Quickly convert torque and RPM into real-world power with our Horsepower Calculator. Whether you’re tuning a car, sizing an electric motor, or analyzing dyno data, this tool gives you accurate horsepower (HP) and kilowatts (kW) in seconds.
What is horsepower and why does it matter?
Horsepower is a measure of how quickly work can be done. In automotive and industrial contexts, it expresses the rate at which an engine or motor can deliver mechanical energy. While torque tells you how much rotational force is available, horsepower combines torque with rotational speed to describe how much total power you can put to the ground or to a load. This is why an engine with moderate torque can still feel strong if it produces that torque at high RPM—power is the product of both.
How the Horsepower Calculator works
The calculator uses the standard relationships between torque, rotational speed, and power. You enter torque (in lb·ft or N·m) and RPM, choose your preferred output units, and the tool returns horsepower and/or kilowatts. Behind the scenes, it applies industry-standard constants and unit conversions for accurate, consistent results.
Formulas used
- Imperial (lb·ft): HP = (Torque(lb·ft) × RPM) ÷ 5252
- Metric (N·m): kW = (Torque(N·m) × RPM) ÷ 9549.296
- Unit conversion: 1 HP (mechanical) = 0.745699872 kW (approximately 745.7 W)
These constants come from the physics of rotational motion. Power equals torque multiplied by angular velocity. Since angular velocity is RPM converted to radians per second, the constants 5252 (for lb·ft) and 9549.296 (for N·m) appear when the unit conversions are applied.
When to use this calculator
- Comparing engine builds and cam profiles at different RPM points
- Converting dyno torque curves into horsepower curves
- Sizing motors for pumps, fans, or conveyors using known torque and speed
- Estimating power demand for racing, towing, or hill-climb scenarios
- Academic and engineering calculations where quick unit switching helps
Step-by-step example
Suppose an engine produces 300 lb·ft at 4,000 RPM. Using the imperial formula, HP = (300 × 4000) ÷ 5252 ? 228.2 HP. Converting to kW, multiply by 0.745699872 to get roughly 170.2 kW. If instead you know torque in N·m, say 400 N·m at 4,000 RPM, compute kW = (400 × 4000) ÷ 9549.296 ? 167.9 kW, which equals about 225.2 HP.
Tips for accurate inputs
- Use steady-state torque values if possible; transient spikes can overstate power.
- Ensure RPM is the shaft speed of the component delivering torque.
- If you have drivetrain losses, remember wheel horsepower will be lower than crank horsepower.
- For electric motors, use rated torque at the rated RPM for continuous power.
Why horsepower and torque both matter
Torque determines the immediate rotational force available, which influences launch and pulling power. Horsepower captures the capacity to sustain work at speed. Vehicles feel “strong” when they combine ample torque with the ability to hold that torque across a broad RPM range, resulting in higher horsepower. The Horsepower Calculator ties these concepts together to give you a complete picture of performance.
Common unit questions
- What’s the difference between HP and kW? They’re both measures of power; kW is SI, HP is imperial. 1 HP ? 0.7457 kW.
- What about PS or CV? Those are metric horsepower variants slightly different from mechanical HP; we convert using mechanical HP.
- Should I use lb·ft or N·m? Use whatever your data source provides; the calculator converts either way.
Get started
Enter your torque and RPM above, pick your units, and click Calculate. The Horsepower Calculator will instantly return power in HP and/or kW with optional rounding, making it ideal for quick comparisons, reports, and engineering checks.