# Pulley Calculator (Belt Drive)

URL: https://speedycalculator.com/calculators/physics/pulley/
Last updated: 2026-10-30

> The pulley calculator finds the driven pulley speed and torque, belt speed, belt length and effective belt tension for a two-pulley belt drive from the power, pulley diameters and driver speed.

## Inputs

- Power transmitted (kW, W, hp)
- Driver pulley diameter (mm, cm, m, in, µm)
- Driver speed (rpm)
- Driven pulley diameter (mm, cm, m, in, µm)
- Centre distance (mm, cm, m, in, µm)

## Results

- Driven pulley speed
- Driver torque
- Driven torque
- Belt speed
- Effective belt pull (tight − slack)
- Belt length

## 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)
```

## Explanation

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

FieldWhat to enterPower transmittedUnits: kW, W, hpDriver pulley diameterUnits: mm, cm, m, in, µmDriver speed (rpm)A numberDriven pulley diameterUnits: mm, cm, m, in, µmCentre distanceUnits: 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)0.755801.135801.55801.885802.255803580

## 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.

## FAQ

**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.


## Sources

- [Shigley's Mechanical Engineering Design (Budynas & Nisbett) – reference text](https://www.mheducation.com/highered/product/shigley-s-mechanical-engineering-design-budynas/M9780073398204.html)
- [The Engineering ToolBox](https://www.engineeringtoolbox.com/)