How to Calculate Horsepower
Five proven methods — from torque and RPM to quarter-mile times — with formulas and examples.
There's no single way to calculate horsepower — the right method depends on what you can measure. This guide covers the five most useful approaches, each with its formula and a linked calculator so you can compute it instantly.
Method 1: From Torque and RPM
This is the gold standard, the same calculation a dyno uses.
Method 2: From Quarter-Mile Trap Speed
If you have a drag-strip timeslip, trap speed (the MPH at the finish line) gives a strong horsepower estimate.
Method 3: From Elapsed Time (ET)
Elapsed time also estimates horsepower, though it's more sensitive to launch quality than trap speed. Use the HP from ET calculator.
Method 4: From Watts or Kilowatts
For electric motors or any power rated in watts, convert directly: divide watts by 745.7, or multiply kilowatts by 1.341. Our HP to kW calculator handles this both ways.
Method 5: From Electrical Input (Motors)
Which Method Should You Use?
| You have… | Use method |
|---|---|
| A dyno sheet (torque + RPM) | Torque & RPM |
| A drag-strip timeslip | Trap speed or ET |
| An electric motor nameplate | Electrical input |
| A power figure in watts/kW | Unit conversion |
Frequently Asked Questions
Measuring torque and RPM on a dynamometer and applying HP = (torque × RPM) ÷ 5252 is the most accurate, since it measures power directly rather than estimating it.
Yes. Trap speed gives HP = weight × (MPH ÷ 234)³, and elapsed time offers another estimate. Both are typically within 5–10% for a well-prepared car.
Divide watts by 745.7 to get mechanical horsepower, or multiply kilowatts by 1.341. This is exact, since it's a unit conversion.
For three-phase: HP = (Volts × Amps × √3 × power factor × efficiency) ÷ 746. For single-phase, remove the √3 term.
Yes. Horsepower depends on how fast the torque is delivered, so RPM is required. The same torque at higher RPM produces more horsepower.