Engine Horsepower
Free Engine Horsepower for automotive. Free online tool with accurate results using verified formulas.
Formula
HP = (Torque × RPM) / 5252
Horsepower equals torque (in lb-ft) multiplied by engine speed (RPM), divided by 5252. This constant comes from the definition of horsepower (33,000 ft-lb/min) and the conversion for rotational motion.
Worked Examples
Example 1: Calculate Horsepower from Dyno Results
Problem:An engine produces 380 lb-ft of torque at 5000 RPM. What is the horsepower at this point?
Solution:HP = (Torque × RPM) / 5252\n\nHP = (380 × 5000) / 5252\nHP = 1,900,000 / 5252\nHP = 361.8 HP\n\nAt 5000 RPM with 380 lb-ft, the engine produces 362 HP.
Result:361.8 HP at 5000 RPM
Example 2: Convert Between Power Units
Problem:A European car is rated at 250 PS. What is this in HP and kW?
Solution:PS to HP: Divide by 1.0139\nHP = 250 / 1.0139 = 246.6 HP\n\nHP to kW: Multiply by 0.7457\nkW = 246.6 × 0.7457 = 183.9 kW\n\nAlternatively, PS to kW directly:\nkW = 250 × 0.7355 = 183.9 kW
Result:250 PS = 246.6 HP = 183.9 kW
Example 3: Find Torque from Horsepower
Problem:An engine makes 500 HP at 6500 RPM. What torque is it producing?
Solution:Rearrange the formula:\nTorque = (HP × 5252) / RPM\n\nTorque = (500 × 5252) / 6500\nTorque = 2,626,000 / 6500\nTorque = 404 lb-ft\n\nNote: At higher RPM, you need less torque to make the same HP.
Result:404 lb-ft of torque at 6500 RPM
Frequently Asked Questions
How is horsepower calculated from torque and RPM?
The formula is HP = (Torque × RPM) / 5252. The constant 5252 is derived from unit conversions (33,000 ft-lb/min ÷ 2π). At exactly 5252 RPM, horsepower and torque are always equal. Below this RPM, torque exceeds HP; above it, HP exceeds torque. This relationship is fundamental to understanding engine performance - torque is the rotational force, while horsepower measures how quickly that force can do work.
What's the difference between horsepower (HP), brake horsepower (BHP), and PS?
HP (Horsepower) is the standard US measurement (1 HP = 745.7 watts). BHP (Brake Horsepower) measures power at the crankshaft via dynamometer - it's the 'real' engine output before drivetrain losses. Wheel HP is typically 15-20% less than crank HP. PS (Pferdestärke) is the metric horsepower used in Europe and Asia (1 PS = 735.5 watts). 1 HP = 1.0139 PS. kW (kilowatts) is the SI unit increasingly used globally (1 HP = 0.7457 kW).
How much power is lost between the engine and the wheels?
Drivetrain losses typically consume 15-25% of engine power. Manual transmissions lose about 15%. Automatic transmissions lose 18-22%. AWD systems lose 20-25% due to additional components. A car rated at 400 crank HP might put 300-340 HP to the wheels. Dyno testing at the wheels provides more accurate real-world power figures. This is why 'wheel horsepower' (WHP) numbers are always lower than manufacturer ratings.
What factors affect engine horsepower besides torque and RPM?
Many factors affect power output: Air density (hot air = less power, altitude reduces power ~3% per 1000 feet), fuel quality (higher octane enables more timing advance), engine temperature (optimal is 180-210°F), intake/exhaust restrictions, ignition timing, air/fuel ratio, compression ratio, and valve timing. Modifications like cold air intakes, exhaust systems, and tunes can add 5-20% power depending on the engine.