Horsepower from ET Calculator
Quarter-mile elapsed time, or ET, is the time a vehicle takes to cover a standing quarter-mile, and together with weight it offers a quick way to estimate engine horsepower. This calculator uses the well-known Fox ET formula: horsepower equals weight divided by the cube of ET divided by 5.825. The constant 5.825 is empirical, chosen so the estimate lines up with measured power across a large set of real quarter-mile runs. You enter the elapsed time in seconds and the vehicle's race weight in pounds, the weight as it ran including the driver and fuel, and the tool returns an estimate of flywheel horsepower. Every figure is computed deterministically from the formula shown below, never guessed, so the worked example reconciles exactly with the result on screen. Because the formula divides by the cube of the scaled ET, small reductions in elapsed time imply large gains in estimated power. ET and trap-speed estimates usually agree closely; comparing the two can spot a run where traction was off. As with the trap-speed method, the result is crank horsepower, higher than the wheel horsepower a chassis dyno reads. Use this tool to estimate power from a time slip or to compare two builds.
The Fox ET formula is HP = weight / (ET / 5.825)^3. For an elapsed time of 12 seconds and a race weight of 3,500 lb, the estimated horsepower is 400.32 hp.
ET horsepower formula
HP = weight / ( ET / 5.825 )^3
ET = quarter-mile elapsed time in seconds
weight = race weight in pounds (with driver)
5.825 = empirical constant
Dividing weight by the cube of the scaled elapsed time captures how much power was needed to accelerate the mass quickly enough to record that time. The result estimates flywheel horsepower for a typical quarter-mile pass.
Worked example
A car runs a 12-second quarter mile at a race weight of 3,500 lb.
- ET / 5.825 = 12 / 5.825 = 2.060086
- (2.060086) cubed = 8.742909
- HP = 3,500 / 8.742909 = 400.32 hp
The estimated flywheel horsepower is about 400.32. These are the calculator's default inputs, so the result above matches the widget exactly.
ET horsepower: frequently asked questions
What is elapsed time (ET)?
Elapsed time, or ET, is how long a vehicle takes to cover the standing quarter-mile, measured in seconds from launch to the finish line. A lower ET means a quicker run. Together with weight, ET can be used to estimate the engine power that produced the run.
What formula does this use?
It uses the Fox ET formula: horsepower equals weight divided by the cube of ET divided by 5.825, written as power equals weight divided by (ET / 5.825) cubed. The constant 5.825 is empirical, calibrated so the estimate matches measured power across many real quarter-mile passes.
Why does a lower ET mean more horsepower?
For a given weight, covering the quarter-mile in less time requires accelerating harder, which takes more power. The formula divides weight by the cube of the scaled ET, so as ET falls the estimated horsepower rises sharply, reflecting how power and quickness are linked.
Should I use ET or trap speed for power estimates?
Both work, and they often agree. Trap speed is generally considered slightly more robust because it is less affected by launch quality and traction, while ET captures the whole run. Comparing the two estimates can flag a pass where the launch or traction was off.
Is the result flywheel or wheel horsepower?
The Fox ET formula estimates flywheel (crank) horsepower, since it is calibrated against manufacturer power figures. Wheel horsepower measured on a chassis dynamometer is lower because of drivetrain losses, typically by around 10 to 15 percent.
Official sources
- Vehicle performance and safety reference: US National Highway Traffic Safety Administration (NHTSA). As at 25 June 2026.
Reviewed by the CalculatorHub team, edited by James Graham, 25 June 2026. See our methodology. This is general information, not financial, tax, legal or investment advice.