limit.
When cranks are too long for your body, the top of the pedal stroke forces your knee and hip into deeper flexion than they handle well under load. The result is that pedaling forces shift away from your muscles and into your joints, tendons, and ligaments. Over time, this can compress blood supply and impinge nerves. The pedal stroke may feel “lumpy” rather than smooth, with a sensation that your power phase ends too quickly, as if you never quite get a full push before the crank comes back around.
Signs Your Cranks Are Too Long
A crank that’s too long causes more problems than one that’s too short. A short crank might cost you a marginal amount of performance, but it’s unlikely to injure you. A long crank is a common source of low-grade, chronic pain that sends riders to physical therapists without lasting relief, because the root cause keeps spinning beneath them.
Watch for these patterns:
- Hip pain or tightness, often on one side only. Hip impingement at the top of the pedal stroke is one of the most common symptoms.
- Recurring knee pain that persists even after you’ve checked saddle height, cleat position, and other fit variables.
- Lower back pain or saddle discomfort that worsens on longer rides.
- Climbing feels unreasonably hard. Seated efforts on hills or during intervals expose the problem because they demand high force through an oversized pedal circle.
- Quad dominance, where your thighs fatigue disproportionately while your glutes and hamstrings seem to contribute nothing.
- Bodywork doesn’t stick. Massage, chiropractic adjustments, or physical therapy provides temporary relief but the pain returns as soon as you ride again.
If several of these apply, and other bike fit factors have been ruled out, crank length is worth investigating. Low-cadence riders tend to notice the problem more, since they push harder per stroke and spend more time in deep flexion.
Shorter Cranks for Aero Positions
Triathletes and time trialists have increasingly moved to shorter cranks, and the reason is straightforward. In an aggressive aero position, your torso is tilted far forward, which closes the angle at your hip every time the pedal comes to the top of the stroke. Shorter cranks reduce the diameter of the pedal circle, which keeps the hip more open at that critical point. The payoff isn’t necessarily more watts. It’s the ability to breathe more easily, stay comfortable, and hold the aero position longer without hip or lower-back strain.
A 2021 study comparing very short 145mm cranks to traditional 175mm cranks found that novice riders at moderate intensity actually produced more power per unit of oxygen consumed on the shorter cranks. The researchers attributed this to slower pedal speeds and more extended joint positions. For triathletes, the practical takeaway is that dropping 5 to 10mm in crank length often lets you maintain a lower, more aerodynamic posture over the full duration of a race, which saves far more time than any marginal power difference measured on a test rig.
How to Choose in Practice
Start with the inseam-based chart above to identify your ballpark range. If you’re between sizes, lean shorter rather than longer. The risk profile is asymmetric: too short is a minor performance trade-off, too long is a potential injury source.
Next, factor in your riding style. Road racers who spend most of their time on the hoods in a moderate position can generally stick with the chart recommendation. Triathletes and time trialists in deep aero should consider going 2.5 to 5mm shorter than the chart suggests. Mountain bikers sometimes prefer slightly shorter cranks for ground clearance on technical terrain, since a smaller pedal circle reduces the chance of striking rocks at the bottom of the stroke.
If you’re experiencing any of the pain patterns described above, try borrowing or buying a set of cranks one size shorter (2.5mm less) before committing to a full bike refit. Many riders report immediate relief from hip impingement simply by dropping from 172.5mm to 170mm. Give the new length at least a few weeks of riding before judging it, since your muscle recruitment patterns need time to adapt. You may need to raise your saddle by the same amount you shortened your cranks to maintain the same leg extension at the bottom of the stroke.
For riders who want precision beyond a chart, a professional bike fit that includes joint-angle measurement with motion capture is the most reliable way to confirm the right length. Fitters can watch your hip and knee angles in real time and identify whether your current cranks are forcing you into ranges that cause trouble.