Horsepower Calculator — Free Online HP Calculator

How to Use the Horsepower Calculator

1. Select the mode: Use the radio buttons to choose between "Calculate HP from Torque & RPM" for computing power from engine measurements, or "Convert Between Units" for translating power values between different unit systems (HP, kW, PS, bhp).
2. Enter your values: In calculation mode, enter the engine torque in pound-feet (lb-ft) and the engine speed in revolutions per minute (RPM). In conversion mode, enter the power value and select the source unit from the dropdown, then select the target unit you want to convert to.
3. Read the result: The result panel displays the calculated horsepower in large text. In calculation mode, the formula is shown with your specific values substituted in, making the math transparent. In conversion mode, the exact conversion is displayed.
4. Review all unit equivalents: Both modes show an "All Units" panel with the power expressed simultaneously in HP, kW, PS, and bhp. This is particularly useful when comparing vehicle specifications from different countries that use different unit systems.

The calculator handles real-time updates as you change inputs, making it easy to explore how varying torque or RPM affects horsepower output. This is useful for understanding engine power curves and comparing different engine configurations.

Horsepower Formulas

Variables Explained

• Torque (lb-ft): The rotational force produced by the engine, measured in pound-feet. Higher torque means stronger pulling ability. Torque varies with RPM and is measured on a dynamometer.
• RPM (revolutions per minute): The rotational speed of the engine's crankshaft. Higher RPM means the engine is spinning faster. Most passenger car engines operate between 600 RPM (idle) and 6,000-8,000 RPM (redline).
• 5,252 (constant): The conversion factor derived from 33,000 / (2π). It represents the RPM at which torque (in lb-ft) and horsepower are numerically equal.

Step-by-Step Example

A sports car engine produces 350 lb-ft of torque at 4,500 RPM. Calculate the horsepower:

1. Apply the formula: HP = (Torque × RPM) / 5,252
2. Substitute values: HP = (350 × 4,500) / 5,252
3. Calculate numerator: 350 × 4,500 = 1,575,000
4. Divide: 1,575,000 / 5,252 = 299.9 HP

At 4,500 RPM with 350 lb-ft of torque, the engine produces approximately 300 HP. Note that this is the power at this specific RPM point. Peak HP typically occurs at a higher RPM where the torque-times-RPM product is maximized.

Practical Examples

Example 1: Comparing Engine Specs Across Countries

Ryan is importing a European car listed at 220 PS and wants to know the equivalent in US horsepower and kilowatts. Using the conversion mode:

• 220 PS × (1 HP / 1.01387 PS) = 217 HP
• 217 HP × 0.7457 = 161.8 kW

The car produces 217 HP or 162 kW. This conversion is essential when shopping for imported vehicles, as European manufacturers use PS, Japanese manufacturers often use kW, and American manufacturers use HP. The differences are small but matter for accurate comparisons.

Example 2: Understanding a Dyno Chart

Nina's car was tested on a dynamometer, producing peak torque of 280 lb-ft at 3,800 RPM and peak HP at 5,400 RPM with 260 lb-ft. She calculates the power at both points:

• At peak torque: HP = (280 × 3,800) / 5,252 = 202.6 HP
• At peak HP: HP = (260 × 5,400) / 5,252 = 267.3 HP

Even though torque is lower at 5,400 RPM, the higher RPM compensates to produce peak horsepower. This illustrates why peak torque and peak horsepower occur at different RPMs. For more engine calculations, try our engine horsepower estimator.

Example 3: Electric Motor for Industrial Application

Greg needs to specify an electric motor for a factory conveyor system. The system requires 15 kW of continuous power. He converts to horsepower to match US motor catalogs:

• 15 kW / 0.7457 = 20.1 HP
• Standard motor sizes: 20 HP or 25 HP

Greg selects a 25 HP motor to provide a safety margin. Industrial motors are typically available in standard sizes (1, 2, 3, 5, 7.5, 10, 15, 20, 25, 30, 40, 50 HP), so engineers often need to convert from metric specifications to find the right standard motor.

Example 4: Motorcycle Engine Comparison

Yuki is comparing two motorcycles: Bike A makes 60 lb-ft at 6,000 RPM and Bike B makes 45 lb-ft at 9,500 RPM. She wants to know which has more peak HP:

• Bike A: (60 × 6,000) / 5,252 = 68.5 HP
• Bike B: (45 × 9,500) / 5,252 = 81.4 HP

Despite having 25% less torque, Bike B produces nearly 19% more horsepower by revving much higher. This is typical of sport bikes versus cruiser motorcycles. Cruisers favor low-RPM torque for relaxed riding, while sport bikes maximize HP through high RPM for speed.

Power Unit Conversion Reference Table

Tips and Complete Guide

Understanding Dynamometer Testing

A dynamometer (dyno) measures engine power by applying a controlled load while recording torque at various RPMs. There are two main types: engine dynos that connect directly to the engine's flywheel (measuring brake HP) and chassis dynos where the vehicle drives its wheels on rollers (measuring wheel HP). Chassis dyno results are typically 10-25% lower than engine dyno results due to drivetrain losses. When comparing vehicle specifications, always note whether the HP figure is at the engine or at the wheels.

HP vs. Torque: What Matters More?

The age-old debate has a nuanced answer. Torque determines how quickly a vehicle can accelerate at any given RPM — it is the instantaneous force. Horsepower determines how fast a vehicle can go and how quickly it can accelerate across the entire RPM range. For towing and off-road driving, torque matters more because you need sustained force at low speeds. For track performance and top speed, horsepower matters more because it measures total work capacity over time. In reality, both are derived from the same measurement, and the relationship HP = Torque × RPM / 5,252 means you cannot have one without the other.

Historical Context of Horsepower

James Watt coined the term around 1782 to market his improved steam engines. He observed that a draft horse could turn a mill wheel 144 times per hour, moving 2.4 feet per revolution with 180 pounds of force. This worked out to 33,000 ft-lbs/min, which became one horsepower. The unit was specifically chosen as a marketing tool to help mine owners understand the value of steam engines by comparing them to the horses they would replace. For other physics calculations, see our Ohm's law calculator.

Common Mistakes to Avoid

• Confusing torque with horsepower: A truck with 500 lb-ft of torque and 300 HP is not more powerful than a sports car with 300 lb-ft and 500 HP. The HP rating is what determines total power output. Torque tells you about low-RPM pulling ability specifically.
• Comparing engine HP to wheel HP: A car rated at 400 HP by the manufacturer will typically show 320-360 HP on a chassis dyno. This does not mean the engine is weak; it reflects normal drivetrain losses of 10-20%.
• Ignoring the RPM factor: Two engines with equal torque but different redlines will produce different peak HP. An engine with 200 lb-ft at 7,500 RPM produces 286 HP, while 200 lb-ft at 4,500 RPM produces only 171 HP.
• Mixing up HP types: There are several types of horsepower. Mechanical HP (745.7W), metric PS (735.5W), and electric HP (746W) are all slightly different. Always specify which type when communicating specifications.
• Assuming more HP always means faster: Vehicle weight matters enormously. A 200 HP car weighing 2,500 lbs will likely outperform a 300 HP car weighing 5,000 lbs. Power-to-weight ratio is the key performance metric.

Frequently Asked Questions