Pulley & Belt Calculator
Driven speed, ratio and belt length for a two-pulley drive.
How it works
Belt drives obey D1 x N1 = D2 x N2 — a large pulley driven by a small one turns slower, in proportion to the diameters.
Belt length for an open drive is approximately
L = 2C + π(D1 + D2)/2 + (D2 - D1)² / (4C)
Wrap angle matters for grip. Below about 120 degrees on the smaller pulley a flat or V-belt starts slipping under load, which is what an idler pulley is there to fix.