How do I calculate horsepower from torque and RPM?

Horsepower = (Torque in lb-ft x RPM) / 5252. For example, an engine producing 300 lb-ft of torque at 4,000 RPM generates 300 x 4000 / 5252 = 228.5 HP. The constant 5252 comes from the relationship between rotational force and power.

How do I calculate HP from quarter mile time?

Using the Hale formula: HP = Weight / (ET / 5.825)^3, where ET is the elapsed time in seconds and Weight is the vehicle weight in pounds. This estimates wheel horsepower based on actual track performance.

What is the difference between HP and kW?

One mechanical horsepower equals 0.7457 kilowatts. Kilowatts are the metric/SI unit of power, while horsepower is commonly used in the US and UK for engines. Electric motors are often rated in both units.

How do I calculate electrical horsepower?

Electrical power in watts = Volts x Amps x Efficiency. Convert watts to horsepower by dividing by 746. For example, a motor running on 240V drawing 10A at 90% efficiency produces 240 x 10 x 0.9 / 746 = 2.9 HP.

Free Horsepower Calculator — HP from Torque

Calculate horsepower using torque & RPM, weight & quarter mile time, or electrical inputs. Convert between HP and kilowatts.

How to Calculate Horsepower

Horsepower is a unit of power that measures the rate at which work is done. Originally defined by James Watt in the 18th century to compare steam engine output to draft horses, it remains the standard unit for rating engines and motors in the United States. This calculator provides three different methods to determine horsepower depending on what information you have available.

Method 1: Torque and RPM

The most common formula is HP = (Torque x RPM) / 5252. Torque is measured in pound-feet (lb-ft) and represents rotational force. RPM is revolutions per minute. The constant 5252 comes from converting foot-pounds per minute to horsepower (33,000 ft-lbs/min divided by 2 x pi). This method is used for engine dynamometer results.

Method 2: Weight and Quarter Mile

The Hale formula estimates horsepower from drag strip performance: HP = Weight / (ET / 5.825)^3, where ET is the elapsed time in seconds. This gives wheel horsepower, which accounts for drivetrain losses. It is useful for estimating the actual output of modified vehicles based on real-world performance.

Method 3: Electrical

For electric motors, power in watts equals Volts x Amps x Efficiency. Divide by 746 to convert watts to horsepower. Efficiency accounts for energy lost as heat in the motor windings. Typical electric motor efficiency ranges from 80-95% depending on size and design.

Horsepower Calculator

How to Calculate Horsepower

Method 1: Torque and RPM

Method 2: Weight and Quarter Mile

Method 3: Electrical

Frequently Asked Questions

Related Calculators