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Automotive

Quarter Mile Calculator

Quarter mile ET and trap speed from weight and power.

Your measurementsEvery assumption editable01

Including driver and fuel.

Quarter mile estimateCalculated live02

Result

12.12sec ET

12.12 sec at 112.5 mph

Weight
3600 lb
Power
400 hp
Power-to-weight
9 lb/hp
Estimated ET
12.12 seconds
Estimated trap speed
112.5 mph

These are empirical estimators, not physics. They were derived from decades of timeslips and they assume a competent launch on a prepared surface — traction, gearing, driver and air density all move the real number.

Trap speed is the more honest indicator of power, because it depends far less on how well the car left the line. Two cars with identical trap speeds and different ETs differ in traction, not output.

Enter flywheel power, not wheel power. Drivetrain losses are already inside the constant.

Enter weight including driver and fuel, and flywheel horsepower. These are empirical estimators derived from decades of timeslips, not physics — they assume a competent launch on a prepared surface.

Why use this tool?What it does differently03

Why use this tool?

ET and trap together

Both figures, since trap speed is the more honest indicator of actual power.

Power-to-weight shown

The ratio that drives everything, itemised so you can see the effect of shedding weight.

Honest about its basis

Empirical, not physical. Traction, gearing, driver and air density all move the real number.

Flywheel power, stated

Drivetrain losses are already inside the constant, so entering wheel power would double-count.

How this worksThe method04

How this quarter mile calculator works

Both estimators work from the power-to-weight ratio. Elapsed time scales with the cube root of pounds per horsepower; trap speed scales with the cube root of horsepower per pound.

These are the long-standing empirical formulas derived from large numbers of real timeslips rather than from first principles. They encode typical traction, gearing and driveline losses, which is why they want flywheel power rather than wheel power.

Trap speed is the more reliable indicator of actual output, because it depends far less on how well the car left the line. Two cars with identical trap speeds and different elapsed times differ in traction, not in power.

How to use itStep by step05

How to use it

  1. Step 1: Weigh the car

    Including driver and the fuel you will run. Race weight, not curb weight.

  2. Step 2: Enter flywheel power

    Not wheel power — drivetrain losses are already inside the formula.

  3. Step 3: Read both figures

    Trap speed is the better guide to real output.

  4. Step 4: Treat it as a ballpark

    Traction, gearing and air density move the real result substantially.

Example usageWorked figures06

Example usage

A modern muscle car
3,600 lb and 400 hp — 9 lb per hp, estimating 12.12 seconds at 112.5 mph.
Shedding weight
The same 400 hp at 3,200 lb improves to 11.65 seconds at 117.0 mph. Four hundred pounds is worth roughly half a second.
A light, low-power car
2,200 lb and 150 hp gives 14.26 seconds at 95.6 mph — weight compensating substantially for modest output.
Frequently asked questionsCommon questions07

Frequently asked questions

How accurate is a quarter mile calculator?

It gives a reasonable ballpark for a competently launched car on a prepared surface. Traction, gearing, driver skill and air density can easily move the real result by several tenths.

Should I enter flywheel or wheel horsepower?

Flywheel. Drivetrain losses are already accounted for inside the empirical constants, so entering wheel power would double-count them.

Why is trap speed a better indicator than ET?

Because it depends far less on the launch. Two cars with the same trap speed and different elapsed times differ in traction, not in power output.

Does this account for gearing?

No. Gearing, tyre compound and suspension setup all affect the real result and none of them are inputs. This works from power-to-weight alone.

What weight should I use?

Race weight — the car as it runs, including driver and fuel. Curb weight will make the estimate optimistic.

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