Pulley Speed Calculator
Calculate driven RPM and ratio from pulley diameters.
Results
Driven RPM
- Speed ratio (driver : driven)
- 2 :1
- Speed reduction: the ratio is driver revolutions : driven revolutions. A 2:1 ratio means half the driver RPM.
- Assumes no belt slip and pulley pitch diameters.
The model assumes no slip and uses pitch diameters, which may differ from outside diameters. It does not size belt width, belt tension, shaft loads or power capacity. Efficiency adjusts torque, not the geometric speed ratio.
Formula · How it works
Formula
n₂ = n₁D₁ / D₂; i = D₂ / D₁ = n₁ / n₂n₂ = n₁ / i; i = D₂ / D₁; T₂,ideal = T₁i; T₂ = T₁iηWith no belt slip, n₁D₁ = n₂D₂. Ratio i = D₂/D₁, driven speed n₂ = n₁/i. Ideal output torque T₂ = T₁i; with efficiency η, T₂ = T₁iη. Efficiency is converted from percent to a fraction.
Worked example
A 100 mm driver turns at 1,800 rpm and drives a 200 mm pulley. The driven speed is 900 rpm, a 2:1 reduction. With 10 N·m input torque and 95% efficiency, ideal output torque is 20 N·m and adjusted output torque is 19 N·m.
How it works
A larger driven pulley reduces rotational speed and increases the ideal torque in the same proportion. A smaller driven pulley increases speed. The ratio is defined from diameters, so it is still meaningful when both shafts are stationary.
Common questions
What does a 2:1 ratio mean?
The driver makes two revolutions for every driven revolution. Output speed is half the input speed. A displayed ratio below 1:1 represents speed increase; 0.5:1 means the driven pulley turns twice as fast.
Should I enter the outside diameter?
Use the pulley pitch or effective diameter specified for the belt system. Outside diameter can give a different speed ratio, particularly for V-belt and timing pulley systems.
Does efficiency reduce the driven RPM?
No. In this no-slip model, diameters determine RPM. Efficiency reduces transmitted power and torque. Belt slip is not modeled.