Pulley Speed Calculator

This professional pulley speed calculator helps mechanical engineers, technicians, and makers determine driven RPM, drive ratio, and belt linear speed for a two-pulley belt system. It supports millimeter and inch units, accounts for slip, and can reverse-solve the driven pulley size for a target RPM.

Calculator Inputs

Fields marked with * are required. Values must be positive real numbers.

Diameter Units
mm
mm

Results

Driven speed — RPM
Speed ratio (driver:driven) —
Belt linear speed —
Required driven diameter (from target) —

Outputs update automatically. Values are rounded for display; calculations use full precision.

Authoritative Content Ecosystem

Data Source and Methodology

Primary reference: Machinery’s Handbook, 30th Edition (Industrial Press, 2016), Section: Belt Drives. Publisher page: industrialpress.com. All equations here follow standard belt-drive kinematics based on pulley pitch diameters.

Tutti i calcoli si basano rigorosamente sulle formule e sui dati forniti da questa fonte.

The Formula Explained

Ideal speed ratio (ignoring slip):

r = N_1 / N_2 = D_2 / D_1

Driven RPM with slip s (fraction):

N_2 = N_1 \cdot \dfrac{D_1}{D_2} \cdot (1 - s)

Belt linear speed (using driver):

v = \dfrac{\pi\, D_1\, N_1}{60}

Required driven diameter for target N_{2,t}:

D_{2,\mathrm{req}} = \dfrac{D_1\, N_1\, (1 - s)}{N_{2,t}}

Use consistent diameter units (mm or in). For linear speed, D must be converted to meters for m/s.

Glossary of Variables

D1 (driver diameter)
Diameter of the driving pulley.
D2 (driven diameter)
Diameter of the driven pulley.
N1 (driver RPM)
Rotational speed of the driver in RPM.
N2 (driven RPM)
Calculated rotational speed of the driven pulley in RPM.
s (slip)
Fractional slip (e.g., 0.02 for 2%).
v (belt speed)
Tangential belt velocity, reported in m/s and ft/min.
D2req
Driven pulley diameter required to meet a target driven RPM.

How It Works: A Step‑by‑Step Example

Inputs: D1 = 120 mm, D2 = 80 mm, N1 = 1750 RPM, s = 2% (0.02).

  1. Compute ideal ratio r = N1/N2 = D2/D1 = 80/120 = 0.6667. Therefore, N2,ideal = N1 · D1/D2 = 1750 · 120/80 = 2625 RPM.
  2. Apply slip: N2 = 2625 · (1 − 0.02) = 2572.5 RPM.
  3. Belt speed (convert D1 to meters: 120 mm = 0.12 m): v = π · 0.12 · 1750 / 60 ≈ 10.99 m/s (≈ 2166 ft/min).

Result: Driven RPM ≈ 2573 RPM; belt speed ≈ 11.0 m/s.

Frequently Asked Questions (FAQ)

Does pulley diameter unit (mm vs in) affect RPM results?
No. RPM ratio depends on the ratio of diameters, which is unitless. Linear speed does depend on units.
Should I use outer diameter or pitch diameter?
For toothed belts and timing pulleys, use pitch diameter for best accuracy. For V-belts, diameter conventions vary—consult the manufacturer.
What is a typical slip value?
Well-tensioned V-belts: 1–3%. Timing belts: effectively 0% (use s = 0). Worn belts can exceed 5%.
Can I enter zero slip?
Yes. Set slip to 0 to compute the ideal kinematic result.
How do I slow down the driven shaft?
Use a larger driven pulley (increase D2) or a smaller driver (decrease D1). The calculator updates results instantly.
Why does the required diameter show “—”?
You must provide a positive Target Driven RPM to see the required driven diameter.
Is belt thickness or center distance considered?
No. This tool addresses ideal kinematics. For tensioning, wrap angle, and belt length, use dedicated sizing tools and manufacturer data.
Strumento sviluppato da Ugo Candido,. Contenuti verificati da, CalcDomain Engineering Editorial Board.
Ultima revisione per l'accuratezza in data: .