Work out belt length instantly with clear inputs, formula shown and shareable results.
Open belt length is 2C + (pi/2)(D + d) + (D - d)² / (4C): twice the centre distance for the straight spans, half a circumference of each pulley, and a small correction because the belt leaves the pulleys at an angle rather than tangentially opposite. Wrap angles follow from the same geometry and govern how much torque the belt can transmit before slipping.
Belt length
L = 2C + (pi/2)(D + d) + (D - d)² / (4C)
Wrap angles
θ = 180 ± 2 arcsin[(D - d) / 2C] degrees
L = 2C + (pi/2)(D + d) + (D - d)² / (4C). Open belt length is 2C + (pi/2)(D + d) + (D - d)² / (4C): twice the centre distance for the straight spans, half a circumference of each pulley, and a small correction because the belt leaves the pulleys at an angle rather than tangentially opposite.
The small one, because it has the least wrap angle and therefore the least friction available. Drive capacity is always calculated on the small pulley.
This calculator takes 3 inputs: Large pulley pitch diameter, Small pulley pitch diameter, Centre distance. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.