Calculate BLDC motor torque using Kt and current, Kv with load and no-load current, or mechanical power and RPM. Get fast torque results in Nm, kg-cm, lb-ft, or oz-in.
Assumptions & Formulas
– Formula: Torque (Nm) = (Power in Watts × 9.5488) / Speed in RPM
– Calculates the direct mechanical torque produced at the motor shaft at a specific operating speed and mechanical power level.
Note: The constant 9.5488 is derived from 60 / (2 × π), which relates power (Watts), torque (Nm), and angular velocity (RPM). Results are mathematically rounded to 2 decimal places.
This calculator estimates or calculates brushless DC (BLDC) motor torque using three distinct methods: mechanical power and speed, torque constant (Kt) and motor current, or motor Kv with load and no-load current. You can view the final output in Newton-meters (Nm), kilogram-centimeters (kg-cm), pound-feet (lb-ft), or ounce-inches (oz-in) to match your project needs.
The Kt mode provides the most direct torque calculation when your motor’s datasheet supplies the torque constant. The Kv mode serves as an estimate when only the velocity constant is available, while the power and speed mode determines the actual shaft torque resulting from mechanical output conditions.
What this BLDC motor torque calculator calculates
| Calculation method | Tool inputs | What the tool returns | Best use case |
|---|---|---|---|
| Mechanical power and speed | Power (W, kW, HP), speed (RPM) | Mechanical shaft torque | When you know output power and shaft speed |
| BLDC torque from Kt and current | Kt (Nm/A), motor current (A, mA) | Calculated motor torque | When the motor datasheet gives torque constant |
| BLDC estimate from Kv and current | Kv (RPM/V), load current, no-load current | Ideal estimated torque | When Kt is not given and only Kv/current data is available |
How to calculate BLDC motor torque with this tool
The calculator offers three distinct paths depending on the motor specifications you have available.
If you know the mechanical power and motor speed, the tool calculates the torque acting on the shaft. This represents pure mechanical output. The formula is:$$T = \frac{P \times 60}{2\pi \times RPM}$$
or$$T = \frac{P \times 9.5488}{RPM}$$
Calculating torque from the torque constant (Kt) is the most direct current-to-torque method. If the manufacturer provides Kt, simply multiply it by the motor current:$$T = K_t \times I$$
When you only have the Kv rating, the tool derives an estimated Kt first. It then subtracts the no-load current from the load current to estimate the torque-producing current. Keep in mind this Kv mode is theoretical. Real-world torque may differ due to electronic speed controller (ESC) behavior, heating, magnetic saturation, and how the manufacturer defines their current ratings.$$K_t \approx \frac{60}{2\pi K_v} = \frac{9.5488}{K_v}$$$$T \approx \left(\frac{9.5488}{K_v}\right) \times (I_{load} – I_{no-load})$$
BLDC torque formulas used in this calculator
| Formula | Used by calculator when | Notes |
|---|---|---|
| $T = \frac{P \times 9.5488}{RPM}$ | Mechanical power and speed mode | Uses power converted to watts |
| $T = K_t \times I$ | Kt and current mode | Direct torque calculation |
| $K_t \approx \frac{9.5488}{K_v}$ | Kv estimate mode | Converts Kv in RPM/V to torque constant in Nm/A |
| $$T \approx \left(\frac{9.5488}{K_v}\right) \times (I_{load} – I_{no-load})$$ | Kv estimate mode | Uses no-load current subtraction |
Inputs and units this calculator accepts
| Input | Accepted unit(s) | Constraint in tool |
|---|---|---|
| Mechanical power | W, kW, HP | Must be greater than 0 |
| Motor speed | RPM | Must be greater than 0 |
| Torque constant | Nm/A | Must be greater than 0 |
| Motor current | A, mA | Must be greater than 0 |
| No-load current | A, mA | Must be 0 or more |
| Kv | RPM/V | Must be greater than 0 |
In Kv mode, the no-load current cannot exceed the load current. The calculator prevents this because the underlying math uses the difference between load and no-load current to determine the active torque-producing current.
Torque output units and conversions
| Output unit | Full name | When users prefer it |
|---|---|---|
| Nm | Newton-meter | Engineering and SI datasheets |
| kg-cm | Kilogram-centimeter | Hobby motors, servos, legacy specs |
| lb-ft | Pound-foot | Imperial mechanical specs |
| oz-in | Ounce-inch | Small motors and RC components |
The calculator computes torque internally in Newton-meters (Nm) and then converts the result to your selected display unit.
When to use Kt vs Kv to calculate BLDC torque
| If you know | Best method | Why |
|---|---|---|
| Kt and current | Use Kt mode | Most direct torque calculation |
| Kv, load current, and no-load current | Use Kv estimate mode | Useful when Kt is not published |
| Mechanical output power and RPM | Use power and speed mode | Best for shaft-side torque |
| Only voltage and Kv | Not enough for this tool | This calculator does not compute torque from voltage alone |
What affects BLDC torque accuracy
The accuracy of your result depends on how closely your inputs match real-world conditions. First, the current you input must match the basis used for the Kt rating. Kv-derived torque is strictly an estimate, not a measured output.
In physical applications, factors like ESC settings, winding resistance, temperature rise, and magnetic saturation can shift real torque away from theoretical calculations. Real measured values often differ materially from simple theoretical models due to these physical tolerances and datasheet conventions.
Example BLDC motor torque calculations
| Example | Inputs | Formula path | Result |
|---|---|---|---|
| Shaft torque from power and speed | 5 kW, 3000 RPM | $T = \frac{5000 \times 9.5488}{3000}$ | 15.91 Nm |
| BLDC torque from Kt and current | Kt = 0.05 Nm/A, I = 6 A | $T = 0.05 \times 6$ | 0.30 Nm |
| BLDC estimate from Kv and current | Kv = 900 RPM/V, I_load = 30 A, I_no-load = 2 A | $T \approx \left(\frac{9.5488}{900}\right) \times (30 – 2)$ | 0.30 Nm |
BLDC motor torque calculator limitations
- Does not accept voltage input.
- Does not factor in winding resistance.
- Does not calculate stall torque.
- Does not include an efficiency or thermal model.
- Does not model propeller thrust, speed constant under load, or controller-specific current behaviors.
- The Kv mode provides a theoretical estimate, not a dynamometer measurement.
BLDC motor torque calculator FAQs
What is torque constant Kt in a BLDC motor?
The torque constant (Kt) defines the amount of torque a motor produces per amp of current, usually expressed in Nm/A. This metric allows for a direct calculation of torque from the motor current. Keep in mind that for the most accurate result, the current you enter must match the basis used by the manufacturer for their Kt rating.
How do you calculate BLDC motor torque from current?
If the torque constant is known, you multiply it by the current using $T = K_t \times I$. If Kt is unknown but you have the Kv rating, you can derive an approximate Kt from Kv and use the Kv-based estimate method.
How do you convert Kv to Kt?
You can convert a Kv rating expressed in RPM/V to Kt in Nm/A using the relationship: $K_t \approx \frac{60}{2\pi K_v} = \frac{9.5488}{K_v}$.
Is torque from Kv and current accurate?
It is an ideal estimate. The actual torque your motor produces depends on controller settings, temperature changes, power losses, and how the manufacturer defines their specific Kv and current ratings.
What is no-load current in a BLDC torque estimate?
This calculator subtracts the no-load current from the total load current in Kv mode. This approximates the current contributing to torque in this model by accounting for baseline mechanical and magnetic losses.
Can I calculate torque from power and RPM?
Yes, you can determine mechanical shaft torque using the relationship: $T = \frac{P \times 9.5488}{RPM}$. This is a primary calculation path supported by the tool.
What units does this torque calculator support?
The tool supports Nm, kg-cm, lb-ft, and oz-in for output results. For inputs, it accepts W, kW, and HP for mechanical power, and A or mA for current.
Can no-load current be higher than load current?
No. This calculator blocks no-load current above load current because the Kv-based estimate assumes load current must be at least as high as no-load current to produce usable torque.
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