Trap Speed Horsepower Calculator
Estimate horsepower from quarter-mile trap speed and vehicle weight.
Trap speed — the MPH recorded at the end of a quarter-mile run — is one of the most reliable ways to estimate horsepower from a timeslip, because it reflects sustained power rather than launch technique.
Trap Speed HP Formula
This is one of Patrick Hale's widely used drag-racing equations. Because trap speed reflects how much power the car carries to the finish line, it's less sensitive to a poor launch than elapsed time — making it the preferred horsepower estimator among racers.
How to Use This Calculator
- Enter trap speed in MPH from your timeslip.
- Enter race weight including driver.
- Read the estimated horsepower.
Worked Example
Compare against an ET-based estimate with the HP from ET calculator, or predict a full run with the quarter mile calculator.
Why Trap Speed Beats ET for Estimating Power
Trap speed is measured at the finish line and reflects the total energy the car accumulated over the run — a clean proxy for engine power. Elapsed time, by contrast, is heavily influenced by the launch: a bad 60-foot, wheelspin, or tall gearing can ruin ET while barely touching trap speed. That's why racers trust trap speed for horsepower estimates and use ET to judge how well the car launched.
This Estimates Flywheel Horsepower
The Hale formula returns flywheel (crank) horsepower, the figure manufacturers advertise — not wheel horsepower. To compare against a chassis-dyno number, subtract drivetrain loss with our wheel horsepower calculator. Accuracy is best on a clean pass; very slippery or very draggy cars deviate from the 234 constant, and density altitude shifts real power and trap speed together.
What the 234 Constant Assumes
Almost every constant on this site is exact unit arithmetic. This one is not — 234 is an empirical fit to observed drag-strip data, and that has consequences for how much you should trust the answer.
It bakes in assumptions about a typical car: a certain aerodynamic drag, a certain drivetrain efficiency, and a reasonably clean run. Deviate from those and the estimate drifts:
| Situation | Effect on the estimate | Why |
|---|---|---|
| Very slippery car (low drag) | Understates power | Traps higher than the constant expects for its power |
| Brick-shaped car, roof box, big wing | Overstates power | Drag holds trap speed down |
| Ran out of gear before the line | Understates power | Engine off the power peak at the trap |
| Lifted early or coasted | Understates power | Not at full throttle through the trap |
| High density altitude | Understates power | Thin air costs real power — see below |
Some racers use constants between 224 and 240 to suit their car's aerodynamics. If you have a known dyno figure, work the constant backwards from one clean pass and reuse it — a personally calibrated constant beats the generic one.
Use Race Weight, Not Curb Weight
The formula scales linearly with weight, so an error here goes straight into the answer. Race weight means the car as it crossed the line:
- Driver included — typically 70–100 kg / 155–220 lb in gear
- Fuel as carried — gasoline is about 6.1 lb per US gallon, so a half tank is not nothing
- Everything still in the car — spare, tools, passenger seat if it stayed in
Using a brochure curb weight on a car that ran with a driver and half a tank understates weight by 200–250 lb, which understates power by roughly 7%. Weigh the car with the driver aboard if the track has scales.
Density Altitude Moves Both Sides
Thin air costs an engine real power, and it also reduces aerodynamic drag slightly. The two do not cancel — the power loss dominates, so trap speed falls on a hot, high day.
Trap Speed vs ET, Side by Side
| Trap speed | Elapsed time | |
|---|---|---|
| Measures | Energy accumulated by the finish | The whole run, launch included |
| Sensitive to launch | Barely | Heavily |
| Sensitive to traction | Low | High |
| Sensitive to gearing | Moderate | High |
| Best used for | Estimating power | Judging how well the car launched |
| Typical accuracy | ±5% on a clean pass | ±10% |
The practical read: if your ET is poor but trap speed is strong, the engine is fine and the launch is costing you. If trap speed is down, the power is genuinely down. That single comparison is the most useful thing a timeslip tells you — more so than either number alone. Cross-check with the HP from ET calculator.
Estimated HP by Trap Speed (3,200 lb)
| Trap (MPH) | Est. Flywheel HP |
|---|---|
| 100 | 250 |
| 110 | 333 |
| 120 | 432 |
| 130 | 549 |
Frequently Asked Questions
Use HP = Weight × (MPH ÷ 234)³, where weight is in pounds including driver and MPH is the quarter-mile trap speed.
Trap speed reflects sustained power at the finish, while ET is heavily affected by launch quality and traction. So trap speed correlates more directly with horsepower.
Typically within 5–10% for a car with reasonable aerodynamics and weight. Very heavy or unusually shaped vehicles show more error.
Race weight — the car as it ran, including fuel and driver. Using an empty curb weight underestimates horsepower.
The estimate approximates flywheel (crank) horsepower, since it's based on the power needed to accelerate the whole vehicle.