October 26, 2026 · DIY & Workshop · 5 min read
Pulley Belts: How to Calculate the Right Belt Length
Belt drives are everywhere — drill presses, lawn mowers, air compressors, washing machines — and the number one mistake people make is guessing the belt length. Too short and it won't fit or will overload the bearings; too long and it flaps, slips and wears out fast. The good news: belt length for two pulleys follows one clean formula.
The belt length formula
For two pulleys of diameter D and d, with a center distance C between their shafts, the belt length L is:
The first term covers the two straight runs between the pulleys, the middle term is the average of the two pulley circumferences (π ÷ 2 ≈ 1.57), and the last term corrects for unequal pulley sizes — when the diameters are equal it drops to zero.
Worked example
A motor drives a machine pulley. Motor pulley: 75 mm diameter. Machine pulley: 150 mm diameter. Center distance: 400 mm.
- Straight runs: 2 × 400 = 800 mm
- Average circumference: 1.57 × (150 + 75) = 353 mm
- Correction: (150 − 75)² ÷ (4 × 400) = 5,625 ÷ 1,600 = 3.5 mm
Total: about 1,157 mm — call it 1,160 mm and pick the nearest standard belt.
What else the drive tells you
Two more numbers are worth knowing before you buy:
- Speed ratio. The driven pulley turns at motor RPM × (D ÷ d). A 75 mm pulley driving a 150 mm pulley halves the speed — and doubles the torque.
- Belt speed. Surface speed is π × pulley diameter × RPM. Standard belts top out around 25–30 m/s; beyond that you need a balanced or cogged belt.
Practical tensioning notes
V-belts work best with roughly 10–20 mm of deflection under thumb pressure at mid-span — tight enough not to slip, loose enough not to strain the bearings. If your computed length falls between two stock sizes, choose the smaller belt and add an adjustable motor mount or idler pulley to take up the slack. Never force-fit a belt that's undersized by more than a couple of percent: the extra tension eats bearings and shafts.
Designing a geared drive instead of a belt drive? Gear Ratios Explained covers how meshing gears trade torque for speed and how efficiency eats into the output.