What this calculator does
Find out how much power an engine makes, in horsepower and kilowatts. If you know the torque at a given engine speed, the calculator gives you the exact power at that speed. Torque is the twisting force on the crankshaft, and RPM is how many times it turns each minute. This is the same math a dyno uses to turn its torque reading into a horsepower curve.
No dyno sheet? You can also estimate power from a drag strip time slip. Enter the car's race weight with either its quarter-mile time or its trap speed. Two well-known drag-racing rules of thumb then give a rough horsepower figure. Those estimates are good for comparing cars and tracking changes, but they are only ballpark numbers.
Where it's used
Why this math matters outside the classroom:
- Dyno sheets. A tuner reads 350 lb·ft at 4,500 RPM and works out 350 × 4,500 ÷ 5,252, which is about 299.9 hp.
- Drag racing. A racer with a 3,000 lb car running a 12.0 second quarter mile estimates 3,000 × (5.825 ÷ 12)³, about 343 hp.
- Physics class. Students find that 400 N·m at 6,000 RPM is 400 × 2π × 6,000 ÷ 60, or about 251.3 kW.
- Car shopping. A buyer converts a 250 kW European rating to about 335 hp by dividing by 0.7457.
- Small engines. A mechanic checks that a mower engine making 8 lb·ft at 3,600 RPM produces about 5.5 hp.
Grade level and standards: Grades 8–9; Common Core State Standards for Mathematics HSN.Q.A.1. © Copyright 2010. National Governors Association Center for Best Practices and Council of Chief State School Officers. All rights reserved.
How to use it
- Choose what you know: torque and RPM, quarter-mile time, or trap speed.
- For torque, enter the value and pick lb·ft or N·m, then enter the engine speed in RPM.
- For the drag-strip methods, enter the race weight, including the driver, and the time or the speed.
- Read the horsepower, kilowatts and metric horsepower (PS), and open the steps to see the working.
What the inputs mean
- Torque
- Use the torque at the exact RPM you enter. Peak torque and peak power usually happen at different engine speeds, so mixing them gives a wrong answer.
- Engine speed
- Engine revolutions per minute, as shown on a tachometer or a dyno sheet.
- Race weight (car plus driver)
- Race weight is the car as it ran, with the driver and fuel on board. A heavier car needs more power to run the same time.
- Quarter-mile elapsed time (ET)
- Elapsed time (ET) is the time from the start line to the end of the quarter mile (402 m). It's printed on the time slip.
- Trap speed at the end of the quarter mile
- Trap speed is the car's speed as it crosses the finish of the quarter mile. It depends less on the launch than ET does, so it often gives a steadier estimate.
Formula and method
hp = torque (lb·ft) × RPM ÷ 5,252.11 kW = torque (N·m) × RPM ÷ 9,549.30 hp ≈ weight (lb) × (5.825 ÷ ET in seconds)³ hp ≈ weight (lb) × (trap speed in mph ÷ 234)³
Power is how fast work gets done: force times speed. For a spinning shaft, that's torque times how fast it turns. One horsepower is 33,000 foot-pounds of work per minute. One turn of the crank is 2π radians, where π (pi) is about 3.1416. So 33,000 ÷ 2π, about 5,252, links lb·ft and RPM to horsepower. In metric units, 60,000 ÷ 2π, about 9,549, does the same job for N·m and kilowatts.
The last two lines are not physics formulas. They are rules of thumb that drag racers fitted to real time slips over many years. The small ³ means the number in brackets is cubed, or multiplied by itself three times. Cubing means a small change in time or speed makes a big change in the horsepower estimate.
Horsepower here is mechanical horsepower, 745.7 watts. Metric horsepower (PS), common in Europe and Japan, is slightly smaller at 735.5 watts.
Worked example
A dyno sheet shows 350 lb·ft of torque at 4,500 RPM.
- Torque times RPM: 350 × 4,500 = 1,575,000.
- Divide by 5,252.113: 1,575,000 ÷ 5,252.113 = 299.88 hp.
- In kilowatts: 299.88 × 0.7457 = 223.62 kW.
Result: 350 lb·ft at 4,500 RPM is 299.9 hp, or about 223.6 kW (304.0 metric horsepower).
Understanding the result
Torque and horsepower in lb·ft and hp are always equal at about 5,252 RPM. Below that speed, the torque number is larger than the horsepower number; above it, horsepower is larger. If a dyno chart doesn't show the two curves crossing there, the axes use different scales.
The drag-strip estimates describe the power the car actually put to use on that run. A poor launch, wheelspin or a headwind will make the ET estimate come out low. Trap speed is usually the better guide of the two.
Use the estimates to compare runs of the same car, before and after a change. Don't treat them as a precise engine rating.
Assumptions and limitations
- The torque and RPM are measured at the same moment, on the same run.
- Horsepower is mechanical horsepower (550 ft·lbf per second, about 745.7 W).
- The drag-strip rules assume a quarter mile (402 m) on a level track, with weight in pounds and speed in mph.
- The quarter-mile formulas are rough rules of thumb, not measurements. They don't separate engine power from power at the wheels.
- Gearing, traction, tires, aerodynamics and altitude all change real times and speeds, and none of them are in the drag-strip formulas.
- It doesn't correct dyno readings for weather. Different correction standards give somewhat different horsepower numbers.
Frequently asked questions
How do I convert torque to horsepower?
Multiply the torque in lb·ft by the RPM, then divide by 5,252. For example, 250 lb·ft at 6,000 RPM is 250 × 6,000 ÷ 5,252, about 285.6 hp. For N·m, multiply by RPM and divide by 9,549 to get kilowatts.
Why do torque and horsepower cross at 5,252 RPM?
Because 5,252 is the number you divide by. When the RPM equals 5,252, torque × 5,252 ÷ 5,252 leaves the torque number unchanged. So at that one speed, horsepower and lb·ft are the same number.
How much horsepower does it take to run an 11.5-second quarter mile?
By the ET rule of thumb, a 3,200 lb car needs about 3,200 × (5.825 ÷ 11.5)³, or roughly 416 hp. Real cars vary a lot with traction and gearing, so treat it as a rough guide.
Which drag-strip estimate is more reliable?
Trap speed, usually. ET depends heavily on the launch, so wheelspin or a slow reaction off the line can throw it off. Trap speed mostly reflects how hard the car pulls over the whole run.
How this calculator is checked
Method: Rotational power P = torque × angular speed: hp = lbf·ft × rpm ÷ (33,000 ÷ 2π) and kW = N·m × rpm ÷ (60,000 ÷ 2π), with 1 hp = 550 ft·lbf/s = 745.69987 W and 1 PS = 735.49875 W. Quarter-mile estimates use the common drag-racing rules of thumb hp ≈ weight (lb) × (5.825 ÷ ET)³ and hp ≈ weight (lb) × (mph ÷ 234)³, which are empirical fits, not derived formulas. The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-26. Found a problem? Tell us.
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