HesapHanem

Pulley Speed Ratio Calculator

Enter both pulley diameters and the driver RPM to find the driven speed.

Result

580.0 RPM
0.400
Driven pulley spins slower (step-down, ratio 1:2.500)

How it is calculated

The driver is the pulley on the motor or power source; the driven pulley is the one it turns. The belt moves at the same speed over both, so Ndriven = Ndriver × (Ddriver ÷ Ddriven), where N is in RPM and D is the diameter in any one unit (mm, inches), as long as both diameters use the same one. The speed ratio shown is Ddriver ÷ Ddriven: above 1 the driven pulley turns faster (step-up), below 1 it turns slower (step-down), and the note also gives the ratio as 1:x or x:1. Both diameters must be above zero. The calculation assumes no belt slip.

Worked example

A motor runs at 1,450 RPM with a 100 mm pulley driving a 250 mm pulley. The speed ratio is 100 ÷ 250 = 0.400. The driven speed is 1450 × 0.400 = 580.0 RPM. The driven pulley is larger, so it turns slower: the note reads step-down, ratio 1:2.500. If the pulleys were swapped (250 mm on the motor, 100 mm driven) the ratio would be 2.500 and the driven speed 3,625.0 RPM.

Try this example →

Common mistakes

  • Swapping the driver and driven diameters. The smaller driven pulley is the one that spins faster, so check which pulley sits on the motor shaft.
  • Mixing units, such as one diameter in mm and the other in inches. Only the ratio matters, so both must share a unit.
  • Using outside diameter instead of pitch diameter for V-belt and timing pulleys. The speed ratio follows the diameter at which the belt runs, which can be a little smaller than the outside size.
  • Expecting the real driven speed to match exactly. The formula has no belt slip, so a loaded belt drive usually turns the driven pulley slightly slower than shown.
  • Treating the ratio as torque. A step-down in speed raises available torque and a step-up lowers it, but this tool reports speed only.

About this calculator

In a belt-and-pulley drive, the driver and driven pulleys are linked by the same belt speed, which means their rotational speeds are inversely proportional to their diameters — the governing relationship is N₁ × D₁ = N₂ × D₂, where N is RPM and D is diameter. A smaller driven pulley spins faster than the driver, and a larger driven pulley spins slower, exactly the trade-off mechanical designers use to step speed up or down between a motor and a driven machine.

The speed ratio shown here is the driver pulley diameter divided by the driven pulley diameter (equivalently the driven RPM divided by the driver RPM), so a value above 1 means the driven pulley turns faster. Machinists and mechanical engineers use this number directly when selecting pulley sizes for conveyors, compressors, and machine tool drives. Both pulley diameters plus the driving pulley's RPM are enough to find how fast the driven pulley turns and the speed ratio between the two.

Was this helpful?

You can pick up to 3 reactions.

Comments (1)

  • Editorial team

    Check which RPM you type for the motor. A nameplate figure such as 1450 is already the loaded speed of an induction motor, while a round 1500 or 3000 is the synchronous speed it never quite reaches under load. The tool also assumes no belt slip, so a loaded V-belt drive can run a little slower than the number shown. Measure the driven shaft with a tachometer if the speed is critical. What machine are you building the drive for?

Popular calculators

Loan Calculator

Monthly payment, total interest and full amortization schedule.

Percentage Calculator

Percent change, percent of, and ratio calculations.

Discount Calculator

Calculate final price, savings, and discount amount.

BMI Calculator

Body mass index from height and weight (metric or imperial).

Age Calculator

Compute your exact age in years, months and days from a birth date.

Compound Interest Calculator

Calculate the final amount and interest earned with compounding.

All Calculators