Pulley Speed and Belt Length Calculator
By Santaji GadepatilinUpdated October 27, 2026
Belt and pulley drives change the speed and torque between a motor and a machine. The pulley diameters set the speed ratio, and the shaft centre distance sets the belt length. This free pulley speed and belt length calculator solves for whichever value you leave blank and shows the formula and steps.
How to use the pulley speed and belt length calculator
Enter the driver speed, driver pulley diameter, driven pulley diameter and centre distance between shafts. You get the driven speed, speed ratio, belt speed and approximate belt length.
Pulley speed and belt length formulas
| Quantity | Formula |
|---|---|
| Driven speed | N₂ = N₁ × D₁ ÷ D₂ |
| Belt speed | v = π × D₁ × N₁ ÷ 60 |
| Belt length | L ≈ 2C + π(D₁ + D₂)/2 + (D₂ − D₁)²/(4C) |
Use pitch diameters for V-belts.
Example
1,440 rpm motor, 100 mm → 250 mm, 500 mm centres:
- Driven speed = 576 rpm
- Belt speed = π × 0.1 × 1,440 ÷ 60 = 7.54 m/s
- Belt length ≈ 1561 mm
Driven speeds from a 1,440 rpm motor
| Driver / driven (mm) | Ratio | Driven speed |
|---|---|---|
| 100 / 100 | 1.00 | 1440 rpm |
| 100 / 150 | 1.50 | 960 rpm |
| 100 / 200 | 2.00 | 720 rpm |
| 100 / 250 | 2.50 | 576 rpm |
| 80 / 300 | 3.75 | 384 rpm |
| 150 / 100 | 0.67 | 2160 rpm |
| 200 / 100 | 0.50 | 2880 rpm |
Belt drive tips
- Keep the speed ratio below about 6:1 for single V-belt stages to maintain wrap angle on the small pulley.
- Do not go below the minimum recommended pulley diameter for the belt section — small pulleys shorten belt life.
- Check alignment and tension; misalignment is a leading cause of belt failure.
- Real speed is 1–3% lower than calculated because of belt slip and creep.
Choosing a standard belt
Round the calculated length to the nearest standard belt size, then adjust the centre distance with the motor slide rails to set tension.
Frequently asked questions
How do I calculate pulley rpm?
Driven rpm = driver rpm × driver diameter ÷ driven diameter. A 1,440 rpm motor with a 100 mm pulley driving a 250 mm pulley gives 576 rpm.
What size pulley do I need for a target speed?
Enter the target as Driven speed and press × on Driven pulley diameter.
What is a good belt speed for V-belts?
Classical V-belts work best at about 5–25 m/s, with around 20 m/s often most efficient. Very low speeds need high tension; very high speeds cause centrifugal losses.
Does torque change with pulley size?
Yes. Torque increases by the same ratio that speed decreases (ignoring losses). A 2.5:1 reduction gives about 2.5 times the torque.