Pulley RPM Calculator

This calculator works out the speed of a belt-driven pulley. Enter the speed of the driving pulley (for example a motor) and the diameters of the driving and driven pulleys; you get the driven pulley speed in rpm, the speed ratio and the belt speed in feet per minute.

Formula

n₂ = n₁ × d₁ ÷ d₂. Speed ratio = d₂ ÷ d₁. Belt speed (ft/min) = π × d₁ (in) × n₁ ÷ 12.

• n₁, d₁: speed and diameter of the driving pulley
• n₂, d₂: speed and diameter of the driven pulley
• Use the pitch diameter where the belt rides for V-belts.

Worked Example

A 1,750 rpm motor with a 4 in pulley drives an 8 in pulley.

1. n₂ = 1,750 × 4 ÷ 8 = 875 rpm
2. Ratio = 8 ÷ 4 = 2.000 : 1
3. Belt speed = π × 4 × 1,750 ÷ 12 = 1,833 ft/min

How to read the result

A bigger driven pulley slows the output and raises torque in the same proportion; a smaller one speeds it up. The belt speed is the same on both pulleys. To pick a pulley for a target speed, rearrange: d₂ = d₁ × n₁ ÷ n₂.

Limits and common mistakes

• The formula ignores belt slip, which makes the real driven speed a little lower.
• Check the machine and belt makers’ maximum speeds before speeding anything up; this page does not rate parts.
• Use the same unit for both diameters.

Frequently Asked Questions

How do I slow a motor down with pulleys?

Use a driven pulley larger than the motor pulley; the speed drops by the ratio of diameters.

What pulley gives 1,000 rpm from 1,750 rpm with a 4 in driver?

d₂ = 4 × 1,750 ÷ 1,000 = 7 in.

Does belt length change the speed?

No. Only the diameters matter.

Is torque affected?

Ideally torque rises by the same ratio that speed falls, minus losses.

in n1 | Driver pulley speed | rpm | 1750 | 0 | 100000
in d1 | Driver pulley diameter | in | 4 | 0.1 | 200
in d2 | Driven pulley diameter | in | 8 | 0.1 | 200
out n2 | Driven pulley speed | rpm | n1 * d1 / d2 | 0
out ratio | Speed ratio (driver : driven) | : 1 | d2 / d1 | 3
out v | Belt speed | ft/min | pi * d1 * n1 / 12 | 0