Horsepower (HP) is a unit of measurement for power, representing the rate at which work is done. One mechanical horsepower is defined as the power required to lift 550 pounds by one foot in one second. It's a common unit for rating the power output of engines and motors in cars, boats, and machinery.

How do I calculate horsepower from torque and RPM?

The most common formula to calculate horsepower from torque and engine speed (RPM) is: HP = (Torque × RPM) / 5252. In this formula, torque must be in pound-feet (lb-ft). This equation is fundamental in engine performance analysis.

What is shaft horsepower?

Shaft Horsepower (SHP) is the power delivered to a propeller or transmission shaft of an engine. It accounts for power losses that occur within the engine and its gearbox. It is calculated by multiplying the engine's brake horsepower (BHP) by the mechanical efficiency of the transmission system.

How do I convert horsepower to kilowatts?

Horsepower and kilowatts (kW) are both units of power. The conversion is straightforward: 1 mechanical horsepower is equal to approximately 0.7457 kilowatts. To convert from HP to kW, you multiply the horsepower value by 0.7457. To convert from kW to HP, you divide by 0.7457.

What is the difference between brake horsepower and wheel horsepower?

Brake Horsepower (BHP) is the power measured at the engine's crankshaft, before any losses from the drivetrain (transmission, driveshaft, etc.). Wheel Horsepower (WHP) is the power that is actually delivered to the wheels. WHP is always lower than BHP due to these drivetrain power losses, typically by 10-20%.

Can I use this calculator for cars, bikes, and boats?

Yes. The physics principles for calculating horsepower are universal. Whether you are working with a car engine, a motorcycle engine, or a marine engine, the relationships between torque, RPM, force, and speed are the same. Simply enter the correct values for your specific application to get an accurate power estimate.

Horsepower Calculator — Compute Engine HP, Torque & Speed

🏎️ Horsepower Calculator

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What Is Horsepower?

Horsepower (HP) is a unit of power that measures the rate at which work is done. It was originally coined by Scottish engineer James Watt to compare the power of steam engines with the power of draft horses. One mechanical horsepower is technically defined as the power needed to lift 550 pounds by one foot in one second. Today, it remains the standard unit for rating the power output of engines in cars, motorcycles, boats, and industrial machinery, providing a common benchmark for performance.

How to Calculate Horsepower from Torque and RPM

The most fundamental relationship in engine performance is between torque, RPM (Revolutions Per Minute), and horsepower. Torque is the rotational force an engine produces, while horsepower is the rate at which it does work. The two are directly linked by the classic formula:

$HP = \frac{Torque \times RPM}{5252}$

In this formula, torque is measured in pound-feet (lb-ft). The number 5252 is a constant derived from the definition of horsepower to make the units work together. This equation shows that for a given RPM, increasing torque increases horsepower. Similarly, for a given torque, increasing RPM also increases horsepower, up to the engine's peak operating speed.

Shaft Horsepower vs Brake Horsepower

When discussing engine power, you may encounter several terms:

Brake Horsepower (BHP): This is the power measured at the engine's crankshaft without any losses from the drivetrain (gearbox, driveshaft, differential, etc.). It represents the "raw" power of the engine itself.
Shaft Horsepower (SHP): This is the power delivered to a propeller shaft (in a boat or aircraft) or a transmission output shaft. It accounts for power losses within the engine's gearbox.
Wheel Horsepower (WHP): This is the power that is actually delivered to the driving wheels of a vehicle. It is always lower than BHP due to frictional losses in the entire drivetrain, typically by 10-20%.

This calculator can estimate Shaft Horsepower by allowing you to input an efficiency percentage, which represents the power losses between the engine and the shaft.

Horsepower to Kilowatts Conversion Explained

The kilowatt (kW) is the standard SI (International System of Units) unit for power. While horsepower is still widely used in the automotive industry, kilowatts are the global standard for scientific and engineering applications, especially for electric vehicles. The conversion is based on a fixed relationship:

1 Horsepower = 0.7457 Kilowatts

To convert from HP to kW, you multiply the horsepower value by 0.7457. To convert from kW to HP, you divide the kilowatt value by 0.7457. Our calculator provides this conversion automatically with every calculation.

Frequently Asked Questions

Why is 5252 the constant in the HP formula? It's a derived constant. One horsepower is 33,000 ft-lbf per minute. The circumference of a one-foot radius circle is 2π feet. So, Power = Torque × 2π × RPM. When you convert this to horsepower, the constant becomes (33,000 / 2π) ≈ 5252.
Can I calculate horsepower from 0-60 times? Not directly with a simple formula. Calculating HP from acceleration is much more complex as it involves vehicle weight, aerodynamic drag, rolling resistance, and gearing. The methods in this calculator are based on direct physical measurements of force, torque, or work.
What is a typical efficiency for SHP calculation? Drivetrain efficiency varies, but a manual transmission is typically 85-95% efficient, while an automatic transmission might be 80-90% efficient.

Disclaimer: This calculator provides estimates only and is not a substitute for professional engineering calculations or dynamometer testing. Results are based on the formulas and inputs provided.