Engine Horsepower Calculator · Practical guide
How to Estimate Horsepower From Quarter Mile Time
Divide vehicle weight by (ET / 5.825)^3 — a 3,500-pound car running 13.5 seconds is making roughly 281 horsepower.
At a glance
The ET Formula
You can estimate horsepower from a quarter-mile elapsed time (ET) with the classic drag racing formula: HP = Weight / (ET / 5.825)^3. Use the car's race weight — curb weight plus driver, fuel, and cargo. A 3,500-pound car that runs 13.5 seconds is making about 281 HP: 13.5 / 5.825 = 2.3176, cubed is 12.446, and 3,500 / 12.446 ≈ 281.2.
Interactive tool
Engine Horsepower Calculator
Enter your own values, then compare the result with the examples below.
Results
Horsepower by Quarter Mile Time (3,500 lb Car)
| Quarter Mile ET | Estimated Horsepower |
|---|---|
| 12.0 seconds | 400 HP |
| 13.0 seconds | 315 HP |
| 13.5 seconds | 281 HP |
| 14.0 seconds | 252 HP |
| 15.0 seconds | 205 HP |
What Affects the Estimate
The ET method rewards traction and launch technique as well as raw power. A car that hooks up perfectly will show a slightly higher ET-based estimate than the same car spinning its tires. Altitude, temperature, and track prep also move ET by tenths, which shifts the estimate noticeably — at 3,500 pounds, each tenth of a second near 13.0 seconds is worth roughly 7-8 HP.
Getting a Realistic Weight
- Start with the published curb weight.
- Add the driver (typically 150-200 lbs) and any passengers.
- Add fuel at about 6.3 lbs per gallon and any cargo or removed seats.
- Truck scales or a drag strip scale give the most accurate race weight.
Cross-check your result with the trap speed method above — the two numbers together bracket your true output.
Frequently Asked Questions
Where does the 5.825 constant come from?
It is an empirical constant derived from decades of drag racing data relating power-to-weight ratio to elapsed time over a quarter mile. Different sources use constants between 5.8 and 6.0, which is why estimates vary slightly.
Should I use curb weight or race weight?
Race weight — the actual weight going down the track, including the driver, fuel, and cargo. Using curb weight alone overstates horsepower because a lighter car runs quicker.
Does the ET method work for street cars?
Yes, as long as you have a real quarter-mile time from a drag strip or a reliable GPS-based performance meter. Magazine road test times also work as inputs.
How accurate is the ET method?
Typically within 5-10% of a dyno reading for rear-wheel-drive cars that launch cleanly. Poor traction, automatic transmission slip, or high altitude widens the error.