Pulley Size Calculator
Enter the driver and driven pulley diameters, the driver speed and the distance between shaft centres. The calculator gives the output speed, ratio, belt speed, belt length and wrap angle.
- Output RPM
- Belt length
- Wrap angle check
Use pitch diameters for V-belts. These are a little smaller than the outside diameter.
Pulley speed and size trade off exactly. Driver diameter × driver RPM equals driven diameter × driven RPM. Double the driven pulley and the shaft turns at half the speed with twice the torque. The belt length then follows from the two diameters and how far apart the shafts are.
Pulley ratio chart
Output rpm from a 1,450 rpm motor. Search for a ratio.
| Driver (mm) | Driven (mm) | Ratio | Output rpm |
|---|---|---|---|
| 50 | 200 | 4 : 1 | 363 |
| 75 | 150 | 2 : 1 | 725 |
| 100 | 150 | 1.5 : 1 | 967 |
| 100 | 100 | 1 : 1 | 1450 |
| 150 | 100 | 1 : 1.5 | 2175 |
| 200 | 100 | 1 : 2 | 2900 |
1,450 rpm is a typical four-pole motor on 50 Hz. On 60 Hz it runs at about 1,750 rpm, so multiply by 1.2.
Driven pulley and output speed
100 mm driver at 1,450 rpm. Slide the driven pulley size.
Speed falls in proportion. A 200 mm driven pulley gives exactly half the motor speed.
How to size pulleys
Ratio first. Step through it.
Set the speeds
Motor speed and wanted speed.
Keep the smallest pulley above the belt maker's minimum diameter. Bending a belt too tightly shortens its life.
V-belt, timing belt or flat belt
Belt types. Compare them.
Wedges into the groove for grip. The workshop standard.
Toothed belt on toothed pulleys. Exact ratio.
Thin and efficient at high speed.
Timing belts never slip, so the output speed is exact. V-belts slip a little but cushion shock loads.
Before fitting
Avoid belt trouble. Tick each item.
Tick each item as you go.
Wrap of at least 120° on the small pulley gives enough grip. Less than that and the belt may squeal and slip.
What you get
Every drive figure. Each tile explains one result.
Driven shaft rpm.
Driven ÷ driver.
Metres per second.
mm and inches.
Grip on the small pulley.
Belt length is the pitch length. Choose the nearest stock belt and adjust the centre distance to suit.
Frequently Asked Questions
No question matches that search.
How do I calculate pulley speed?
Driven rpm = driver rpm × driver diameter ÷ driven diameter.
How do I work out belt length?
L = 2C + π(D + d)/2 + (D − d)²/(4C), where C is the centre distance.
What pulley size halves the speed?
A driven pulley twice the diameter of the driver.
Does a bigger pulley give more torque?
Yes. Torque rises by the same ratio as speed falls, less a little friction loss.
What is pitch diameter?
The diameter where the belt's neutral line runs. Slightly less than the outside diameter on V-pulleys.
What is a safe belt speed?
Up to about 30 m/s for standard V-belts.
What wrap angle do I need?
At least 120° on the small pulley.
The right speed from any motor
Pick a ratio, size the pulleys and check the belt length and wrap. A well-designed drive runs quietly at the speed you need, with no slip and long belt life.
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