Pulley Speed Ratio Calculator Online

How to calculate belt and pulley speed

Calculate pulley speed, speed ratio and belt speed for a belt-driven system.

Enter the driver and driven pulley diameters along with the driver speed. The calculator determines the resulting driven pulley speed and linear belt speed.

You can also enter driver torque and belt drive efficiency to calculate the estimated torque at the driven pulley.

How to use the calculator

Enter the:

• Driver pulley diameter (D₁) in mm
• Driven pulley diameter (D₂) in mm
• Driver speed (N₁) in rpm
• Driver torque (T₁) in Nm, if required
• Belt drive efficiency (η), if required

The calculator returns the speed ratio, driven pulley speed (N₂) and belt speed (V). When driver torque is entered, it also calculates the estimated driven torque (T₂).

How the calculations work

For a belt drive, pulley speed changes according to the ratio between the driver and driven pulley diameters.

Speed ratio:

SR = D₂ ÷ D₁ = N₁ ÷ N₂

Driven pulley speed:

N₂ = N₁ × (D₁ ÷ D₂)

Belt speed:

V = π × D × N ÷ 60,000

where belt speed is in m/s when pulley diameter (D) is entered in mm and rotational speed (N) is in rpm.

If driver torque is entered, the calculator estimates driven torque using:

Driven torque:

T₂ = T₁ × SR × η

where η is the belt drive efficiency expressed as a ratio.

Understanding the results

A larger driven pulley rotates more slowly than the smaller driver pulley. A smaller driven pulley rotates faster.

For example, a 100 mm driver pulley running at 1,750 rpm with a 250 mm driven pulley has a speed ratio of 2.5:1. The calculated driven pulley speed is:

N₂ = 1,750 × (100 ÷ 250) = 700 rpm

The calculator also determines the corresponding belt speed.

These calculations provide a useful starting point when checking belt-driven pumps and other rotating equipment. Actual operating conditions can vary due to belt slip, pulley condition, belt tension and other mechanical losses.

Last Updated on September 2, 2026 by TPE