Engine Horsepower Calculator

How Does the Formula Work?

This calculator has three modes. The first turns torque and engine speed into power, the second converts between mechanical horsepower (HP), kilowatts (kW) and metric horsepower (PS), and the third estimates quarter-mile time and trap speed from power and weight with the formulas Patrick Hale published in 1986.

Three Units for One Quantity

Horsepower, kilowatts and PS all measure power, the rate at which work is done. NIST's conversion tables put the mechanical horsepower, defined as 550 foot-pounds-force per second, at 745.6999 W, and the metric horsepower at 735.4988 W. The metric one is written PS in German, CV in Spanish and cv in Portuguese. The kilowatt is the SI unit itself. A rating of 200 PS therefore equals 147.1 kW or 197.3 HP. The calculator converts with these defined values instead of rounded factors, so the last decimal it prints is the true one.

Torque, Engine Speed and the 5,252 Crossing

Power is torque times angular speed. With torque in newton-metres and engine speed in revolutions per minute, multiplying by 2π and dividing by 60 gives watts. In foot-pounds the same relation reads HP = torque × RPM ÷ 5,252.1, since one horsepower is 33,000 foot-pounds per minute and each revolution sweeps 2π radians. That divisor is why a dyno chart drawn in lb-ft and HP shows the two curves meeting near 5,252 RPM: below it the torque number is larger, above it the horsepower number. An engine making 500 lb-ft at 2,000 RPM delivers 190.4 HP.

What Hale's Formula Can and Cannot Tell You

Patrick Hale, a drag racer and engineer, released his Quarter and Quarter jr programs in 1986 and published a pair of short estimates alongside them. Elapsed time is 5.825 × (weight/HP)^⅓ and trap speed is 234 × (HP/weight)^⅓, where weight means the car plus driver in pounds. Stealth 316, the page that documents those constants, set them against Road & Track tests of 243 street cars. Hale's times came out quicker than the cars really ran, and the speed formula matched the tests much better than the time formula did (R² of 0.91 against 0.62). Rimac lists its Nevera at 1,914 hp and 2,300 kg with a 402-metre time of 8.25 seconds; for those two figures the formula gives 8.1 seconds. Read the elapsed time as an optimistic first guess and the trap speed as the steadier number.

Power-to-Weight Ratio

Dividing power by mass in metric tonnes gives horsepower per ton, the figure the quarter-mile mode prints beside the time. The Nevera's 1,914 hp over 2,300 kg comes to 832 HP/ton. Hale's formula sees only this ratio, so two cars that share it get the same elapsed time and the same trap speed whatever their size.