Start with the drivetrain instructions
The chain-length calculator gives a starting estimate from chainstay length and sprocket sizes. The final fit depends on the derailleur, suspension movement, axle adjustment and connecting link. Check your drivetrain maker's sizing procedure before removing links.
Measure and count consistently
- Measure chainstay length from the center of the crank axle to the center of the rear axle.
- For a derailleur estimate, enter the largest chainring and largest rear sprocket.
- One pitch is half an inch, or 12.7 mm, between adjacent pins. An inner-and-outer pair spans one inch. A 110-pitch chain measures 55 inches.
- Include the connecting link in the final pitch count. Use a connector compatible with the chain.
A calculator example
With a 16 inch chainstay, a 50 tooth chainring and a 34 tooth rear sprocket, the common derailleur estimate is 2 × 16 + 50 ÷ 4 + 34 ÷ 4 + 1 = 54 inches. That is 108 half-inch pitches. This estimates the starting length; it does not replace checking the installed fit.
Derailleur fit
A chain that is too short can overextend the rear derailleur in the largest gear combination. Excess length can leave slack in small gear combinations. Full-suspension frames may need extra length at the point where the rear axle is farthest from the crank. Modern drivetrain procedures can specify different allowances, so use the manufacturer's method for the final cut.
Single-speed and BMX fit
A single-speed chain must suit the available axle adjustment. After changing a sprocket, check tension through a full crank revolution and look for the tightest point. A half-link changes the adjustment options, but its suitability depends on the chain and drivetrain. The BMX gearing calculator helps compare ratios before changing tooth counts.
Replacing an existing chain
Check wear before copying the old chain's length. Count its pitches rather than matching stretched end-to-end lengths. Use the chain-wear checker to interpret a ruler measurement, then verify the new chain's fit on the bicycle.
Reference: Park Tool chain length sizing.