Calculator input
Tip: horsepower depends on both torque and rotational speed. Higher torque at the same speed produces more power.
Plotly graph
This chart shows horsepower versus RPM using your converted torque and selected efficiency.
Example data table
| Torque | Speed | Efficiency | Approx. mechanical hp | Use case note |
|---|---|---|---|---|
| 40 lb-ft | 2500 RPM | 100% | 19.04 hp | Simple shaft comparison example. |
| 120 N·m | 1500 RPM | 100% | 25.28 hp | Common industrial motor estimate. |
| 75 lb-ft | 3600 RPM | 95% | 48.84 hp | Useful for efficiency-adjusted delivered output. |
| 180 lb-ft | 5252 RPM | 100% | 180.00 hp | Classic torque-horsepower crossover reference. |
Formula used
Base power relation: Power = Torque × Angular Speed
SI form: Power (W) = Torque (N·m) × Speed (rad/s)
Mechanical horsepower: hp = Power (W) ÷ 745.69987158227
Metric horsepower: hp(M) = Power (W) ÷ 735.49875
With efficiency: Delivered Power = Ideal Power × Efficiency ÷ 100
Shortcut with N·m and RPM: hp ≈ Torque (N·m) × RPM ÷ 7120.909237 × Efficiency factor
Shortcut with lb-ft and RPM: hp ≈ Torque (lb-ft) × RPM ÷ 5252.113122 × Efficiency factor
How to use this calculator
- Enter the torque value from your source data.
- Select the matching torque unit.
- Enter the rotational speed.
- Choose the correct speed unit.
- Add efficiency if you want delivered output instead of ideal shaft power.
- Select your preferred decimal precision.
- Press the convert button to show results above the form.
- Use the chart and export buttons for review or reporting.
Frequently asked questions
1) Why are both torque and speed required?
Horsepower is a rate of doing work. Torque alone does not define power. Rotational speed determines how often that torque is applied each second, so both values are necessary.
2) What does efficiency change in the result?
Efficiency reduces ideal power to a more realistic delivered output. This is useful for gearboxes, belts, drive systems, and machines where losses prevent all input power from reaching the load.
3) Which horsepower value should I use?
Mechanical horsepower is usually used in engines, motors, and shaft power comparisons. Metric horsepower appears in some international specifications. Electrical horsepower can help compare with electrical machine ratings.
4) Why does horsepower rise with RPM on the chart?
The graph keeps torque constant. When torque stays fixed, higher rotational speed increases angular velocity, and power grows proportionally. If torque changed with speed, the curve shape could look different.
5) Can I use this for engines and electric motors?
Yes. The same physical relation applies to both. Just use the torque at a given operating speed and include an efficiency estimate when you want a delivered output instead of an ideal value.
6) What happens if speed is zero?
The calculator returns zero horsepower because no rotational work is being completed over time. Torque may still exist at stall, but power output becomes zero when angular speed is zero.
7) Why do N·m and lb-ft give the same answer after conversion?
They represent the same physical quantity in different unit systems. Once converted correctly into a common base unit, the power result should match apart from rounding differences.
8) Is the result exact?
The math is exact within the chosen constants and rounding rules. Real machines may differ because torque varies with load, speed fluctuates, and efficiency changes across the operating range.