Pulley and Belt Length Calculator

Find the belt length for two pulleys, or the center distance for a belt you have, along with the driven speed and belt speed.

What Do You Want to Find?
Pitch diameter, on the motor.
Belt Layout
Fractions and decimals both work.
–
Inch Belt
Belt length
Center distance
Driven speed
Speed ratio
Belt speed
Wrap on small pulley
Direction

Saved Drives

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DrivePulleysCenterBeltDriven RPMRemove

How to Calculate Belt Length and Pulley Speed

The belt wraps half of each pulley and spans the gap between them twice, plus a correction for the pulleys being different sizes.

Open belt: L = 2C + π(D + d) ÷ 2 + (D − d)² ÷ 4C
Crossed belt: L = 2C + π(D + d) ÷ 2 + (D + d)² ÷ 4C
Driven RPM = driver RPM × driver diameter ÷ driven diameter   Belt speed (ft/min) = π × D × RPM ÷ 12

To find the center distance for a belt you already have (open belt): let b = 4L − 2π(D + d); then C = [b + √(b² − 32(D − d)²)] ÷ 16.

Worked Example

Tips

Use pitch diameters for V-belts; the pulley's outside diameter is larger. Belt makers list pitch length, and the number stamped on a belt may be an outside or inside length depending on the section, so match against the maker's chart. Leave room to adjust the center distance for installing and tensioning the belt. Wrap on the small pulley under about 120° tends to slip.