A belt drive changes speed and torque in the ratio of the pulley diameters. A small motor pulley driving a large one slows the output and increases torque, as in table saws and fans.
How to use the Pulley Calculator
- Power transmitted
- Driver pulley diameter
- Driver speed (rpm)
- Driven pulley diameter
- Centre distance
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Enter the transmitted power, the driver pulley diameter and speed, the driven pulley diameter and the centre distance.
Inputs explained
| Field | What to enter |
|---|---|
| Power transmitted | Units: kW, W, hp |
| Driver pulley diameter | Units: mm, cm, m, in, µm |
| Driver speed (rpm) | A number |
| Driven pulley diameter | Units: mm, cm, m, in, µm |
| Centre distance | Units: mm, cm, m, in, µm |
What each result means
- Driven pulley speed = [Driver speed (rpm)]×[Driver pulley diameter]÷[Driven pulley diameter]
- Driver torque = [Power transmitted]÷(2×pi×[Driver speed (rpm)]÷60)
- Driven torque = [Power transmitted]÷(2×pi×[Driver speed (rpm)]×[Driver pulley diameter]÷[Driven pulley diameter]÷60)
- Belt speed = pi×[Driver pulley diameter]×[Driver speed (rpm)]÷60
- Effective belt pull (tight − slack) = [Power transmitted]÷(pi×[Driver pulley diameter]×[Driver speed (rpm)]÷60)
- Belt length = 2×[Centre distance] + pi×([Driver pulley diameter] + [Driven pulley diameter])÷2 + ([Driven pulley diameter] – [Driver pulley diameter])^2÷(4×[Centre distance])
Example values
How the main result changes when Power transmitted changes and everything else stays at the example values above:
| Power transmitted (kW) | Driven pulley speed (rpm) |
|---|---|
| 0.75 | 580 |
| 1.13 | 580 |
| 1.5 | 580 |
| 1.88 | 580 |
| 2.25 | 580 |
| 3 | 580 |
Formula
n₂ = n₁·D₁/D₂, T = P/ω, v = π·D₁·n₁/60, F_eff = P/v, L = 2C + π(D₁ + D₂)/2 + (D₂ − D₁)²/(4C)
Worked example
Inputs — Power transmitted: 1.5 kW; Driver pulley diameter: 100 mm; Driver speed (rpm): 1,450; Driven pulley diameter: 250 mm; Centre distance: 500 mm.
Results — Driven pulley speed = 580 rpm; Driver torque = 9.8786 N·m; Driven torque = 24.6965 N·m; Belt speed = 7.5922 m/s; Effective belt pull (tight − slack) = 197.5717 N; Belt length = 1,561.02871 mm.
Key terms
- Speed ratio
- Ratio of driver to driven speeds, equal to D₂/D₁.
Frequently asked questions
How do I slow a motor down with pulleys?
Use a larger pulley on the driven shaft.
Is power lost in belts?
Typically 2–5% for V-belts.
What is a good belt speed?
V-belts work best at about 5–30 m/s.
What is effective belt pull?
The difference between tight-side and slack-side tension.