Use this motor power calculator to find shaft power, torque, speed, or mechanical output from electrical inputs. It supports DC, single-phase AC, and three-phase AC motors with kW, HP, Nm, lb-ft, voltage, current, PF, and efficiency inputs.
Assumptions & Formulas
– Power (W) = (Torque in Nm × Speed in RPM) / 9.5488
– Torque (Nm) = (Power in W × 9.5488) / Speed in RPM
– Speed (RPM) = (Power in W × 9.5488) / Torque in Nm
Note: The constant 9.5488 is derived from 60 / (2 × π), converting RPM to radians per second.
Note: Results are mathematically precise based on inputs and are rounded to 2 decimal places. 1 HP = 745.7 Watts. 1 kW = 1000 Watts.
This motor power calculator finds mechanical shaft power, motor torque, or motor speed using the standard torque-speed-power relationship. It also estimates mechanical output power from electrical inputs using voltage, current, efficiency, and power factor for DC, single-phase AC, and three-phase AC motors.
You can use this tool to calculate power from torque and RPM, torque from power and RPM, speed from power and torque, or output power from voltage, current, efficiency, and power factor. Calculating these values accurately helps ensure the numerical relationships between your inputs are mathematically sound based on standard industrial formulas.
The calculator supports multiple output units, including watts (W), kilowatts (kW), and horsepower (HP) for power, alongside Newton-meters (Nm), kilogram-centimeters (kg-cm), and pound-feet (lb-ft) for torque.
Whether you need a motor torque calculator, a shaft power calculator, a power torque speed calculator, or an electric motor power calculator, this tool gives you the exact numbers required.
What this motor power calculator calculates
Knowing which mode to select depends on the nameplate data you have available and the specific unknown value you need to find.
| Calculation mode | Inputs required | Output shown | Notes |
|---|---|---|---|
| Mechanical shaft power | Torque + motor speed | Power | Finds running power based on shaft torque and RPM. |
| Motor torque | Mechanical shaft power + motor speed | Torque | Useful for finding rated running torque when power and RPM are known. |
| Motor speed | Mechanical shaft power + torque | Speed | Useful for back-solving RPM from known power and torque. |
| Mechanical output from electrical inputs | Voltage + current + efficiency + power factor for AC, or voltage + current + efficiency for DC | Mechanical output power and electrical input power | Supports three-phase, single-phase, and DC motor inputs. |
This tool is strictly for power–torque–speed relationships and electrical input power conversion. It is not designed for conveyor load sizing, pump total dynamic head calculations, NEC conductor sizing, or complete motor selection.
Motor power formula used by this calculator
Motor power represents the rate at which work is done at the spinning shaft. It is calculated from torque and speed using:$$P(W) = \frac{T(Nm) \times RPM}{9.5488}$$
An equivalent standard form of this mathematical relationship is:$$P(W) = \frac{2\pi \times RPM \times T}{60}$$
In these formulas: $P$ is mechanical shaft power in watts $T$ is running torque in newton-meters $RPM$ is shaft speed in revolutions per minute $9.5488$ is a conversion constant derived from $60 / (2\pi)$ to convert revolutions per minute into radians per second.
Torque formula from power and RPM
Torque is the rotational twisting force the motor applies. To find torque when power and speed are known, the formula is:$$T(Nm) = \frac{P(W) \times 9.5488}{RPM}$$
If your power is already listed in kilowatts, the formula is:$$T(Nm) = \frac{9550 \times P(kW)}{RPM}$$
This calculation helps verify the torque output when your motor nameplate provides rated kW and rated RPM.
Speed formula from power and torque
To calculate shaft speed from known power and measured torque, the formula is:$$RPM = \frac{P(W) \times 9.5488}{T(Nm)}$$
For power expressed in kilowatts, the formula becomes:$$RPM = \frac{9550 \times P(kW)}{T(Nm)}$$
You need this formula when checking the mathematical RPM relationship at a specific power and torque limit.
Electrical input to mechanical output power formulas
For three-phase AC motors, electrical input power is calculated as:$$P_{in}(W) = \sqrt{3} \times V_{LL} \times I \times PF$$
For single-phase AC motors:$$P_{in}(W) = V \times I \times PF$$
For DC motors:$$P_{in}(W) = V \times I$$
To find the actual mechanical output power available at the shaft, multiply the electrical input by the motor’s efficiency:$$P_{out}(W) = P_{in}(W) \times \frac{\eta}{100}$$
When using the electrical mode, the calculator assumes three-phase voltage is line-to-line voltage ($V_{LL}$). Efficiency ($\eta$) must be entered as a percentage.
Inputs and units supported
This tool handles conversions internally so you can input values as they appear on your equipment.
| Input / output | Units supported | Notes |
|---|---|---|
| Power | W, kW, HP | HP is converted internally to watts. |
| Torque | Nm, kg-cm, lb-ft | Converted internally to Nm for all math processing. |
| Speed | RPM | Required for shaft power, torque, and speed modes. |
| Voltage | V, kV | Three-phase electrical mode requires line-to-line voltage. |
| Current | A, mA | Used in electrical mode to calculate input power. |
| Efficiency | % | Must be greater than 0 and up to 100. |
| Power factor | 0 to 1 | Used to account for phase shifts in AC modes. |
Unit conversions used in this calculator
The calculator converts all supported power inputs to watts and all torque inputs to newton-meters before solving the core formulas.
| Conversion | Value used |
|---|---|
| 1 kW | 1000 W |
| 1 HP | 745.699872 W |
| 1 lb-ft | 1.35581795 Nm |
| 1 kg-cm | 0.0980665 Nm |
How to use this motor power calculator
Follow these steps to get precise results based on your available data:
- Choose the correct calculation method based on your unknown variable.
- Enter the known variables only for that specific mode.
- Pick the correct input units to match your available data.
- In electrical mode, choose the motor type first: DC, single-phase AC, or three-phase AC.
- Enter efficiency and power factor where required by the selected motor type.
- Read the result in your preferred output unit.
Worked examples for motor power, torque, speed, and output power
| Example | Inputs | Formula path | Result |
|---|---|---|---|
| Shaft power from torque and speed | 15.9 Nm, 3000 RPM | $P = T \times RPM / 9.5488$ | about 4,995 W or 5.00 kW |
| Torque from power and speed | 5 kW, 3000 RPM | $T = 9550 \times P / RPM$ | about 15.92 Nm |
| Speed from power and torque | 5 kW, 15.9 Nm | $RPM = 9550 \times P / T$ | about 3003 RPM |
| Three-phase output power | 400 V line-to-line, 15 A, PF 0.85, efficiency 90% | $P_{in} = \sqrt{3} V I PF$, then $P_{out} = P_{in} \eta$ | input about 8.83 kW, output about 7.95 kW |
| DC output power | 48 V, 50 A, efficiency 90% | $P_{in} = V I$, then $P_{out} = P_{in} \eta$ | input 2.40 kW, output 2.16 kW |
When to use shaft power vs electrical input power
Mechanical shaft power is the physical twisting energy the shaft delivers. Electrical input power is the total energy the motor draws from the power supply.
Electrical input power is usually higher than mechanical output power because of losses within the motor. The calculator’s electrical mode estimates both values simultaneously to give you a clear view of both the input requirement and the output result.
Three-phase, single-phase, and DC motor power differences
The calculator adjusts its required inputs based on the motor type because the core math changes depending on the power supply. Three-phase calculations require a $\sqrt{3}$ multiplier to account for the three overlapping power phases. Both AC modes require a power factor (PF) input to calculate true power from apparent power. DC circuits do not have this alternating wave phase shift, so the DC calculation uses a simpler direct voltage and current multiplier without power factor.
Limits, assumptions, and validation rules
To prevent impossible results, this calculator enforces strict mathematical rules based on real-world constraints.
- Torque, speed, power, voltage, and current must be greater than zero in active calculations.
- Efficiency must be strictly greater than 0 and less than or equal to 100.
- Power factor must be greater than 0 and up to 1 when AC mode is used.
- DC mode ignores the power factor completely.
- Speed warnings will appear above 20,000 RPM.
Common use cases for this motor power calculator
| Use case | Best mode to use |
|---|---|
| Find continuous shaft power from running torque and RPM | Mechanical Shaft Power |
| Back-calculate running torque from known kW and speed | Motor Torque |
| Estimate expected RPM at a given shaft power and torque | Motor Speed |
| Estimate mechanical motor output from volts and amps | Mechanical Output |
| Compare total drawn electrical power to output shaft power | Mechanical Output |
Motor power calculator FAQ
What is the formula for motor power from torque and speed?
Motor power is calculated as $$P(W) = T(Nm) \times RPM / 9.5488$$. This formula directly links the torque and rotational speed to the mechanical power being generated.
How do you calculate motor torque from power and RPM?
Motor torque is calculated as $T(Nm) = 9550 \times P(kW) / RPM$
. If your power is in watts instead of kilowatts, use the equivalent formula $T = P(W) \times 9.5488 / RPM$.What is the difference between shaft power and electrical input power?
Shaft power is the mechanical work delivered at the motor’s physical shaft. Electrical input power is the electrical energy drawn from the power source. Input power is usually higher than output shaft power because of inherent motor losses.
Why is my calculated electrical input power higher than my mechanical output?
If you calculate the electrical input using volts, amps, and power factor, the number will be higher than the actual output power. You must multiply that input power by the motor’s efficiency percentage to find the true mechanical output power available at the shaft.
How do you calculate three-phase motor power?
Three-phase motor input power is calculated as $P = \sqrt{3} \times V_{LL} \times I \times PF$. This calculator performs this math, and then multiplies the total input power by the efficiency percentage to estimate the usable mechanical output power.
Do I use line-to-line voltage or phase voltage for three-phase motor calculations?
This calculator uses line-to-line voltage ($V_{LL}$) in three-phase mode.
Does power factor apply to DC motors?
No. DC electricity flows in a single direction without alternating waves, meaning power factor does not apply. Therefore, the DC mode in this calculator only uses voltage, current, and efficiency to estimate output power.
Is this a full motor sizing calculator?
No. This tool calculates precise power, torque, speed, and output relationships based on standard running conditions. It does not size a motor for application loads, starting inertia, duty cycles, or required service factors.
Related Tools & Calculators: