Engine Horsepower Calculator

Precautions

These formulas were fitted to drag-strip results and give a rough estimate of the power reaching the wheels, not a dyno-accurate figure. They assume a reasonably good launch, a typical drivetrain and normal air density. Wheelspin, a slow shift, a headwind, altitude, hot air, or an automatic transmission that is slow off the line all make the estimate too low, while a tailwind or a cool, dense day can make it too high.

Use the real race weight: the car with its driver and fuel, ideally weighed on a scale. Never try to measure acceleration on public roads; a sanctioned drag strip or test day is the only safe and legal place, and many tracks offer inexpensive test-and-tune sessions with accurate timing.

What Is Horsepower?

Horsepower is a measure of how fast an engine can do work. James Watt defined one horsepower as 33,000 foot-pounds of work per minute, the rate he estimated for a draft horse. For engines, horsepower is calculated from torque, the twisting force on the crankshaft, and engine speed: horsepower = torque in lb-ft × rpm ÷ 5,252. Because of that relationship, torque and horsepower curves always cross at 5,252 rpm when plotted in those units.

An engine's power varies with speed. The figure in specifications is the peak, reached at a particular rpm, often close to the redline for modern gasoline engines.

Measuring Horsepower

Manufacturers measure engine power on an engine dynamometer, with the engine on a test stand, which gives brake horsepower at the crankshaft. Tuners usually use a chassis dynamometer, where the car's driven wheels spin a heavy roller; this gives wheel horsepower, which is lower because some power is lost in the transmission, driveshafts and differential, typically 10% to 20%. Acceleration-based estimates like the ones above also measure power at the wheels.

ET method: hp = weight ÷ (ET ÷ 5.825)³ · Trap speed: hp = weight × (speed ÷ 234)³

weight

vehicle plus driver, in pounds

ET

quarter-mile elapsed time in seconds

speed

quarter-mile trap speed in mph

Gross or Net Horsepower

Before 1972, US manufacturers advertised gross horsepower, measured on a bare engine without air filter, alternator, water pump load or full exhaust system. Since then they have used net horsepower, measured with all the accessories and emissions equipment a production car carries, under the SAE J1349 standard. Net figures are much lower and more realistic, which is part of why some classic muscle cars appear more powerful on paper than they were on the road.

Horsepower Versus Torque

Torque is the rotational force the engine produces; horsepower is that force multiplied by how fast the engine turns. Torque determines how hard a car pulls at a given engine speed, which is what you feel when you press the accelerator at low rpm. Horsepower determines how much work can be done per second and therefore the top speed and the acceleration achievable when the gearbox keeps the engine near its power peak.

Diesel engines and turbocharged engines typically produce strong torque at low rpm, which makes them feel effortless for towing and city driving. High-revving naturally aspirated engines may have modest torque but produce high horsepower by turning fast. Electric motors deliver maximum torque from a standstill, which is why electric cars accelerate so quickly from a stop.

Gearing multiplies torque on its way to the wheels, so a car's acceleration depends on the power available across the rev range, the gear ratios and, crucially, the weight being moved. For comparing cars, the power-to-weight ratio is more meaningful than horsepower alone.

Horsepower and Performance

The table applies the formulas above to show the quarter-mile performance to expect at different power-to-weight ratios, measured in pounds per horsepower. Halving the weight per horsepower does not halve the time, because the elapsed time depends on the cube root of the ratio.

Frequently asked questions