Calculate the speed of a driven pulley from the motor RPM and both pulley diameters, or find the pulley size you need for a target speed. Add the center distance to get the belt length and wrap angle, and check the belt speed.
Last updated:
Find
RPM
Use the pitch diameter for V-belts
RPM
Distance between shaft centers; needed for belt length
%
0 for timing belts; 1–3% typical for V-belts
Enter your values to see the result.
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How to use the Pulley Belt and Speed Calculator
Choose what to find: the driven RPM or the driven pulley size.
Pick mm or inches and enter the motor (driver) RPM and pulley diameter.
Enter the driven pulley diameter or the target RPM.
Add the center distance between the shafts for the belt length.
Read the results and compare other pulley sizes in the table.
Pulley and belt formulas
Find
Formula
Driven speed
N2 = N1 × D1 ÷ D2
Driven diameter
D2 = D1 × N1 ÷ N2
Speed ratio
i = D2 ÷ D1 = N1 ÷ N2
Belt length (open belt)
L = 2C + π(D1 + D2) ÷ 2 + (D2 − D1)² ÷ 4C
Belt speed
v = π × D1 × N1 ÷ 60 (m/s with D in m)
Wrap angle (small pulley)
θ = 180° − 2 × asin((D − d) ÷ 2C)
Driven torque (no losses)
T2 = T1 × D2 ÷ D1
Common motor speeds
Motor poles
Synchronous RPM 60 Hz
Typical full-load RPM 60 Hz
Synchronous RPM 50 Hz
Typical full-load RPM 50 Hz
2
3,600
about 3,450
3,000
about 2,900
4
1,800
about 1,725–1,750
1,500
about 1,420–1,450
6
1,200
about 1,140
1,000
about 960
8
900
about 860
750
about 720
Induction motors run slightly below synchronous speed (slip). Use the RPM on the motor's nameplate for the most accurate result.
Frequently asked questions
How do I calculate pulley RPM?
Driven RPM = driver RPM × driver diameter ÷ driven diameter. A 1,450 RPM motor with a 100 mm pulley driving a 250 mm pulley turns the driven shaft at 1,450 × 100 ÷ 250 = 580 RPM.
How do I calculate belt length?
For an open belt: L = 2C + π(D1 + D2) ÷ 2 + (D2 − D1)² ÷ (4C), where C is the center distance. With 100 and 250 mm pulleys 500 mm apart, L = 1,000 + 549.8 + 11.25 ≈ 1,561 mm. Choose the nearest standard belt and adjust the center distance.
What pulley size do I need for a certain RPM?
Driven diameter = driver diameter × driver RPM ÷ target RPM. To slow a 1,750 RPM motor with a 3 in pulley to 700 RPM you need a 3 × 1,750 ÷ 700 = 7.5 in driven pulley.
How do I calculate belt speed?
Belt speed = π × driver diameter × driver RPM. In metric: v (m/s) = π × D (m) × RPM ÷ 60. A 100 mm pulley at 1,450 RPM gives about 7.6 m/s (1,495 ft/min). Classical V-belts usually run below about 30 m/s.
What is the wrap angle and why does it matter?
It is the angle of contact between the belt and the small pulley. Less wrap means less grip. Keep it at 120° or more by increasing the center distance or reducing the ratio.
Which diameter should I use for V-belts?
Use the pitch diameter, where the belt's neutral layer runs, not the outside diameter of the sheave. For timing belts use the pitch diameter given by the tooth count and pitch.
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