Horsepower Calculator
Horsepower from torque and RPM, from a quarter-mile time and weight with the Fox and Hale formulas, or from a 0–60 time, with kilowatts and power-to-weight.
Horsepower Calculator: with the default inputs, horsepower is 314.2 hp.
At the RPM you are quoting. Peak torque and peak power rarely happen at the same RPM.
Race weight — the car as it runs, driver and fuel included. Both empirical methods are very sensitive to this.
- In kilowatts
- 234.3 kW
- Second formula's estimate
- —Horsepower. Quarter-mile mode only: the other of the two published equations.
- Power to weight
- 165.3Horsepower per US ton (2,000 lb) of vehicle.
- Pounds per horsepower
- 12.1The number drag racers use; lower is quicker.
- Watts per kilogram
- 135.9
- Method
- Torque × RPM ÷ 5252 — an exact identity, not an estimate
Assumptions
- Horsepower means mechanical horsepower, 745.6999 W. Metric PS is about 1.4% smaller.
- The torque method is exact and returns crank power only if the torque figure was measured at the crank.
- Fox and Hale are empirical fits to drag-strip data with roughly ±10–15% scatter, and assume a competent launch.
- The 0–60 method assumes half of peak power is effectively used across the run, which is adjustable and is the largest source of error.
| RPM | Horsepower | Kilowatts |
|---|---|---|
| 1,000 | 57.1 | 42.6 |
| 2,000 | 114.2 | 85.2 |
| 3,000 | 171.4 | 127.8 |
| 4,000 | 228.5 | 170.4 |
| 5,000 | 285.6 | 213 |
| 6,000 | 342.7 | 255.6 |
| 7,000 | 399.8 | 298.2 |
| 8,000 | 457 | 340.8 |
Real engines do not hold peak torque everywhere, so this is the ceiling rather than a power curve. It does show why power keeps climbing after torque has peaked: the RPM term is still growing.
How this is worked out
The formula
From torque (exact — this is a definition, not a fit) hp = torque (lb·ft) × RPM ÷ 5252 5252 = 33,000 ÷ 2π P (W) = torque (N·m) × 2π × RPM ÷ 60 From the quarter mile (empirical, ±10–15%) Fox: ET = 6.290 × (weight ÷ hp)^⅓ → hp = weight ÷ (ET ÷ 6.290)³ Hale: ET = 5.825 × (weight ÷ hp)^⅓ → hp = weight ÷ (ET ÷ 5.825)³ weight in pounds, with driver and fuel. From 0–60 mph (physics plus one fudge factor, ±20–25%) hp = ½ m v² ÷ (t × η × 745.7), v = 26.8224 m/s, η ≈ 0.5
Open How it’s calculated above to see this worked through with your own numbers.
What you enter
- What do you know?
- Choose one of 3 options.Torque and engine RPM (exact) · Quarter-mile elapsed time and weight (empirical) · 0–60 mph time and weight (roughest)
- Torque
- At the RPM you are quoting. Peak torque and peak power rarely happen at the same RPM.from 0 to 5000 · defaults to 300
- Torque units
- Choose one of 2 options.pound-feet (lb·ft) · newton-meters (N·m)
- Engine speed
- A number.from 0 to 20000 · defaults to 5500
- Quarter-mile elapsed time
- A number.from 0 to 60 · defaults to 13
- 0–60 mph time
- A number.from 0 to 60 · defaults to 5.5
- Vehicle weight with driver
- Race weight — the car as it runs, driver and fuel included. Both empirical methods are very sensitive to this.in kg, lb, t, ton · from 0 to 40000 · defaults to 3800
- Power actually used in the 0–60 run(under More options)
- Accounts for time off peak power, gear changes, drivetrain loss and drag. Real cars land between 40% and 60%.a percentage · from 10 to 100 · defaults to 50
What you get back
- Horsepowermain answer
- In kilowatts
- Second formula's estimate
- Horsepower. Quarter-mile mode only: the other of the two published equations.
- Power to weight
- Horsepower per US ton (2,000 lb) of vehicle.
- Pounds per horsepower
- The number drag racers use; lower is quicker.
- Watts per kilogram
- Method
What this assumes
- Horsepower means mechanical horsepower, 745.6999 W. Metric PS is about 1.4% smaller.
- The torque method is exact and returns crank power only if the torque figure was measured at the crank.
- Fox and Hale are empirical fits to drag-strip data with roughly ±10–15% scatter, and assume a competent launch.
- The 0–60 method assumes half of peak power is effectively used across the run, which is adjustable and is the largest source of error.
About this calculator
Three ways to arrive at a horsepower number, in descending order of how much you should trust them.
From torque and RPM — exact
Power is torque times how fast the shaft is turning, and hp = torque × RPM ÷ 5252 is that relationship in imperial units. It is a definition, not an approximation: the 5252 is 33,000 foot-pounds per minute per horsepower divided by 2π radians per revolution. James Watt picked 33,000 by measuring what a strong dray horse could lift, and every horsepower figure since has inherited it.
This is also why every torque and power curve on a dyno chart crosses at 5,252 RPM — at that speed the two numbers are equal by construction. And it is why an engine can keep making more power after peak torque: torque is falling, but RPM is climbing faster.
Enter torque in either pound-feet or newton-meters. The result is crank power if the torque is a crank figure, and wheel power if it came off a chassis dyno; the calculator cannot tell which, and drivetrain losses between them run roughly 10–15% for a manual and 15–20% for an automatic.
From the quarter mile — good to about ±10–15%
Two published equations relate elapsed time, weight and power. Fox's (Geoffrey Fox, Hot Rod) says ET = 6.290 × (weight ÷ hp)^⅓; Hale's (Patrick Hale, of the drag-racing software firm Racing Systems Analysis) uses 5.825 in the same form. The calculator rearranges both.
They disagree by around 20%, and the disagreement is informative rather than a defect: Hale's smaller constant assumes a car that converts more of its power into forward motion — slicks, a prepared surface, a driver who can launch it. Fox's larger constant fits a street car on street tyres. If your run was on a prepared strip with a good 60-foot time, lean towards Hale; if it was a stock car on all-season tyres, Fox is closer.
Both are cube laws, which makes them brutally sensitive: a tenth of a second at 13 seconds is worth about 2.3% of the power, and a full second nearly doubles it. Weight matters linearly, and it must be race weight — the car as it ran, with driver and fuel, not the brochure figure.
From 0–60 — treat as a sanity check only
Kinetic energy at 60 mph is ½mv², and dividing by the time gives the average power the car actually delivered. Peak crank power is higher, because the engine spends much of the run away from its power peak, loses time to gear changes, and gives up 10–20% in the drivetrain plus whatever the tyres slip. The calculator divides by a single efficiency figure to cover all of that, defaulted to 50% — the value that fits measured cars best across a wide range.
Real cars land between roughly 40% and 60%, and the table shows what that spread does to the answer. Combined with how much a 0–60 time varies with launch technique, surface and rollout, ±20–25% is an honest error bar.
Power to weight
All three modes report horsepower per ton, pounds per horsepower and watts per kilogram, because acceleration depends on the ratio rather than on power alone. Under 10 lb/hp is quick; under 6 is supercar territory; a family hatchback is usually 20–30.
Frequently asked questions
▸Why is horsepower torque times RPM divided by 5252?
Because one horsepower is 33,000 foot-pounds of work per minute, and one revolution is 2π radians. 33,000 ÷ 2π = 5,252.1, so that constant converts pound-feet at a given RPM into horsepower exactly.
▸Why do torque and horsepower curves always cross at 5252 RPM?
Because at exactly 5,252 RPM the formula multiplies torque by 1. Above that speed the horsepower number exceeds the torque number, below it the reverse — it is an artefact of the units, not a property of the engine.
▸How accurate is estimating horsepower from a quarter-mile time?
About ±10–15% for a well-launched run. The two published formulas, Fox's and Hale's, disagree by roughly 20% themselves because they assume different levels of traction. A poor launch makes any car read low.
▸Which should I use, Fox or Hale?
Fox for a street car on street tyres, Hale for a prepared car on slicks with a good 60-foot time. Hale's smaller constant assumes more of the power reaches the ground.
▸Is horsepower from 0–60 reliable?
Not really — ±20–25%. The energy calculation is exact, but the fraction of peak power a car actually uses during a launch varies from about 40% to 60%, and 0–60 times themselves vary hugely with surface, tyres and technique.
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