Belt length calculator

Belt length, ratio, driven speed and the wrap angle that decides whether it slips.

The formula

L = 2C + π(D₁ + D₂) ÷ 2 + (D₁ − D₂)² ÷ 4C

This is the open belt case, where both pulleys turn the same way. Speed ratio is simply D₁ ÷ D₂, so a 6" driver on a 3" driven doubles the speed.

Wrap angle is the number that matters

Belt length tells you what to buy. Wrap angle tells you whether the drive will actually work. A V-belt transmits through friction over the arc it touches, so as the pulleys get further apart in diameter, or closer together in center distance, the small pulley loses contact arc. Below roughly 120° of wrap the belt starts slipping under load, which shows up as glazing, squeal and heat rather than as an obvious failure. Increase center distance or add an idler.

Round the calculated length up to the nearest belt you can actually buy, then set the center distance to suit it, leaving take-up adjustment.

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