Measure from the center of the crank axle to the center of the pedal hole

Crank length is the distance from the middle of the axle (where the crank attaches to the bike frame) to the middle of the pedal hole. You measure in a straight line, not along the curve of the crank arm itself. Most bikes come with cranks between 165 mm and 180 mm long, and getting the right length matters for comfort, power transfer, and knee health.

The easiest way to find your crank length is to look at the crank arm itself. Most manufacturers stamp the measurement directly onto the metal — usually on the inside of the arm where it faces the frame. If the number is there, you have your answer. If not, you will need to measure it yourself.

Key Takeaways

  • Crank length is stamped on the inside of most crank arms; check there first before measuring.
  • If no marking exists, measure from the center of the axle bolt to the center of the pedal hole using a ruler or caliper.
  • Standard sizes run from 165 mm to 180 mm, with 170 mm and 175 mm being the most common.
  • Crank length affects how your legs move through each pedal stroke and can cause knee pain if mismatched to your body.

Finding the stamped measurement on your crank

Remove the pedal from the crank arm you want to measure. You do not need to remove the crank from the bike. Look at the inside face of the crank arm — the side facing toward the frame — and search for a number followed by "mm". It may be small and faint, especially on older bikes or after years of dirt and grime.

If the marking is hard to read, clean the crank arm with a damp cloth and look again. The number will usually appear near the middle of the arm or closer to the axle end. Once you find it, that is your crank length. Write it down so you have it for future reference.

Measuring the crank yourself with basic tools

If there is no marking, you can measure the crank with a ruler, measuring tape, or digital caliper. A caliper is most accurate, but a ruler works fine for this job. You need to find two points: the center of the axle bolt and the center of the pedal hole.

Start by locating the axle bolt — the bolt that holds the crank to the bike frame. It sits at the center of the crank arm where it connects to the frame. Mark or note this spot. Then find the pedal hole at the other end of the crank arm. Measure the straight-line distance between the center of the axle bolt and the center of the pedal hole. That distance is your crank length.

If you are using a ruler, lay it along the crank arm from the axle to the pedal hole and read the measurement in millimeters. If you are using a caliper, place one end at the center of the axle bolt and the other at the center of the pedal hole, then read the display. Round to the nearest 5 mm — cranks are manufactured in 5 mm increments, so your measurement will be something like 165, 170, 175, or 180 mm.

Why crank length matters for your riding

Crank length affects your leg extension and the arc your legs travel through with each pedal stroke. A longer crank gives you more leverage and can feel smoother on flat terrain, but it also means your legs extend further and move through a larger circle. A shorter crank lets your legs move faster and can feel more natural if you have shorter legs or a shorter torso.

Using the wrong crank length can cause knee pain, especially over long rides. If your cranks are too long, your knees may overextend at the bottom of the pedal stroke. If they are too short, your legs may not reach full extension and you may feel like you are spinning without power. Most riders find their best crank length falls within a range based on their inseam measurement, though personal preference plays a role too.

Standard crank sizes and what fits most riders

Crank lengths come in 5 mm increments. The most common sizes are 170 mm and 175 mm, which fit the majority of adult riders. Smaller riders or those with shorter legs often use 165 mm or 167.5 mm cranks. Taller riders or those with longer legs may prefer 177.5 mm or 180 mm cranks. Some specialty bikes, especially children's bikes or compact road bikes, may use 155 mm or 160 mm cranks.

Your inseam measurement is a rough guide. Riders with an inseam under 28 inches often do well with 165 mm or 167.5 mm cranks. Those with an inseam between 28 and 32 inches typically fit 170 mm or 175 mm cranks. Riders with an inseam over 32 inches may prefer 177.5 mm or 180 mm cranks. These are starting points, not rules — comfort and how the bike feels matter more than a formula.

What to do if you need to change your crank length

If your current cranks feel wrong — if your knees hurt, your legs feel cramped, or you feel like you are spinning without power — you may need different cranks. Changing cranks is a straightforward job if you have the right tools: a crank puller (a specialized tool that costs $15 to $40) and a wrench. Many bike shops will change cranks for you for a small labor fee if you provide the new cranks.

When you buy replacement cranks, make sure they match your bike's bottom bracket type. Cranks attach to the frame through the bottom bracket, and different bikes use different bracket standards. Your current crank will tell you what type you have — take a photo of it or bring the bike to the shop so the staff can match it correctly. Buying the wrong type will not work, so this step is important.

Frequently Asked Questions

Can I measure crank length without removing the pedal?

Yes. The pedal does not affect the measurement — you are measuring from the axle to the pedal hole, not to the pedal itself. Removing the pedal just makes it easier to see the hole clearly and measure to its center. If the pedal is in the way, you can measure around it.

What if my two cranks are different lengths?

Cranks should always be the same length on both sides. If they are not, one crank may be damaged or bent, or the bike may have been assembled incorrectly. Check both measurements to be sure. If they truly differ, the shorter one is the problem and should be replaced to avoid uneven pedaling and potential knee injury.

Does crank length affect how fast I can go?

Crank length affects how your legs move, not your top speed directly. Longer cranks give more leverage, which can help on climbs or when accelerating from a stop. Shorter cranks let your legs spin faster, which some riders prefer on flat ground. The best crank length is the one that feels natural and does not cause pain.

Is 170 mm the right size for everyone?

No. While 170 mm and 175 mm are the most common sizes, the right length depends on your inseam, leg length, and personal preference. Riders with shorter legs often need 165 mm or 167.5 mm cranks. Taller riders may need 177.5 mm or 180 mm. Start with your inseam as a guide, then adjust based on how the bike feels.