This quarter-mile calculator estimates your car's 1/4-mile elapsed time (ET) and trap speed (MPH) from horsepower and weight — and works in reverse to estimate horsepower from a measured ET or trap speed off your time slip. It uses the classic Roger Huntington and Patrick Hale drag-racing equations trusted by racers since the 1950s.
Quarter-Mile Formula
MPH = 234 × (HP ÷ Weight)^(1/3)
These formulas provide estimates for optimal conditions: an excellent launch on a prepped track with drag radials or slicks. Real-world street tires will result in slower ETs. The horsepower figure is treated as flywheel (crank) horsepower, not wheel horsepower.
The Three Quarter-Mile Formulas
Drag racers use three well-known empirical equations, each calibrated from real time-slip data. They agree closely for typical street cars and differ only at the extremes:
- Roger Huntington (1950s): the original. ET = 6.290 × (W ÷ HP)^⅓ and MPH = 224 × (HP ÷ W)^⅓. Huntington plotted elapsed times and trap speeds from many cars and fit a curve — the foundation of every formula since.
- Patrick Hale (1980s): ET = 5.825 × (W ÷ HP)^⅓ and MPH = 234 × (HP ÷ W)^⅓, refit for later tires and aerodynamics. Hale's ET constant predicts a quicker time than Huntington's, so this calculator keeps Huntington's more conservative 6.290 for ET and uses Hale's 234 for trap speed.
- Geoffrey Fox: ET = 6.269 × (W ÷ HP)^⅓ and MPH = 230 × (HP ÷ W)^⅓ — a middle-ground variant some racers prefer.
Estimating Horsepower from a Time Slip
Run the math in reverse to back-calculate horsepower from track data — useful when you don't have a dyno. From trap speed: HP = Weight × (MPH ÷ 234)³. From ET: HP = Weight × (6.290 ÷ ET)³. Trap-speed-based estimates are usually closer to true power because trap speed reflects the energy the car actually accumulated, while ET also absorbs launch mistakes, wheelspin, and reaction time.
Worked Example
Quarter-Mile ET & Trap Speed by Power-to-Weight
Reference estimates for a 3,200 lb car at various horsepower levels (ideal conditions, flywheel HP):
| Horsepower | Power-to-Weight | Est. ET (sec) | Est. Trap (MPH) | Class feel |
|---|---|---|---|---|
| 200 | 0.063 hp/lb | 15.9 | 93 | Economy / daily |
| 300 | 0.094 hp/lb | 13.9 | 106 | Hot hatch / sport sedan |
| 400 | 0.125 hp/lb | 12.6 | 117 | Muscle car |
| 500 | 0.156 hp/lb | 11.7 | 126 | Modern performance |
| 700 | 0.219 hp/lb | 10.5 | 141 | Supercar |
Tested figures vary with prep and weather — published Model 3 Performance runs alone range from about 10.9 s to 11.8 s — so treat this column as representative, not definitive.
These are mathematical ideals. Street tires, a soft launch, tall gearing, or high density altitude will all add tenths to your real ET — which is exactly why trap speed is the better proxy for raw engine power.
This calculator provides estimates based on standard mathematical formulas. Real-world results will vary based on mechanical condition, environmental factors, and other variables.
Sanity Check Against Real Cars
The best way to judge an empirical formula is against published time slips. Here is this calculator's estimate (Huntington ET, Hale trap speed) versus widely reported quarter-mile results (manufacturer/press figures, driver ~180 lb added):
| Car (approx. specs) | HP / Race Wt | Formula ET @ MPH | Typical Tested |
|---|---|---|---|
| Mazda MX-5 (ND, 181 hp) | 181 / 2,520 lb | 15.1 @ 97 | ~14.5 @ 95 |
| Ford Mustang GT (480 hp) | 480 / 4,010 lb | 12.8 @ 115 | ~12.4 @ 112 |
| Chevrolet Corvette Z06 (670 hp) | 670 / 3,610 lb | 11.0 @ 134 | ~10.6 @ 131 |
| Tesla Model 3 Performance (510 hp) | 510 / 4,230 lb | 12.8 @ 115 | ~11.7 @ 115 |
Gas cars land within a few tenths — the formula slightly under-predicts modern cars with launch control and sticky tires. The Tesla beats its estimate by a full second in ET while matching the trap speed almost exactly: instant electric torque and all-wheel-drive launch dominate the first 60 feet, but trap speed still tracks power-to-weight. That pattern — trap speed obeys the math, ET rewards the launch — is the single most useful thing to remember when reading a time slip.
1/8-Mile to 1/4-Mile Conversion
Many local tracks run eighth-mile. To project a quarter-mile result, the accepted rule of thumb is ET × 1.56 and MPH × 1.25 (the ratio isn't 2× because the car is already moving at the eighth-mile mark):
| 1/8-mile ET | ≈ 1/4-mile ET | 1/8-mile MPH | ≈ 1/4-mile MPH |
|---|---|---|---|
| 6.50 | 10.14 | 105 | 131 |
| 7.00 | 10.92 | 98 | 123 |
| 7.50 | 11.70 | 92 | 115 |
| 8.00 | 12.48 | 86 | 108 |
| 8.50 | 13.26 | 81 | 101 |
| 9.00 | 14.04 | 76 | 95 |
| 10.00 | 15.60 | 68 | 85 |
Use our Eighth Mile Calculator for the reverse direction and power estimates from 1/8-mile slips.
Sources: G. T. Fox, "On the physics of drag racing," American Journal of Physics 41, 311 (1973); Roger Huntington's time-slip regression analyses (Hot Rod, 1950s); Patrick Hale, Quarter Mile Math. Constants shown are the commonly published values.
Trap speed is the more reliable half of a timeslip for estimating power, because it is far less sensitive to launch quality than elapsed time — see trap speed horsepower.
Trap Speed and Top Speed Are Different Numbers
This calculator returns quarter-mile trap speed — how fast the car is traveling after 1,320 feet of acceleration from rest. That isn't the car's top speed, and the two diverge sharply as power rises, because they're limited by completely different things.
Trap speed is limited by how much work the engine can do over a fixed distance, so it depends on power divided by weight. Top speed is limited by aerodynamic drag, which doesn't care about weight at all — it depends on power divided by frontal area and drag coefficient. A heavy, slippery car can have a modest trap speed and a very high top speed; a light, boxy one does the opposite.
| Power | Quarter-mile trap speed | Estimated top speed | Gap |
|---|---|---|---|
| 150 HP | 84 MPH | 135 MPH | +51 |
| 200 HP | 93 MPH | 149 MPH | +56 |
| 300 HP | 106 MPH | 171 MPH | +65 |
| 400 HP | 117 MPH | 188 MPH | +71 |
| 600 HP | 134 MPH | 215 MPH | +81 |
| 1000 HP | 159 MPH | 255 MPH | +96 |
Trap speeds assume a 3,200 lb car; top speeds assume a typical sedan's drag area with 85% of crank power reaching the road. Real top speeds are often lower still. Most road cars are gear-limited rather than drag-limited, hitting the rev limiter in top gear before they run out of power. That's a deliberate choice by the manufacturer, not a shortfall.
What the Cube Root Costs You
Both figures scale with the cube root of power, and that single fact governs the economics of chasing speed. Doubling power doesn't double speed; it multiplies it by the cube root of two, about 1.26.
| Power increase | Speed increase | What it means in practice |
|---|---|---|
| +10% | +3.2% | Roughly 3 MPH on a 100 MPH trap speed |
| +25% | +7.7% | Noticeable but far less than the power figure suggests |
| +50% | +14.5% | A major build for a modest speed gain |
| +100% | +26.0% | Twice the engine for a quarter more speed |
| +300% | +58.7% | Four times the power, not quite 60% more speed |
This is why speed records demand such extraordinary power, and why the last few miles per hour cost more than everything before them. It also explains a common disappointment. Add 30 horsepower to a 300 horsepower car and trap speed rises by around 3 MPH. That's real, but far less dramatic than the dyno sheet makes it feel.
Frequently Asked Questions
The formula assumes perfect traction, gearing, and driver skill. Wheelspin off the line will drastically hurt ET without changing trap speed much.
Yes. Hotter, more humid air or higher elevation increases density altitude (DA), meaning less oxygen for the engine and less horsepower.
Predictions are typically within a few tenths for a car with average traction and gearing. Track conditions, tires, and launch technique cause most of the variation.
ET is the time to cover the quarter mile; trap speed is how fast you're going at the finish line. Trap speed correlates more closely with horsepower, while ET also depends on launch and traction.
Both matter through power-to-weight ratio. Reducing weight and adding power both lower ET, but traction often limits how much of that power reaches the track.
Yes. Use HP = Weight × (MPH ÷ 234)³ from your trap speed, or HP = Weight × (6.290 ÷ ET)³ from your elapsed time. The trap-speed method is generally more accurate because it is less affected by launch quality and wheelspin.
Engineer Roger Huntington first derived them in the 1950s by plotting real drag-strip data and fitting a curve. Patrick Hale and Geoffrey Fox later refined the constants for modern tires and aerodynamics. They are empirical, not exact physics, so treat results as well-grounded estimates.