Motor Calculator
Calculate motor current, power and operating values for drive-system planning.
View CalculatorCalculator Setup
Enter your drive parameters below.
mm
Diameter of motor/power pulley
mm
Diameter of machine pulley
mm
Distance between shaft centers
Estimate torque changes across the drive.
Drive Results
Enter parameters and calculate to see results.
How the Pulley & Belt Calculator Works
When two pulleys are connected by a belt, they transmit rotational power from a driver (usually a motor) to a driven machine. Because the pulleys are linked by the belt, the belt travels at the exact same linear speed over both pulleys (assuming no slip).
This physical link means that a smaller pulley will spin faster than a larger pulley to keep up with the belt. This relationship allows us to change speeds and torque purely by altering pulley diameters.
Pulley RPM Formula
The driven RPM is inversely proportional to the diameter ratio.
Driven RPM = Motor RPM × (Drive Dia ÷ Driven Dia)
Example: A 1450 RPM motor with a 100mm drive pulley is connected to a 200mm driven pulley.
1450 × (100 ÷ 200) = 725 RPM.
Torque Change Explained
Speed and torque share an inverse relationship. If you reduce the speed via pulleys, you increase the torque (turning force) by the same ratio, minus slight efficiency losses from belt friction.
Torque Multiplier = (Driven Dia ÷ Drive Dia) × Eff%
Belt Length Formula
Calculating the exact length of an open belt (un-crossed) requires accounting for the straight runs between the pulleys and the curved wrap around each pulley. The standard geometric formula is:
L = 2C + 1.5708(D + d) + (D - d)² / 4C
- LTotal pitch length of the belt
- CShaft centre-to-centre distance
- DDiameter of larger pulley
- dDiameter of smaller pulley
How to Choose Pulley Sizes When You Only Know the RPM
If you know the motor RPM and the RPM required by the pump, fan, compressor or machine, the first step is to calculate the speed ratio. The calculator then tests common pulley sizes and suggests practical pulley combinations that come closest to the target RPM.
Why Shaft Size Affects Pulley Selection
A pulley ratio calculates the speed, but shaft size affects whether a pulley is practical to fit. A large shaft needs enough pulley hub material for the bore, keyway and locking bush. This is why very small pulleys should not be selected for large industrial shafts, even when the RPM calculation is correct. Always confirm pulley bore, keyway, belt type and machine safe speed before buying parts.
Where This Calculator Is Used
- Air compressors and blowers
- HVAC fans and extraction systems
- Water pumps and hydraulics
- Conveyor belt drives
- Lathes, mills and drill presses
- Agricultural machinery
- Custom workshop generator builds
Common Mistakes
Measuring outside diameter: Calculations should technically use the Pitch Diameter (where the belt sits inside the groove), not the raw outside diameter of the pulley.
Ignoring belt slip: Actual driven speed can differ from the theoretical result because slip depends on belt type, installation, tension, wear and load.
Too short centres: If the pulleys are too close, the belt won't have enough "wrap angle" on the smaller pulley, causing slippage.
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View CalculatorWorked Drive Check
For 1,450 rpm with a 100 mm drive pulley, 200 mm driven pulley and 500 mm centre distance, the ideal driven speed is 725 rpm and the estimated open-belt length is 1,476.24 mm. Treat the torque change as an ideal planning ratio and verify belt section, pulley diameter, power rating, tension and guarding with the manufacturer.
Frequently Asked Questions
How is driven pulley speed calculated?
For an ideal belt drive, driven RPM equals drive RPM multiplied by drive pulley diameter and divided by driven pulley diameter. Actual speed can differ because of slip and load.
What does the pulley ratio mean?
The ratio compares driven diameter with drive diameter. It indicates the ideal speed change and ideal torque multiplication before efficiency losses.
How is open-belt length estimated?
The calculator uses the standard two-pulley open-belt approximation based on both pulley diameters and shaft centre distance. Confirm the selected belt section and take-up range with the belt manufacturer.
Does the calculator include belt slip?
No. Reported RPM and belt speed are geometric estimates. Belt type, tension, wear, contamination and load can change actual speed.
Can the calculated torque multiplier be treated as delivered torque?
No. It is an ideal ratio adjusted only where the calculator explicitly applies the entered efficiency. Confirm motor, belt and pulley ratings for the real duty.
Why does pulley alignment matter?
Misalignment can cause tracking problems, edge wear, heat and reduced belt life. Follow the drive manufacturer’s alignment procedure.
How should belt tension be selected?
Use the belt manufacturer’s tensioning method and values for the belt section, span and transmitted power. Excessive or insufficient tension can damage the drive.
What safety checks are needed before operating a belt drive?
Provide required guarding, isolate energy before inspection, verify pulley condition and fasteners, and stay within manufacturer speed and power ratings.
Technical References
Use the applicable manufacturer instructions, approved drawings and site procedures together with these primary safety references.
Calculation method, worked example and reference
What this calculator does and formula: Ignoring slip, driver speed×driver diameter = driven speed×driven diameter. Belt speed follows π×diameter×rotational speed.
Variables and units: Enter values in the units named by the controls. The calculator converts through its internal base unit or applies the displayed geometric or financial relationship before rounding the presented result.
Verified worked example: A 100 mm driver at 1500 rpm driving a 300 mm pulley gives 500 rpm.
Result interpretation, assumptions and practical limits: Real drives require belt profile, wrap, tension, service factor, alignment and manufacturer ratings. Guard rotating machinery before operation.
Professional check: Treat the result as a planning estimate. Where safety, regulated work, contracts or significant money are involved, verify inputs and the result with the governing standard, manufacturer data and an appropriately qualified professional.
Authoritative reference
Pulley ratio and belt geometry
Use this schematic to identify the calculator inputs and how they relate. It is explanatory and not a fabrication, installation or lift-plan drawing.