Belt Length Calculator

The belt length calculator will tell you the length of a belt spread out between two pulleys.

Belt length1,248.82 mm1.2488 m, 49.17 inches

Speed ratio2.5:1diameters, not circumferences — the ratio is the same either way

Wrap around the two pulleys439.82 mmexactly half the sum of the circumferences, for equal pulleys

Straight runs800 mmplus the 9 mm correction for the unequal diameters

Centre distance400 mm

Belt speed6.074 m/s1,196 feet a minute — V-belts are usually happiest between about 5 and 25 m/s

Driven pulley speed580 rpm

Belt passes per minute291.8each pass flexes the belt twice — the main driver of fatigue life

Wrap angle on the small pulley162.75°less than 180° — the smaller the wrap, the less torque the belt can transmit before slipping

Wrap angle on the large pulley197.25°

The formula

L = 2C + π(D+d)/2 + (D−d)² ÷ 4C

Where the formula comes from

A belt around two pulleys is two straight runs plus the arcs where it wraps each pulley. For equal pulleys the wrap is exactly half of each circumference, so the length is twice the centre distance plus π times the diameter — clean and exact.

Unequal pulleys make the straight runs slightly longer than the centre distance, because they are not parallel, and shift the wrap angles away from 180°. The (D−d)²/4C term corrects for both, and it is accurate to well within manufacturing tolerance unless the centres are unusually close relative to the diameters.

The wrap angle matters as much as the length in practice. Torque capacity depends on how far the belt wraps the smaller pulley, so a large ratio on short centres — which reduces that wrap — is where belts start slipping. Increasing the centre distance brings both wrap angles back towards 180°.