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Pulley Speed Ratio Calculator

Work out pulley speed ratio instantly with clear inputs, formula shown and shareable results.

About Pulley Speed Ratio Calculator

Belt drives conserve surface speed, so the product of diameter and speed is the same at both pulleys: N2 = N1 x D1 / D2. Torque is multiplied by the inverse of the speed change, and belt speed in feet per minute is pi x diameter x rpm divided by 12.

Formulas

Speed relation

N2 = N1 x D1 / D2; torque multiplier = D2 / D1

Belt speed

V fpm = pi x D in x rpm / 12

Worked example

With the default values below, Pulley Speed Ratio Calculator returns Driven pulley speed of 1,050.0. Change any input above and every figure here is recalculated in your browser.

Inputs used

Driver pulley diameterminimum 0.1 in
6 in
Driver speedminimum 1 rpm
1,750 rpm
Driven pulley diameterminimum 0.1 in
10 in

Results

Driven pulley speed
1,050.0 rpm
Drive ratio
1.6667
Belt speed
2,749 fpm
Torque multiplication
1.6667

Step by step

  1. 1Values used
  2. 2Speed relation
  3. 3Belt speed
  4. 4Driven pulley speed= 1,050.0 rpm
  5. 5Drive ratio= 1.6667
  6. 6Belt speed= 2,749 fpm
  7. 7Torque multiplication= 1.6667

Assumptions and limits

  • The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

How is the Pulley Speed Ratio figure worked out?

N2 = N1 x D1 / D2; torque multiplier = D2 / D1. Belt drives conserve surface speed, so the product of diameter and speed is the same at both pulleys: N2 = N1 x D1 / D2.

Is there a belt speed limit?

Yes. Classical V-belts are usually kept below about 5000 fpm; beyond that centrifugal tension reduces the grip available and special belt construction is needed.

What values do I need to enter?

This calculator takes 3 inputs: Driver pulley diameter, Driver speed, Driven pulley diameter. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.

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