Skip to content

Horsepower Calculator

Calculate horsepower from torque and RPM, convert between HP, kW, PS, and watts, or estimate engine HP from quarter-mile elapsed time and trap speed.

Torque & RPM Method

Engine horsepower from dyno torque and engine speed. The canonical horsepower formula.

rpm

Power Unit Converter

Convert between mechanical HP, kilowatts, metric HP (PS/CV), and watts.

Elapsed Time (ET) Method

Estimate from vehicle weight and quarter-mile elapsed time.

seconds

Trap-Speed Method

Estimate from vehicle weight and speed at the quarter-mile finish (not average speed).

The Horsepower Formula

One mechanical horsepower equals 33,000 foot-pounds of work per minute — the figure James Watt chose in the late 1700s to compare steam engines to draft horses. From that definition, the working formula for an engine is:

HP = Torque (lb-ft) × RPM ÷ 5252

The constant 5252 is 33,000 divided by 2π. It also explains why every dyno chart shows the horsepower and torque curves crossing at exactly 5252 RPM: at that engine speed, the multiplier reduces to 1.

In metric units the equivalent formula is:

kW = Torque (Nm) × RPM ÷ 9549

Worked Example

An engine producing 285 lb-ft of torque at 5,400 RPM:

HP = 285 × 5400 ÷ 5252 ≈ 293.0 HP ≈ 218.5 kW ≈ 297.1 PS.

Power Unit Conversions

Different regions and industries use different units for power. The conversions are:

FromTo HPTo kWTo PS
1 HP (mechanical)10.74571.0139
1 kW1.341011.3596
1 PS (metric HP / CV)0.98630.73551
1,000 W1.341011.3596

Mechanical HP (745.6999 W) is the standard in the United States and United Kingdom. Metric HP — PS in German, CV in French and Spanish, pk in Dutch — equals 735.49875 W and is the legal unit for vehicle power in most of continental Europe. The two differ by about 1.4%, which is why a Porsche advertised at 450 PS is 443.7 mechanical HP.

Drag-Strip Horsepower Estimation

When dyno torque isn't available, two empirical formulas estimate crank HP from quarter-mile data. Vehicle weight should include the driver, passengers, fuel, and anything else in the car during the run.

Elapsed Time (ET) Method

Roger Huntington's 1958 formula uses the total time to cover a quarter mile (402.3 m):

HP = Weight ÷ (ET ÷ 5.825)³

Works best when the car launches cleanly and applies maximum power from start to finish.

Trap-Speed Method

Patrick Hale's formula uses speed at the finish line, not average speed:

HP = Weight × (Speed ÷ 234)³

Trap speed is less affected by reaction time and launch quality, so this method often agrees more closely with dyno results when traction is marginal.

Example

A 3,800 lb vehicle with a 14.5-second quarter mile:

  • ET Method: 3800 ÷ (14.5 ÷ 5.825)³ ≈ 246.3 HP

Same vehicle with a 95 mph trap speed:

  • Trap Method: 3800 × (95 ÷ 234)³ ≈ 254.2 HP

Factors That Affect Accuracy

  • Drivetrain loss: Drag-strip formulas estimate crank HP. Wheel HP is typically 10–20% less due to transmission and differential friction.
  • Altitude & weather: Higher altitude and hotter temperatures reduce air density and engine output. Sanctioning bodies apply correction factors; these formulas don't.
  • Traction: Wheel spin wastes energy, inflates ET, and reduces trap speed, leading to underestimated HP.
  • Aerodynamics: At higher speeds drag becomes significant; the simple ET and trap formulas don't account for vehicle frontal area or drag coefficient.
  • Launch technique: The ET method is sensitive to reaction time and launch consistency.

Typical Power Outputs & Quarter-Mile Times

Vehicle TypeHP / kWET (sec)Trap (mph)
Economy car100–150 / 75–11217–1975–85
Mid-size sedan200–300 / 149–22414–1690–105
Sports car300–500 / 224–37312–14105–120
Muscle car / tuned500–800 / 373–59710–12120–140
Top Fuel dragster11,000+ / 8,200+3.6–3.8330+

Related Calculators

Age Calculator — Years, Months, Days, Hours Exactly

Calculate your exact age in years, months, and days. Find the time between any two dates down to seconds.

How Old Was I Calculator — Age on Any Past Date

Find out how old you were on any date. Calculate age in years, months, days, hours, minutes, and seconds.

Add Times Calculator — Sum HH:MM:SS Durations

Add multiple time durations together in hours, minutes, and seconds. Get results in HH:MM:SS and decimal hours.

Time Until Calculator — Live Countdown to Any Date

Calculate the time remaining until a specific time and date. Live countdown in days, hours, minutes, and seconds.

End of School Countdown — Live Days, Hours, Minutes

Live countdown to the end of the school year. See remaining days, hours, minutes, and seconds in real time.

Weeks Between Dates Calculator — Plus Days & Months

Calculate the number of weeks between two dates. Also shows total days, months, weekdays, and weekend days.

Frequently Asked Questions

What is the formula for horsepower?

The canonical formula is HP = Torque × RPM ÷ 5252, where torque is in pound-feet and RPM is engine revolutions per minute. The constant 5252 comes from converting torque and angular speed (rad/min) into the 33,000 ft-lb/min that defines one mechanical horsepower.

Why does horsepower equal torque at 5252 RPM?

Because 5252 ≈ 33,000 ÷ (2π). At that exact RPM, the formula HP = T × RPM ÷ 5252 collapses to HP = T, so on a dyno chart the HP and lb-ft curves always cross at 5252 RPM.

How many watts in one horsepower?

One mechanical (or imperial) horsepower equals 745.6998715822702 watts, or about 0.7457 kW. Metric horsepower (PS, also written as CV or pk) is slightly smaller at 735.49875 W ≈ 0.7355 kW.

What is the difference between HP and PS (metric HP)?

Mechanical HP is the U.S./U.K. standard (745.7 W). PS — Pferdestärke, also CV in French — is the European metric HP at 735.5 W. A 200 PS engine is roughly 197 mechanical HP, which is why European spec sheets often list slightly higher numbers than U.S. equivalents.

How accurate are the drag-strip horsepower estimates?

These are empirical approximations based on weight and quarter-mile data. Real dyno results vary due to drivetrain loss, altitude, temperature, tire grip, and driver skill. Expect ±10–15% variance from actual wheel or crank horsepower.

What is wheel HP vs crank HP?

Crank HP is measured at the engine flywheel. Wheel HP is what reaches the wheels after drivetrain losses — typically 10–15% for manual transmissions, 15–20% for automatics. The torque-and-RPM formula gives whatever you feed it: dyno torque at the wheels yields wheel HP; engine-bench torque yields crank HP.

What weight should I use in the drag-strip methods?

Use the total weight during the run: vehicle curb weight plus driver, passenger, fuel, and any cargo. This is sometimes called "race weight" or "as-tested weight."

When should I use ET vs trap speed?

The ET method is sensitive to launch and reaction time. The Trap-Speed method uses finish-line speed, which is generally less affected by traction and launch technique, so it tends to be more accurate when the car spins or has a bad launch.