Pulley Calculator

Written by Great Medico

Updated September 25, 2026 •2 min read

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Small input changes can matter a lot in pulley, especially when angles, squared dimensions, ratios, or logarithms are involved. A useful machine calculation shows both the ideal relationship and the assumptions that separate it from a real component under load. That is why the useful part of this calculator is the relationship as well as the headline value.

What this calculator does

The Pulley Calculator turns Transmitting power, Pulley centers distance, Driver pulley — Diameter into the page’s driven pulley angular velocity using the specific relationship described below. Supporting values appear only where this calculator derives them, which makes the headline easier to audit against the same model.

How to use it

Start with the fields that drive this result: Transmitting power, Pulley centers distance, Driver pulley — Diameter, Driver pulley — Angular velocity, Driven pulley — Diameter. Use the unit menu beside each quantity instead of converting by eye; the page normalizes supported units before calculating. Then change one value at a time and watch whether the headline and supporting values move as the formula predicts.

How the calculation works

For an ideal belt drive, driven RPM = driver RPM·D1/D2. Belt speed is πD1·RPM1/60, torque is power/angular speed, and belt tension is power/belt speed. The page also estimates open-belt length from both pulley diameters and center distance.

Example

Using the default example on the page (Transmitting power = 1000 W; Pulley centers distance = 0.5 m; Driver pulley — Diameter = 0.1 m; Driver pulley — Angular velocity = 1800 rpm), the calculator returns driven pulley angular velocity of 600 rpm. That default case is a convenient baseline: alter only one field and compare the new result before substituting a completely different setup.

How to interpret the result

Interpret the driven pulley angular velocity as the result of this page’s idealized machine relationship. Check the supporting RPM, torque, speed, diameter, force, or power values for consistency before carrying the result into a separate design calculation.

Limitations and notes

The belt-drive relationships assume ideal no-slip speed ratio and simplified belt geometry. The page’s power/belt-speed ‘tension’ is an effective force, not a resolved tight-side/slack-side tension pair. Belt elasticity, wrap angle, pretension, friction, pulley inertia, efficiency, and transient loads are not modeled.