Motor Torque Calculator
Determine electric motor shaft torque using mechanical output power and rotational speed. Supports standard kW and horsepower equations with instant conversion.
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Motor Torque Calculator
Active Formula: Torque (Nm) = (9550 × Power kW) ÷ RPM
Calculated torque represents theoretical shaft torque based on mechanical output power and speed. Actual torque can vary due to efficiency, service factor, load conditions, and motor losses.
How to Use Motor Torque Calculator
Determining the rotational force of an electric motor helps mechanical and electrical engineers size gears, couplings, pulleys, and shafts correctly. Computing nominal shaft torque ensures that industrial equipment like a pump motor, fan motor, or conveyor motor operates within safe mechanical limits. Use the following simple steps to calculate your motor's torque output:
- Step 1: Select power unit. Choose either Kilowatts (kW) or Horsepower (HP) based on your motor's nameplate specification.
- Step 2: Enter motor power. Input the rated mechanical output power. You can verify input power requirements using a motor power calculator if needed.
- Step 3: Enter motor speed. Input the rated rotational speed of the motor in revolutions per minute (RPM).
- Step 4: Click Calculate. Press the "Calculate Torque" button to execute the formulas.
- Step 5: Read torque output. View the computed shaft torque in Newton-meters (Nm) and pound-feet (lb-ft) instantly.
These results are vital when analyzing an induction motor or an AC motor under constant load. If you also need to estimate winding configurations or full-load current ratings, refer to our motor FLA calculator or check running currents using a standard motor current calculator.
How to Calculate Motor Torque
Calculating motor torque relies on the mechanical relationship between power, speed, and rotational force. Torque represents the twisting moment delivered at the motor shaft. In physics, mechanical power is defined as the product of torque and angular velocity. Re-arranging this relationship allows us to solve for torque based on motor power and RPM.
Formula 1 — Sizing in Kilowatts (kW)
When motor output power is given in kilowatts (kW), the torque in Newton-meters (Nm) is calculated using the following formula:
Where:
- Power (kW) represents the motor's mechanical output power.
- RPM represents the motor's rotational speed in revolutions per minute.
- 9550 is a rounded constant derived from converting units: (60 seconds/min × 1000 watts/kW) ÷ (2 × π radians/revolution) ≈ 9549.297.
Formula 2 — Sizing in Horsepower (HP)
When motor output power is given in horsepower (HP), the torque in Newton-meters (Nm) can be calculated using the alternative constant:
Where:
- Power (HP) represents the motor's horsepower rating.
- 7127 is a constant that converts imperial horsepower to Newton-meters based on rotational speed.
Unit Conversion to Pound-Feet (lb-ft)
To convert Newton-meters to imperial pound-feet (lb-ft), the standard conversion factor is applied:
Step-by-Step Engineering Worked Example
Let's calculate the shaft torque for a motor with the following nameplate values:
- Motor Output Power: 15 kW
- Motor Speed: 1450 RPM
Step 1 — Calculate Torque in Newton-meters (Nm)
Use Formula 1 to calculate metric torque:
Torque (Nm) = 143,250 ÷ 1450 = 98.79 Nm
Step 2 — Convert Torque to Pound-Feet (lb-ft)
Apply the imperial unit conversion factor:
Torque (lb-ft) = 72.86 lb-ft
Final Verified Results
- Metric Shaft Torque: 98.79 Nm
- Imperial Shaft Torque: 72.86 lb-ft
- Speed Rating: 1450 RPM
Motor Torque Chart
This reference chart displays theoretical motor torque values in Newton-meters (Nm) for common motor power ratings at a standard speed of 1440 RPM (typical for 4-pole induction motors running on 50Hz grids).
| Power (kW) | RPM | Torque (Nm) |
|---|---|---|
| 0.75 kW | 1440 RPM | 4.97 Nm |
| 1.5 kW | 1440 RPM | 9.95 Nm |
| 3.0 kW | 1440 RPM | 19.90 Nm |
| 5.5 kW | 1440 RPM | 36.48 Nm |
| 7.5 kW | 1440 RPM | 49.74 Nm |
| 11.0 kW | 1440 RPM | 72.95 Nm |
| 15.0 kW | 1440 RPM | 99.48 Nm |
| 22.0 kW | 1440 RPM | 145.90 Nm |
| 30.0 kW | 1440 RPM | 198.96 Nm |
| 45.0 kW | 1440 RPM | 298.44 Nm |
Note: All calculations in the table are theoretical estimates based on standard mechanical formulas. Actual torque delivered to the load will vary based on mechanical efficiency, gearbox ratios, service factors, and electrical supply stability.
Motor Torque Calculator Frequently Asked Questions
To calculate motor torque from power in kilowatts (kW), use the standard engineering formula: Torque (Nm) = (9550 x Power in kW) / Speed in RPM. This equation stems from the mechanical relationship between rotational power, speed, and torque, where the constant 9550 accounts for unit conversions between kilowatts, revolutions per minute, and Newton-meters.
The torque of a 15 kW motor depends entirely on its rotational speed (RPM). For instance, a typical 4-pole induction motor running at 1450 RPM will produce approximately 98.79 Nm (72.86 lb-ft) of shaft torque. If the same 15 kW motor runs at a lower speed, such as 960 RPM, its torque output increases to about 149.22 Nm.
For a constant power output, torque and rotational speed (RPM) are inversely proportional. This is because mechanical power is the product of torque and angular velocity. If the speed of the motor decreases, the torque must increase to maintain the same power rating. Conversely, higher speeds result in lower torque for the same electrical power input.
Torque represents the rotational force or twisting effort produced by the motor shaft, measured in Newton-meters (Nm) or pound-feet (lb-ft). Power represents the rate at which work is performed over time, measured in kilowatts (kW) or horsepower (HP). In short, torque is the mechanical strength, whereas power is the speed at which that strength is applied.
Yes, motor shaft torque can be calculated directly from horsepower (HP) and speed (RPM) using the formula: Torque (Nm) = (7127 x HP) / RPM. To obtain the torque in pound-feet (lb-ft), the alternative formula Torque (lb-ft) = (5252 x HP) / RPM can be used. These constants reconcile the differences between metric and imperial measurement units.
The standard International System of Units (SI) unit for motor torque is the Newton-meter (Nm). In the United States and other regions using the imperial system, the pound-foot (lb-ft) is commonly utilized. One Newton-meter is equal to approximately 0.73756 pound-feet, allowing for straightforward conversion between the two measurement systems.
Yes, motor efficiency affects the actual torque delivered at the shaft. The torque formulas use the mechanical output power of the motor. If you are calculating torque based on electrical input power, you must multiply the input power by the motor efficiency rating (expressed as a decimal) to determine the true mechanical output power before calculating torque.
Motor torque is a critical parameter in mechanical design and industrial applications because it determines the motor's ability to start and drive a load. Gearboxes, couplings, shafts, belts, and driven equipment like pumps, fans, and conveyors must be rated to handle the motor's peak and continuous torque output to prevent mechanical failure and ensure reliable operation.