How do you choose the right crank length?

Cyclists pay a lot of attention to frame size and saddle height, but one component that’s often overlooked is the crank. Yet this part has a huge impact on comfort and performance. We explain exactly what a crank is, why its length is crucial, and how to determine the ideal size for your body type and riding style.

Written by: Dieter Devriendt | May 29, 2026 | Reading time: 6minutes

Learn more about the author: Dieter Devriendt

Journalist Dieter writes passionately and expertly about the world of cycling. Under the motto "we write – we ride," he gladly shares his experience with cyclists to make their lives easier, both on and off the saddle.

Key findings

The crank converts the force from your legs into a rotational motion that propels the bike forward.
The right crank length has a significant impact on performance, cadence, comfort, and the strain placed on the knees and hips.
Current research shows that shorter cranks are often more comfortable and can help prevent injuries in many cyclists.

What is a crank, and what is it used for?

The crank—also known as the “crank arm”—is the component that connects the bottom bracket (the pivot point in the frame) to the pedal. Every bicycle has two cranks: a left and a right one, positioned 180 degrees apart. The right crank is usually connected to the chainrings (the gears located at the front). Together with the bottom bracket and the chainrings, this assembly forms what is known as the “crankset.”

The crank’s function is purely mechanical and fundamental to cycling: it converts the linear up-and-down motion of the legs into a rotational motion that drives the chain. You can think of the crank as a lever. When you push down on the pedal, the crank arm rotates the bottom bracket. Without this component, it would be impossible to efficiently convert the muscle power of your thighs and calves into pure speed on the road or in the woods.

The Importance of Crank Length

The length of a crank is measured in millimeters, from the exact center of the bottom bracket axis to the exact center of the pedal axle. Most standard bikes are equipped with 170 mm, 172.5 mm, or 175 mm cranks. Although a difference of 2.5 or 5 millimeters may seem minor, it has a huge impact on the body’s biomechanics while pedaling.

The impact on performance and pace

The length of the crank determines the diameter of the circle your feet trace when you pedal. A shorter crank (for example, 160 mm to 167.5 mm) creates a smaller circle. This means your feet cover a shorter distance with each revolution. This makes it easier to maintain a high cadence.

Since the lever is shorter, you do have to apply a little more force to generate the same torque, but this is offset by a higher cadence. A shorter crank also creates a more “open” hip angle at the top of the pedal stroke (when the knee is highest). This improves aerodynamics and breathing, especially on time trial and triathlon bikes.

Longer cranks (e.g., 175 mm to 180 mm) increase the crank arm length. This provides greater leverage, allowing you to generate more torque with less raw muscle power at low speeds. This is useful on very steep climbs or when accelerating from a standstill (such as in BMX or MTB). The downside is that your feet have to cover a greater distance, making it harder to maintain a very high cadence. There is also more stress on your joints, as your knees and hips have to bend further.

Comfort and injury prevention

The wrong crank length is a common cause of injury. If your cranks are too long relative to the length of your legs, your knee will be too close to your chest at the top of the pedal stroke. This causes excessive bending of the knee and hip under heavy load. This can lead to stress on the kneecap, lower back pain, and tightness in the hip flexors.

Cranks that are too short rarely cause injuries, but they can make you feel like you’re losing power or cause an unusually jerky pedaling motion.

Step-by-Step Guide: How to Determine the Right Crank Length

Choosing the right crank length isn’t an exact science, as it depends on your body type, flexibility, and cycling discipline. However, with the guide below, you’ll be able to determine exactly which length is best for you.

Step 1: Measure your inseam

Your overall height says surprisingly little about the ideal crank length; what really matters is the length of your legs.

Stand with your back against a wall, without shoes, with your feet about 15 centimeters apart. Place a level or a thick book upright between your legs and lift it slightly (as if you were sitting on a saddle). Ask someone to measure the distance between the top of the level/book and the floor. This measurement in centimeters is your inseam length.

Step 2: Use traditional rules as a starting point

Next, you can use your inseam measurement in a general reference chart.

If your inseam is less than 75 cm, a crank length of 165 mm or less will work.
Between 75 and 80 cm: 167.5 mm crank.
Between 80 and 85 cm: 170 mm crank.
Between 85 and 90 cm: 172.5 mm crank.
Over 90 cm: a crank of 175 mm or longer.

Step 3: Adapt to your discipline and riding style

Modern knowledge of biomechanics (particularly from professional bike fitters) goes beyond just leg length. Your discipline plays a major role.

A road cyclist who specializes in climbing and prefers a smooth ride will likely choose the recommended size, or even a slightly shorter one, to facilitate a high cadence.

Time trialists and triathletes deliberately choose cranks that are 2.5 to 5 mm shorter than those on their standard road bikes. Because their riding position is highly aerodynamic, a shorter crank prevents the abdomen and rib cage from being compressed as the knee rises. This also preserves the muscles used for running.

Mountain bikers and gravel riders often need significant power at low speeds to clear obstacles or climb steep hills. A standard crankset or one that’s slightly longer (such as 175 mm) can be advantageous in this case, provided you’re careful not to hit the ground or rocks with your pedals.

Step 4: Assess your flexibility

Do you quickly experience lower back pain, stiff hips, or knee pain? If so, it’s almost always advisable to choose a shorter crank. A shorter crank reduces the range of motion in your joints. This means you need less flexibility to achieve a smooth, pain-free pedaling motion.

Conclusion

Crank length is a small detail with major implications. While in the past, standard cranks of 172.5 mm or 175 mm were almost always the norm, modern scientific research shows that many cyclists benefit from shorter cranks. They optimize hip angle, increase cadence, and reduce stress on sensitive joints such as the knees and lower back.

Use your inseam length as a starting point, but always take your flexibility and riding style into account. Can’t decide between two sizes? In today’s cycling world, the general rule is often this: when in doubt, shorter is usually safer and more comfortable.

For the perfect fit, a dynamic bike fitting with a specialist is still the best investment you can make.

FAQ

What is the purpose of a crank on a bicycle?

The crank connects the pedal to the bottom bracket and converts the force from your legs into a rotational motion that drives the chain.