Crank Length Calculator
Switching crank length moves three things at once. Raise your saddle by the same amount you shorten the crank to keep knee extension. Going from a 172.5 mm to a 165 mm crank raises the saddle 7.5 mm, makes the gears feel about 4.5% harder, and opens your hip angle 7.5 mm for a more aero, back-friendly position.
Enter your current crank, your target crank, and your saddle height to get the exact new saddle height, the gearing change to expect, and the hip-opening benefit. We call the output your Crank Fit Plan.
Enter your current and target crank lengths plus your saddle height and hit calculate. Your new saddle height, gearing change, and hip-opening gain appear here.
Gear up
Short-crank swap gear
Changing crank length is a low-torque job with tight specs. Measure twice and torque to spec. Test any new fit in training before race day.
- Short crankset (Shimano 165 mm)A drop-in 165 mm option to open the hip angle without changing your whole groupset.View on Amazon →
- Short crankset (SRAM 165 mm)SRAM short-crank option for riders moving to a more aero, lower-back-friendly fit.View on Amazon →
- Precision bike torque wrench (2 to 14 Nm)Crank bolts and stem clamps have low torque specs. A click wrench prevents over-tightening.View on Amazon →
- Saddle height and setback measuring toolReproduce your new saddle height and fore-aft position exactly after the swap.View on Amazon →
- Bike fit calipersMeasure crank arm length and small saddle moves to the millimeter.View on Amazon →
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Crank length calculator FAQ
Do I need to raise my saddle with shorter cranks?
Yes. A shorter crank means the pedal does not travel as far down at the bottom of the stroke, so your knee would over-bend if the saddle stayed put. Raise the saddle by the same amount you shortened the crank to keep the same knee extension. Going from a 172.5 mm crank to a 165 mm crank shortens the arm by 7.5 mm, so you raise the saddle 7.5 mm. This calculator does that math for you and gives you the exact new saddle height.
How much should I raise my saddle going from 172.5 to 165 cranks?
Raise it 7.5 mm. The crank arm got 7.5 mm shorter, so the pedal sits 7.5 mm higher at the bottom of the stroke. Moving the saddle up by the same 7.5 mm restores your original knee extension and keeps your power and comfort consistent. If your saddle was at 740 mm, set it to about 747.5 mm, then fine-tune on the bike. Recheck fore-aft setback too, since a small saddle rise also moves you slightly back.
Do shorter cranks make gears feel harder?
Slightly. The crank arm is a lever, so a shorter arm gives a little less mechanical advantage at the same gear. Going from 172.5 mm to 165 mm is about a 4.5% loss of leverage, so the same gear feels a touch harder, mostly on steep climbs. The fix is simple: shift down one sprocket when it bites, or fit a marginally easier cassette or chainring. Many riders make up the difference with a slightly higher cadence and never notice it on flat or rolling terrain.
What are the benefits of short cranks?
Shorter cranks open the hip angle at the top of the pedal stroke. That lets you hold a lower, more aero position without your thigh crowding your torso, eases strain on the lower back, and frees the diaphragm so breathing is easier at the top of the stroke. They also reduce knee flexion at the top, which many riders with hip or knee restriction find more comfortable. The trade is a small loss of leverage, easily handled with gearing, and most riders adapt within a few rides.
Will shorter cranks change my power?
For most riders, no meaningful loss. Research on crank length over the common 165 to 175 mm range shows power output is largely unaffected once you adjust saddle height and let cadence settle. You are trading a slightly longer lever for a slightly faster spin, and the two roughly cancel. The bigger wins from short cranks are positional: a more sustainable aero posture and less hip and back strain, which often let you produce that same power for longer.
How do I pick a crank length from my inseam?
A common starting guideline is about 20% of your inseam in millimeters, rounded to the nearest manufactured length. An 800 mm inseam points to roughly a 160 mm crank by that rule. Treat it as a reference, not a verdict: fit goals matter more than the formula. Many fitters now run shorter than the old inseam-based numbers to open the hip angle. Enter your inseam in the optional field and the calculator shows this reference alongside your conversion.
See also: do shorter cranks actually make you faster, the fit and power evidence behind the switch.
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