Horsepower Calculator

About Horsepower Calculator

The Horsepower Calculator computes engine power from torque and RPM using the standard mechanical horsepower formula. Enter any two of the three variables—torque, RPM, or horsepower—and the calculator instantly solves for the unknown with step-by-step math and comprehensive unit conversions.

The Horsepower Formula

The relationship between torque, RPM, and horsepower is defined by:

This formula can be rearranged to solve for any variable:

• Torque: Torque (lb-ft) = HP × 5,252 / RPM
• RPM: RPM = HP × 5,252 / Torque (lb-ft)

For metric units (Nm and kW), the equivalent formula is: kW = (Torque in Nm × RPM) / 9,549.

Why 5,252?

How to Use This Calculator

1. Choose what to solve for: Leave the unknown field blank—either Torque, RPM, or Horsepower.
2. Enter two known values: Fill in the two values you know. Select the torque unit (lb-ft or Nm).
3. Click Calculate: The calculator applies the appropriate formula and solves for the missing value.
4. Review results: See the answer with unit conversions (HP, kW, PS, Watts, BTU/hr), step-by-step solution, power gauge, and vehicle class indicator.

Horsepower vs. Torque

Torque measures rotational force—how hard the engine twists the crankshaft. Horsepower measures the rate of doing work—how fast that twisting force moves the vehicle. In practical terms:

• Torque determines acceleration from a standstill and towing capacity.
• Horsepower determines top speed and sustained high-speed performance.
• At 5,252 RPM, the HP and torque values (in lb-ft) always cross—below this RPM, torque is numerically higher; above it, HP is higher.

Types of Horsepower

Vehicle Power Classes

Frequently Asked Questions

What is the formula for horsepower?

The standard formula for mechanical horsepower is HP = (Torque in lb-ft × RPM) / 5,252. The constant 5,252 comes from the definition of one horsepower as 33,000 foot-pounds per minute divided by 2π. For metric units, kW = (Torque in Nm × RPM) / 9,549.

Why is 5252 used in the horsepower formula?

The constant 5,252 is derived from the original definition of horsepower. James Watt defined 1 HP as 33,000 foot-pounds of work per minute. Since torque is a rotational force, the conversion uses 33,000 divided by 2π, which equals approximately 5,252. This is also the RPM at which horsepower and torque values are numerically equal when measured in HP and lb-ft.

What is the difference between HP, kW, and PS?

HP (horsepower) is the Imperial unit of power equal to 745.7 watts. kW (kilowatt) is the SI metric unit equal to 1,000 watts. PS (Pferdestärke, or metric horsepower) is used in Europe and Japan and equals 735.5 watts. 1 HP equals approximately 0.7457 kW or 1.01387 PS.

What is the relationship between torque and horsepower?

Torque measures rotational force while horsepower measures the rate at which that force does work over time. They are related by RPM: HP = Torque × RPM / 5,252. Torque determines how much force an engine can exert, while horsepower determines how quickly that force can be applied. At exactly 5,252 RPM, the numerical values of HP and torque in lb-ft are equal.

How do I convert between lb-ft and Nm torque?

To convert from lb-ft to Nm, multiply by 1.35582. To convert from Nm to lb-ft, divide by 1.35582 (or multiply by 0.7376). For example, 300 lb-ft equals approximately 406.75 Nm, and 400 Nm equals approximately 295.04 lb-ft.

Additional Resources

• Horsepower - Wikipedia
• Torque - Wikipedia