Pulley RPM Calculator
Calculate driven pulley RPM and belt speed from driver RPM and pulley diameters. Inch and metric units.
Introduction
Belt drives trade speed for torque through pulley diameters, and the Pulley RPM Calculator gives you the three numbers that define any such drive: driven-pulley speed, speed ratio, and belt velocity. Enter the driver RPM and both pulley diameters in inches or millimeters and it solves the drive instantly — whether you are slowing a 3450 RPM motor to a usable spindle speed, checking that a fan stays inside its rated speed after a pulley swap, or verifying belt velocity against the maximum the belt manufacturer allows. It is the calculation behind every step-pulley chart on a drill press, executed for arbitrary diameters.
How it Works
Driven speed follows N2 = N1 × D1 ÷ D2: pulley surface speeds must match through the belt, so RPM scales inversely with diameter. The speed ratio reported is D1/D2 — values below 1 mean reduction, above 1 mean overdrive. Belt velocity is computed from the driver pulley as π × D1 × N1 ÷ 12 in ft/min for inch inputs, or π × D1 × N1 ÷ 1000 in m/min for millimeter inputs. A 2 in driver at 3450 RPM with a 6 in driven pulley yields 1150 RPM and about 1806 ft/min of belt speed.
Usage Scenarios
- Slowing a 3450 RPM bench-grinder motor to roughly 1150 RPM for a belt sander conversion by selecting a 1:3 pulley pair.
- Re-rating a dust-collector impeller after replacing a worn 4 in motor pulley with a 4.5 in spare — the calculator shows the 12.5% overspeed before you commit.
- Building a step-pulley speed chart for a lathe or drill press by running each diameter pair through the calculator.
- Checking belt velocity against a V-belt's rated maximum (commonly around 6500 ft/min for classical sections) before speeding up a drive.
- Sizing a driven pulley for a target speed: rearranging the ratio mentally, then confirming the exact diameter with the calculator.
FAQ
Do I measure pulley outside diameter or pitch diameter?
Pitch diameter is the accurate input — it is where the belt's tension cord rides. For V-belts, pitch diameter sits a few millimeters below the OD; for flat belts OD plus belt thickness is close.
Does the calculation account for belt slip?
No, it assumes perfect traction. Real V-belt drives lose roughly 1–3% to slip and creep, so measured driven speed runs slightly below the computed N2 = N1 × D1 ÷ D2 value.
Why does belt speed matter?
Belts have centrifugal and fatigue limits — exceeding the manufacturer's rated velocity shortens life dramatically. Belt speed also sets surface feet per minute on belt-driven grinders and sanders.
Does the same math apply to timing belts?
Yes, with teeth instead of diameters: N2 = N1 × T1 ÷ T2. Timing belts add zero slip, so the computed ratio is exact rather than approximate.