What This Calculator Does
The Torque to Horsepower Calculator converts an engine's rotational force (torque) at a given speed into power output, expressed in both horsepower (hp) and kilowatts (kW). It works with the two most common torque units worldwide — pound-feet (lb-ft), typical in the United States and the UK, and newton-metres (N-m), used across most metric countries. Whether you are tuning an engine, comparing dyno results, or studying for a mechanics exam, this tool gives you a quick, accurate power figure from torque and RPM.
The Inputs You Enter
- Torque — the twisting force your engine produces, measured at a specific speed.
- Engine Speed (RPM) — revolutions per minute at which that torque is measured.
- Torque Unit — choose lb-ft or N-m; the calculator applies the correct constant for whichever you select.
The Formula Explained
For pound-feet, the calculator uses the classic relationship:
$$\text{HP} = \frac{\text{Torque (lb-ft)} \times \text{RPM}}{5252}$$The number 5252 comes from dividing the conversion between work and power (33,000 ft-lb/min per hp) by \(2\pi\). It is also why torque and horsepower curves always cross at exactly 5252 RPM on a dyno chart.
When you select N-m, the tool divides by 7124.59 instead, which is the equivalent constant for metric torque. Finally, the result is converted to kilowatts by multiplying horsepower by 0.7457.
Worked Example
Suppose your engine produces 300 lb-ft of torque at 4000 RPM:
- $$\text{Horsepower} = \frac{300 \times 4000}{5252} = 228.5 \text{ hp}$$
- $$\text{Kilowatts} = 228.5 \times 0.7457 = 170.4 \text{ kW}$$
If you had entered 407 N-m (the same torque in metric) at 4000 RPM, dividing by 7124.59 gives roughly the same 228 hp.
Frequently Asked Questions
Why is the constant always 5252? It is fixed by the definition of horsepower and the math of converting rotational torque into power. Because of it, horsepower and torque are equal at 5252 RPM for any engine measured in lb-ft.
Does more torque always mean more horsepower? Not necessarily. Horsepower depends on both torque and RPM. A high-torque engine spinning slowly can make less power than a lower-torque engine that revs higher.
Can I convert horsepower back to torque? Yes — rearrange the formula: \(\text{Torque} = \frac{\text{Horsepower} \times 5252}{\text{RPM}}\) for lb-ft. This calculator focuses on the torque-to-power direction with automatic kW output.
