Belt length calculator
Calculate the required length of an open belt running on two pulleys, from the pulley diameters and the center distance.
How it is calculated
L = 2C + π(D + d) ÷ 2 + (D − d)² ÷ (4C), with D and d the pulley pitch diameters and C the center distance, all in the same unit.
Worked example
D = 200 mm, d = 100 mm, C = 400 mm: L = 800 + 471.24 + 6.25 = 1277.49 mm.
Steps
- Enter the pitch diameters of both pulleys.
- Enter the center distance between shaft centers.
- Read the length and choose the nearest standard belt length.
Frequently asked questions
Is this for V-belts or timing belts?
It is the open-belt approximation. Use the pitch diameter, not the outside diameter, and check the belt maker for the standard length.
Which length should I buy?
Round to the nearest standard belt length, then adjust the center distance to take up the difference.
What about crossed belts?
Crossed belts use a different formula. This calculator covers open belts only.
Related calculators
References
Open-belt length formula from the standard belt drive geometry (Shigley, Mechanical Engineering Design). Check the pitch length of the chosen belt section.
Estimates for information only. Verify critical values with a qualified engineer or your supplier standards.