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Gurukrupa Enterprises
Mechanical Calculators

Pulley RPM Calculator

Calculate driven pulley speed from driver pulley speed and pulley diameters.

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About this calculator

In a belt drive, pulley speed is inversely proportional to pulley diameter — a smaller driven pulley spins faster than a larger one, and vice versa. This calculator finds the driven pulley's speed from the driver's speed and the two pulley diameters.

Formula

N₂ = N₁ × (D₁ ÷ D₂)
  • N₁Driver pulley speed — RPM
  • D₁Driver pulley diameter — mm
  • D₂Driven pulley diameter — mm
  • N₂Driven pulley speed — RPM

Worked Example

Given: A 100mm driver pulley at 1440 RPM drives a 200mm pulley.

Result: N₂ = 1440 × (100 ÷ 200) = 720 RPM.

How to Use

  1. Enter the driver (motor-side) pulley speed in RPM.
  2. Enter the driver pulley diameter.
  3. Enter the driven pulley diameter.
  4. Click Calculate to get the driven pulley's speed.

Important Assumptions

  • Assumes no belt slip — real V-belt drives typically have 1–3% slip under load.
  • Both diameters must be in the same unit.

Common Mistakes

  • Mixing diameter and radius, or mixing units (mm with inches) between the two pulleys.

Frequently Asked Questions

Does belt slip affect this result?

Yes — this calculator gives the theoretical speed assuming no slip. Actual driven speed under load is typically 1–3% lower with V-belts.

Can I use pitch diameter instead of outer diameter?

Yes, and it is more accurate — pitch diameter is the effective diameter where the belt centreline sits.

These calculators are provided for informational and estimation purposes. Results depend on the assumptions and input values used. Always verify critical engineering calculations against applicable standards, manufacturer specifications, site measurements, and qualified professional advice.