Two-Pulley Belt Length Calculator

About this calculator

Sizing a flat or V-belt for a two-pulley drive means finding the belt length that lets it wrap both pulleys and run taut across the center distance between them, without being so tight it strains the bearings or so loose it slips. The standard formula is L = 2C + π(D + d)/2 + (D − d)²/(4C), where C is the center distance, D is the large pulley diameter, and d is the small pulley diameter — the π(D+d)/2 term accounts for the belt wrapped around both pulleys, and the last term corrects for the extra length needed when the pulleys are different sizes.

Mechanical designers and machinists use this formula (or its close variants) when specifying a belt for conveyors, compressors, motors, and other belt-driven machinery, since ordering the wrong belt length means either a drive that can’t be tensioned properly or one that won’t fit the pulleys at all. This calculator takes your center distance and both pulley diameters to compute the required belt length directly.