How to Measure Bike Crank Length | Center-to-Center Method

Bike crank length is the distance from the bottom-bracket center to the pedal-axle center, usually stamped on the arm’s backside.

Getting this measurement right matters more than most riders realize. Crank length changes your knee angle, hip position, and even how much clearance you have over obstacles. The good news? You can verify it in about thirty seconds, whether you’re shopping for replacements or just checking your current setup.

The key is knowing where to measure — and avoiding the mistakes that lead to wrong numbers.

What Exactly Is Crank Length?

Crank length is the distance from the center of the bottom-bracket axle to the center of the pedal axle, measured along the effective line of the arm. It is not the physical outside edge of the arm, which is a common point of confusion.

Most modern cranks have the length stamped or engraved on the backside of the arm, near the pedal hole. You’ll typically see numbers like 165, 170, 172.5, or 175. The 170 mm size is widely considered the most common, and 2.5 mm increments are standard across major manufacturers.

While some references cite a broader 160 mm to 185 mm range, production cranks cluster tightly between 165 mm and 175 mm. One caveat: some older British cranks use inch measurements, so double-check the unit before you assume.

How To Measure Crank Length In 3 Steps

If your crank arm has no visible marking, the measurement takes two minutes with a metric ruler.

1. Position the crank arm horizontal — parallel to the ground, pointing forward or backward. This gives you straight-line access to both axle centers.
2. Find the bottom-bracket center — the middle of the spindle where the crank arm attaches to the frame. Place your ruler’s zero mark there.
3. Measure to the pedal-axle center — the middle of the pedal spindle at the far end of the arm. Read the value in millimeters.

That center-to-center distance is your crank length. Write it down before you change any components, so you can match or intentionally adjust your setup later.

What success looks like: a reading between 160 mm and 180 mm for most adult bikes, with 170 mm being the most frequent result. If the crank is unmarked and your measurement lands in that zone, you’re done.

Three Mistakes That Skew Your Reading

Even experienced riders get this wrong. The most common error is measuring the outside edge of the arm instead of center-to-center between the two axle centers. That extra metal on the pedal end adds a few millimeters to your result.

The second mistake is assuming the printed number represents the arm’s total physical length. It doesn’t — it represents only the distance between the two axle centers. The arm itself is slightly longer.

Third, don’t assume longer is better. Multiple fit guides note that optimal crank length depends on the rider, not on maximizing the number. A longer crank changes your knee bend at the top of the pedal stroke and your hip angle, which can affect comfort and injury risk on an individual basis.

Some sizing charts use rider height or inseam formulas to estimate ideal crank length. These are starting points, not official standards — nothing in the gathered sources dictates one universal crank length for all riders. BikeRadar’s crank-length guidance covers how fit interacts with frame geometry and clearance.

When Crank Length Actually Matters

Crank length doesn’t exist in isolation. It interacts with saddle height, frame geometry, and your own inseam. Off-road riders sometimes prefer slightly shorter cranks than road riders to preserve pedal clearance over obstacles, but that’s a fit preference, not a rule.

If you’re changing cranks for fit reasons, keep a few things in mind:

• Knee angle — shorter cranks reduce knee bend at top dead center; longer ones increase it.
• Toe overlap — longer cranks can bring your front wheel closer to your toes on small frames.
• Hip angle — extreme changes in length can affect your position over the saddle.

If you’re replacing cranks entirely — especially on a BMX or kids’ bike where the arms are often one piece — checking the current length first saves you from buying the wrong size. Our tested roundup of the best BMX cranks breaks down which lengths suit different riding styles.

One honest note: if you’re measuring to match an existing bike, the stamped number is more reliable than any manual measurement. Your ruler work is the fallback, not the primary source. For most riders, the printed number on the arm’s backside settles the question in seconds.