Calculator Inputs
Example Data Table
| Power | RPM | Efficiency | Base Torque | Design Factor | Design Torque |
|---|---|---|---|---|---|
| 5 kW | 1500 | 100% | 31.831 N·m | 1.25 | 39.789 N·m |
| 10 hp | 1750 | 92% | 37.403 lb·ft | 1.15 | 43.013 lb·ft |
| 2,000 W | 3000 | 88% | 5.602 N·m | 1.50 | 8.403 N·m |
Formula Used
Angular speed: ω = 2 × π × RPM ÷ 60
Torque: T = P ÷ ω
Metric form: T(N·m) = P(W) × 60 ÷ (2 × π × RPM)
Horsepower shortcut: T(lb·ft) = HP × 5252 ÷ RPM
Design torque: design T = calculated T × service factor
Efficiency is applied as effective power = input power × efficiency ÷ 100.
How to Use This Calculator
- Enter the power value from your motor, engine, or shaft data.
- Select the matching power unit from the list.
- Enter actual shaft RPM, not only nameplate synchronous speed.
- Set efficiency to 100 when power is already output power.
- Enter a service factor for shock, duty, or design margin.
- Choose your required torque unit and decimal places.
- Press the calculate button and read the result above the form.
- Download the CSV or print the page for records.
Understanding Torque From RPM and Power
Why This Calculation Matters
Torque matters because it shows twisting force. Motors and engines may list power first. Speed usually appears as revolutions per minute. Those two values can reveal the torque at a shaft. This calculator connects them in one clear workflow. It is useful for pumps, gearboxes, conveyors, fans, cutters, vehicles, and generators.
Power is the rate of doing work. RPM is rotational speed. Torque is the turning effort that creates rotation or resists load. A high speed with the same power gives lower torque. A low speed with the same power gives higher torque. This relationship explains why gear reduction increases usable pulling force.
How the Formula Works
The core formula converts RPM into angular velocity. Angular velocity uses radians per second. The calculator then divides power by angular velocity. That gives torque in newton meters. Other units are converted after that step. Mechanical horsepower uses a familiar shortcut. Torque in pound feet equals horsepower times 5252 divided by RPM.
Advanced checks need more than one final number. Efficiency can adjust input power to estimated shaft power. A service factor can raise torque for design checks. This helps when loads start hard, shock suddenly, or run for long hours. The result should not replace manufacturer data. It gives a structured estimate for early sizing.
Choosing Practical Units
Unit choice is also important. Kilowatts are common in motor plates. Horsepower appears on many engines. Watts work well for small machines. Newton meters suit metric design. Pound feet suit many automotive and industrial references. Pound inches are useful for small shafts, couplings, and fasteners.
Always check the RPM source. Motor synchronous speed is not always actual shaft speed. Slip, belts, gearboxes, and variable drives can change the real RPM. Power ratings can also be peak, continuous, brake, input, or output values. The best torque estimate uses continuous output power and measured shaft speed.
Design Notes
Use the example table to compare realistic values. A small motor can spin fast yet produce modest torque. A large low speed drive may deliver heavy torque. These comparisons help users notice mistakes. They also help users select safer factors before buying parts or sizing shafts.
For critical equipment, confirm thermal limits, duty cycle, mounting, shaft diameter, coupling strength, bearing load, and braking needs. Torque is only one design value. Machines also need safety margins. Use this calculator to organize the first calculation. Then verify the design with standards, suppliers, and qualified engineers.
Good records make repeated sizing easier. Save the entered power, RPM, efficiency, and service factor with each result. Compare several operating points when speed varies. Many drives deliver constant power over one range and constant torque over another range. That behavior changes the final answer. When a machine accelerates, starting torque may exceed running torque. When a machine stalls, torque can rise sharply. These cases need protective devices, proper controls, and conservative mechanical choices.
Review values carefully before applying them to costly rotating equipment.
FAQs
What does this calculator find?
It finds torque from power and RPM. It also adjusts for efficiency and service factor when those values are entered.
Which formula is used?
It uses torque equals power divided by angular speed. Angular speed equals 2 times pi times RPM divided by 60.
Why does torque fall when RPM rises?
For the same power, faster rotation spreads work over more revolutions. Each revolution then carries less turning effort.
Can I use horsepower?
Yes. Choose mechanical horsepower or metric horsepower. The calculator converts it internally before finding torque.
What RPM should I enter?
Enter the actual shaft speed. Nameplate speed, gearbox output speed, belt speed, or measured tachometer speed may differ.
What does efficiency change?
Efficiency reduces input power to estimated shaft power. Use 100 percent when your power value is already output power.
What is service factor?
Service factor is a design multiplier. It raises calculated torque for load shock, frequent starts, or long duty.
Is the horsepower shortcut exact?
The 5252 shortcut is based on standard mechanical horsepower and pound feet. It is a convenient rounded engineering form.
Can this size a gearbox?
It can support early gearbox checks. You must also check ratio, duty, thermal rating, service class, and manufacturer limits.
Why are multiple torque units offered?
Different industries use different units. Metric designs often use newton meters. Automotive and shaft work may use pound feet or pound inches.
Can I trust the result for final design?
Use the output as an estimate for initial planning. Always verify values before using torque for equipment decisions.