Drag Race Calculator

How a bracket race is scored

Bracket racing handicaps the start instead of the car. Each driver predicts an elapsed time, the dial-in, and the slower dial gets a head start equal to the difference between the two. If both cars run exactly their dials and both drivers leave exactly on the green, they reach the stripe together. Everything else in the round is measured against that ideal. Running under your dial is a breakout and a loss, because the number was supposed to be a promise. Leaving before the green is a red light and a loss, and by the NHRA rulebook a red light is the worse infraction, so a red-lit driver loses even when the opponent breaks out. When both break out, the smaller breakout wins regardless of who crossed first. When both are clean, the finish-line clocks decide, and because the head start already equals the difference of the dials, the margin at the stripe is exactly the difference between the two packages.

Why racers talk about the package

Package is reaction time plus how far over the dial the car ran. A .012 light on a car that ran .009 over is a .021 package, and the lower package wins a clean round. The word matters because it separates the two things a driver controls. The reaction time is the driver at the tree; the margin over the dial is the driver at the finish, lifting or dumping at the stripe to run as close to the number as possible without going under. A perfect .000 package is a perfect light and the dial to the thousandth. Close rounds are decided by a few thousandths, which is why the "what it took" line on this page is written in thousandths: that is the resolution the sport is decided at.

How the air moves the dial

A naturally aspirated engine makes power in proportion to the mass of air it swallows, and density altitude is the one number that says how much air there is. The textbook link from there to the timeslip is the constant-power model behind every quarter mile formula: elapsed time scales with the cube root of weight over power, trap speed with the cube root of power over weight. So a day that takes 3% of the air takes 3% of the power and adds 1% to the ET, which on a 12-second car is 0.118 seconds per thousand feet of density altitude, or about a hundredth per hundred feet. That is the rule of thumb racers quote; this page derives it from your own baseline instead of quoting it, and shows the whole day in a table so you can read it against the weather board. If you have a barometer and a thermometer but no board, the density altitude calculator is the other half of this tool, and the quarter mile calculator is where the 1.555 to 1.60 eighth-to-quarter factors used here are documented.

Why your run log beats the physics

The physics is right about direction and roughly right about size, for a naturally aspirated car that is never traction-limited. Real cars are messier. A car that spins in good air can run quicker on a worse day; a hot, thin day is also a hot, greasy track; a converter flashes higher in cool air; a turbo car with a boost controller notices a thousand feet far less. The published rules of thumb show the spread: about 0.02 seconds per thousand feet for a very fast car, around 0.15 for a slow junior dragster, and the textbook 0.118 for a naturally aspirated twelve-second car sits between them. Only a log of that one combination says where your car falls, and the log is right, because it is that car. Paste your runs into the log and the tool fits a line through them, quotes the scatter as the honest width of the prediction, and draws it against the physics line so the disagreement is visible rather than hidden. Two runs define a line, not a trend; three or more take over the headline. The log is also the reason a boosted car on a single baseline is flagged rather than corrected: there is no published boosted slope worth trusting, only yours.

What this tool will not do

It will not set a throttle stop timer, predict a 60-foot, or tell you how much to pad the dial. Throttle stop math is specific to the stop and the car, 60-foot times carry no air information at all, and the pad is a judgement about how the round will be driven, which is yours. It also does not model rollout, staging depth or a pro tree, because the timeslip already has: the reaction time and the ET printed there are what this page scores. If the timeslip is from a track that still prints a .500-perfect full-tree reaction time, switch the convention and the arithmetic follows. For the horsepower behind a trap speed, the horsepower from trap speed calculator is the neighbour to use.