Mechanical calculator

Pulley Ratio Calculator

Find belt-drive speed relationship from driver and driven pulley diameters.

Use calculator View formulas FAQ

Calculator

Enter mechanical values

Select units beside each input. Results update automatically.

Mechanical

Driver pulley diameter m
Driven pulley diameter m
Input speed rpm
Input torque Nm

Result

About this calculator

How it works

Pulley ratio relates speed and torque for belt drives. A larger driven pulley reduces speed and increases ideal torque.

Formulas

Formulas used

Pulley ratio

R = driven diameter / driver diameter

Output speed

n2 = n1 / R

Worked example

Example calculation

Output speed

100 mm driver to 200 mm driven gives half speed

Guide

How to use this calculator

Enter the known machine, motion, power, or geometry values.
Select units beside inputs where unit menus are shown.
Use the highlighted result first, then review supporting values for design context.

Reference

What the results mean

Pulley ratio

Diameter ratio that controls belt-drive speed change.

Output speed

Estimated driven shaft RPM.

Assumptions and limits

Before using the result

Calculations are simplified engineering estimates for idealized geometry and operating conditions.
Real systems may require efficiency, fatigue, slip, vibration, lubrication, thermal, tolerance, and manufacturer data checks.
Use applicable standards, safety factors, test data, and qualified engineering judgement for final machine design.

FAQ

Pulley Ratio Calculator questions

Can I use different units?

Yes. Select the unit beside each input. The calculator converts values internally before solving.

Are these results suitable for final machine design?

Use the results for education, screening, and early design checks. Final designs should include verified data, safety factors, standards, fatigue, tolerances, and manufacturer guidance.

Why can real mechanical systems differ from the result?

Real systems include friction, slip, wear, vibration, misalignment, efficiency losses, and manufacturing variation.