Quarter Mile Calculator

ET (elapsed time): The time in seconds to travel the selected distance (usually 1/4 mile). Lower ET means a quicker run.

Trap speed (finish-line speed): The estimated speed (mph) at the end of the run. It often follows horsepower more than ET does.

Horsepower (hp): A unit of power (how fast work is done). Higher horsepower can lower ET and raise trap speed. [1][2]

Wheel horsepower (WHP): Power delivered to the driven wheels (often measured on a chassis dyno). This is the horsepower the formulas use.

Crank horsepower (engine/flywheel): Power at the engine before drivetrain losses, so it is usually higher than WHP.

Drivetrain loss (percent): The share of crank horsepower that does not reach the wheels due to the drivetrain.

Traction and launch factor (ET only): A multiplier applied to ET to show how wheelspin or a poor launch can slow the time. Example: 1.05 means about 5% slower ET.

Power-to-weight (hp/lb): Horsepower used in formulas (wheel) divided by Vehicle weight (lb). Higher is better.

Weight-to-power (lb/hp): Vehicle weight (lb) divided by Horsepower used in formulas (wheel). Lower is better.

Drag racing: A type of racing where vehicles accelerate over a short straight distance and are timed. [3][4]

Mistake: Using curb weight (empty car) instead of race weight in Vehicle weight (lb) .
Fix: Include driver, fuel, and anything carried in the car. Understating weight makes ET and trap speed look too optimistic.

Mistake: Selecting Horsepower type = Crank horsepower (engine) but leaving Drivetrain loss (percent) blank or typing 100.
Fix: Enter a loss between 0 and less than 100 so the tool can compute Horsepower used in formulas (wheel) and avoid divide-by-zero.

Mistake: Mixing power types, like entering WHP while Horsepower type is set to crank (or vice versa).
Fix: Match the dropdown to the number you have, then confirm the conversion using Horsepower used in formulas (wheel) and Horsepower equivalent (crank) .

Mistake: In Solve modes, trying to enter both Target ET (seconds) and Target trap speed (mph) and expecting the calculator to satisfy both at once.
Fix: Check What do you want to solve for? and then recalculate. Use Performance target to use to pick one target method (ET or trap speed). Use the other result only as a cross-check.

Mistake: Expecting Traction and launch factor (ET only) to change trap speed too.
Fix: Use traction factor only to adjust ET. If you want a different trap speed, change horsepower or weight instead.

Mistake: Comparing your result to another site without matching constants.
Fix: Set the same Formula constants set before deciding two calculators disagree.

Power-to-weight only: This calculator uses a simplified cube-root model based mainly on wheel horsepower and race weight. It does not model tire grip, suspension, gearing, shift time, driver reaction, or aerodynamics in detail.

ET vs trap speed: ET is highly affected by the first part of the run (launch and traction). Trap speed is usually more tied to horsepower. So you can see a realistic trap speed but an ET that is too quick (or too slow).

Race weight is required: Vehicle weight (lb) should include driver and fuel. If weight is too low, both ET and mph will look better than real life.

Wheel vs crank horsepower: The formulas use wheel horsepower. If you enter crank horsepower, results depend on your Drivetrain loss (percent). A single percent is an approximation, and real drivetrain loss is not perfectly constant.

Constants are not universal: Different Formula constants set options can shift results. No one constant pair matches every car, track surface, and setup.

1/8 mile is approximate: If you select 1/8 mile, the result is a rough estimate derived from the 1/4-mile style model, so accuracy can be worse than a tool tuned specifically for 1/8-mile data.

Extreme inputs: Very low or very high weight, horsepower, ET, or mph may describe vehicles outside the range where this kind of estimate is useful. Treat such outputs as a math check, not a prediction.

This calculator uses a common drag racing estimate that assumes quarter-mile performance is mostly driven by power-to-weight, using cube-root equations for ET and trap speed. [3][4]

1) Choose constants

From Formula constants set, the calculator picks two numbers: kET (for time) and kMPH (for speed). Different sites use slightly different constants, which is why results can vary.

2) Convert horsepower to wheel horsepower (if needed)

The core equations use wheel horsepower (WHP). If you selected crank horsepower, the calculator estimates WHP using drivetrain loss.

wheel_hp = crank_hp * (1 - drivetrain_loss_pct/100)

If drivetrain loss is provided and valid, the calculator can also show the crank equivalent for comparison:

crank_hp = wheel_hp / (1 - drivetrain_loss_pct/100)

3) Predict ET and trap speed (or solve by inverting)

Let weight_lb be Vehicle weight (lb), hp_used be the wheel horsepower used in the formulas, and traction_factor be the optional ET-only multiplier (blank means 1.00).

ET_seconds = kET * (weight_lb / hp_used)^(1/3) * traction_factor

trap_mph = kMPH * (hp_used / weight_lb)^(1/3)

Traction factor is applied only to ET, because ET depends strongly on the launch, while trap speed is more tied to power. [3][4]

For Solve modes, the same equations are rearranged:

hp_used = weight_lb / ( (ET_seconds/(kET*traction_factor))^3 )

hp_used = weight_lb * (trap_mph / kMPH)^3

weight_lb = hp_used * (ET_seconds/(kET*traction_factor))^3

weight_lb = hp_used / (trap_mph / kMPH)^3

4) Extra outputs

Power-to-weight and weight-to-power are computed from the same hp_used and weight_lb:

power_to_weight = hp_used / weight_lb

weight_to_power = weight_lb / hp_used

Mini example (Predict)

If weight_lb = 3500 lb, hp_used = 400 WHP, kET = 5.825, kMPH = 234, and traction_factor = 1.00, then ET_seconds is about 12.00 s and trap_mph is about 113.56 mph.