Crank length is the distance from the bottom bracket spindle’s center to the pedal spindle’s center, and it controls your pedaling circle’s size.
Whether you’re chasing a faster cadence or trying to ease knee strain, understanding crank length matters more than most riders think. Your crank arms act as levers — and their length directly shapes how your legs move through every rotation. Getting it right starts with knowing what the number means, how to find it, and which measurement suits your body and riding style.
How Crank Length Works and Why It Matters
Crank length is the lever arm your legs push against, and it’s typically stamped in millimeters on the inside of the arm near the pedal hole. Typical adult lengths range from about 165 mm to 175 mm, with 172.5 mm being common on mid-sized road bikes and 170 mm or 175 mm typical on mountain bikes.
Shorter cranks reduce the size of your pedaling circle, which lets you spin a higher cadence more easily. Longer cranks increase leverage, giving you more torque for climbing but requiring more hip and knee flexion — that’s why your body proportions and riding discipline matter. The difference between sizes might feel small on paper, but experienced riders notice it quickly.
One detail worth knowing: some bikes ship with mismatched crank arms. Check both sides if your bike’s had repairs or part swaps, since the lengths should always match.
The Standard Sizing Chart for Crank Length
While one-size-fits-all doesn’t exist for crank length, sizing charts help narrow your options. The most common approach starts with your inseam — the distance from your crotch to the floor in bare feet. Here’s a widely published starting point:
| Inseam | Suggested Crank Length |
|---|---|
| Under 75 cm | 150 mm or less |
| 75–79 cm | 155 mm |
| 79–82 cm | 160 mm |
| 82–85 cm | 165 mm |
| 85–89 cm | 170 mm |
| 89–94 cm | 175 mm |
| Over 95 cm | 180 mm if preferred |
Another chart suggests base lengths starting slightly differently: under 72 cm inseam gets 160 mm, moving up through 165 mm, 170 mm, 172.5 mm, 175 mm, 177.5 mm, and 180 mm for riders over 96 cm. The variation between charts is normal — it reflects different rider populations and fit philosophies.
For riders with inseams over 96 cm, a second chart suggests 180 mm as the base, while the first chart mentions 180 mm only “if preferred.” Both are reasonable starting points; professional bike fit remains the gold standard. This isn’t a settle-it-once-and-done number — many riders deliberately choose different lengths for road, mountain, or track riding.
How to Measure or Find Your Crank Length
Finding your current crank length takes just a minute. Start by looking for the stamped or printed number on the inside of the crank arm near the pedal hole — the most common location is right where the arm meets the pedal. If nothing’s marked, measure from the center of the bottom bracket spindle to the center of the pedal spindle using a tape measure.
BMX riders, for instance, use crank lengths from 160 mm to 180 mm depending on whether they’re racing or riding freestyle. If you’re shopping, our tested roundup of top-rated 165mm bike cranks covers solid options for riders who land in the shorter range.
The practical reality is that most adult riders fall into the 165–175 mm band, and moving within that range is more about feel than right-or-wrong. Try your current length first, then test a size up or down on a trainer or borrow a bike — you’ll know within a few rides whether your knees and cadence prefer a change.
FAQ
Does crank length affect knee pain?
Yes, it can. Longer cranks require more hip and knee flexion for every pedal stroke, which some riders find aggravates knee discomfort. Shorter cranks reduce that range of motion, which is why many riders with sensitivity prefer 165 mm or 170 mm options. But crank length is one factor — saddle height and cleat position matter equally.
What’s the most common crank length for adults?
170 mm and 172.5 mm are the most common lengths for adult riders. The 172.5 mm size shows up frequently on mid-sized road bikes, while 170 mm is a staple on mountain bikes. Riders under roughly 5’6″ often prefer 165 mm, and taller riders in the 6’0″ range usually ride 175 mm.
Can I ride different crank lengths on the same bike?
You can, but the handling will feel slightly off, and the difference is rarely intentional. Some bikes do ship with mismatched crank arms after poor repairs or part swaps. Since crank length affects how your legs move through the pedal circle, identical lengths on both sides is the safe choice.
References & Sources
- BikeRadar. “Cranksets — Everything You Need To Know.” Covers typical adult crank lengths, common sizes, and measurement basics.