Horsepower & Torque Calculator
Turn torque and RPM into horsepower (or back), convert hp, PS and kW, estimate quarter-mile times from power and weight, and work out wheel versus crank horsepower.
Horsepower and torque (in lb-ft) are always equal at 5,252 RPM, which is why the two curves cross there on every imperial dyno chart.
Power at other RPM with the same torque
| RPM | hp | kW |
|---|
Formula constants
Defaults are the widely used Hale/Fox values. Roger Huntington's older trap-speed rule uses a constant of about 224–225, which predicts more horsepower for the same trap speed.
Losses are rules of thumb. Automatics and AWD systems usually lose more than manuals; modern drivetrains often lose less than older ones.
How to use the Horsepower & Torque Calculator
- Pick a tab: HP from torque, Torque from HP, Unit converter, Quarter mile or Wheel vs crank.
- Enter your figures and choose units. Changing a unit converts the value you typed so the quantity stays the same.
- Read the results instantly, with the formula worked out underneath.
- For drag estimates, enter race weight with the driver and flywheel horsepower, or switch to HP from ET or trap speed.
- Press Copy result to paste the numbers into a forum post or build sheet.
Horsepower and torque formulas
Power is torque multiplied by rotational speed. With the units engine builders use, that becomes:
- hp = lb-ft × RPM ÷ 5,252 (5,252.1 = 33,000 ÷ 2π, from James Watt's 33,000 ft·lbf per minute)
- lb-ft = hp × 5,252 ÷ RPM
- kW = N·m × RPM ÷ 9,549 (9,549.3 = 60,000 ÷ 2π)
- N·m = kW × 9,549 ÷ RPM
Example: an engine making 400 lb-ft at 4,000 RPM produces 400 × 4,000 ÷ 5,252 = 304.6 hp (227.2 kW). Because the constant is 5,252, an engine's horsepower is lower than its torque figure below 5,252 RPM and higher above it.
Power and torque unit conversions
| Unit | Equals |
|---|---|
| 1 mechanical horsepower (hp) | 745.70 W = 1.0139 PS |
| 1 metric horsepower (PS, CV, ch, pk) | 735.50 W = 0.9863 hp |
| 1 kilowatt (kW) | 1.3410 hp = 1.3596 PS |
| 1 lb-ft | 1.3558 N·m |
| 1 N·m | 0.7376 lb-ft = 0.1020 kgf·m |
| 1 kgf·m | 9.8067 N·m = 7.2330 lb-ft |
European and Japanese spec sheets often quote PS or kW, while US sheets use hp (SAE net). A car rated at 150 kW makes 201 hp or 204 PS.
Quarter-mile estimates
The widely used Hale/Fox relationships estimate quarter-mile performance from race weight (car plus driver, in lb) and flywheel horsepower:
- ET (seconds) = 5.825 × (weight ÷ hp)1/3
- Trap speed (mph) = 234 × (hp ÷ weight)1/3
| Weight | Horsepower | lb per hp | Estimated ET | Estimated trap |
|---|---|---|---|---|
| 3,500 lb | 300 hp | 11.7 | 13.21 s | 103.2 mph |
| 3,000 lb | 400 hp | 7.5 | 11.40 s | 119.5 mph |
| 3,800 lb | 500 hp | 7.6 | 11.45 s | 119.0 mph |
Rearranged, they estimate horsepower from a time slip: hp = weight ÷ (ET ÷ 5.825)³ or hp = weight × (mph ÷ 234)³. Poor launches, wheelspin, altitude and heat make real ETs slower than predicted, which is why the trap-speed version is usually the more reliable power estimate.
Crank vs wheel horsepower
Manufacturers quote crank (flywheel) horsepower, measured at the engine. A chassis dyno measures wheel horsepower, after the gearbox, driveshaft and differential have taken their share. Wheel hp = crank hp × (1 − loss). With a 15% loss, a 400 hp engine shows about 340 hp at the wheels; a dyno reading of 340 whp suggests about 400 crank hp. Different dynos (inertia vs load-bearing) also read differently, so compare results from the same dyno.
Frequently asked questions
How do you convert torque to horsepower?
Horsepower = torque (lb-ft) × RPM ÷ 5,252. For example, 300 lb-ft at 5,000 RPM is 300 × 5,000 ÷ 5,252 = 285.6 hp. In metric, kW = torque (N·m) × RPM ÷ 9,549.
Why do horsepower and torque cross at 5,252 RPM?
Because the conversion constant is 5,252 (33,000 ÷ 2π). At exactly 5,252 RPM the formula becomes hp = lb-ft, so on any dyno chart plotted in hp and lb-ft the two curves meet there. With kW and N·m the curves cross at about 9,549 RPM instead.
What is the difference between hp, PS and kW?
One mechanical horsepower is 745.7 W, one metric horsepower (PS, CV or ch) is 735.5 W, and one kilowatt is 1,000 W. So 100 kW equals 134.1 hp or 136.0 PS.
How accurate are quarter-mile ET formulas?
They give a reasonable estimate for a street car with decent traction, but real times depend on launch, gearing, tires, aerodynamics and the weather. Trap speed is a better indicator of power than ET because it is less affected by traction.
How much horsepower is lost through the drivetrain?
A common rule of thumb is about 10% for front-wheel drive, 15% for rear-wheel drive and 20% or more for all-wheel drive, with automatics usually losing more than manuals. Loss is not strictly a fixed percentage, so treat these as estimates.
Is the quarter-mile horsepower crank or wheel horsepower?
The Hale/Fox formulas are generally used to estimate flywheel (crank) horsepower. Use the Wheel vs crank tab to translate the result to approximate wheel horsepower.