What formula does this calculator use?

It uses the Wallace/NHRA rule-of-thumb: ET = 5.825 × (Weight ÷ HP)^(1/3) and Trap Speed = 234 × (HP ÷ Weight)^(1/3). Roger Huntington popularised these in the 1960s and they remain the standard back-of-envelope method used by NHRA bracket racers and engine builders worldwide.

How accurate is the Wallace formula?

For well-prepared rear-wheel-drive cars on slicks in ideal conditions it is accurate to within 0.2–0.5 seconds. Street cars on radials typically run 0.3–0.6 s slower than predicted due to traction loss. Altitude, temperature, humidity, and drivetrain type (manual vs automatic) all shift the result.

Should I use flywheel or wheel horsepower?

The formula was calibrated against flywheel (crankshaft) horsepower, so use that if you have a dyno sheet or manufacturer spec. If you only have a wheel-horsepower figure, divide it by 0.83 (assuming 17% drivetrain loss for a rear-wheel-drive car) to estimate flywheel HP before entering it.

What is a good quarter-mile time for a street car?

A stock family saloon typically runs 15–17 s. A hot hatch or muscle car runs 13–15 s. A modified performance car hits 11–13 s. Under 10 s requires serious engine work; under 9 s demands a full drag build. The calculator labels your predicted time with a performance class so you can see at a glance where you stand.

Can I use kilograms instead of pounds?

Yes. Select "kg" next to the weight field and the calculator converts to pounds internally before applying the formula. Results are still shown in seconds and mph because the Wallace constants are calibrated to imperial units.

How is the 60-foot time estimated?

The 60-ft time shown is an approximation: ET ÷ 5.5. This is a rough rule of thumb used in bracket racing. Real 60-ft times depend heavily on launch technique, tyre prep, and traction — a good launch can be 0.5 s faster than a poor one even at identical overall ET.

Quarter-Mile Calculator

The quarter-mile — 1,320 feet of strip — is the definitive test of straight-line performance. Before you bolt on a power adder, pull weight, or book a track day, knowing your predicted elapsed time (ET) and trap speed helps you set realistic expectations, choose the right tyre, and plan your build intelligently. This calculator applies the Wallace formula, the same industry standard that NHRA bracket racers, engine builders, and motorsport journalists have used since the 1960s.

How the formula works

The two core equations are elegantly simple:

ET (seconds) = 5.825 × (Weight ÷ HP)^(1/3)

Trap speed (mph) = 234 × (HP ÷ Weight)^(1/3)

Both follow a cube-root relationship, which means doubling horsepower does not halve your ET — it cuts it by about 20%. To go from a 13-second pass to a 10-second pass you need roughly 3.4× more power (or the equivalent weight reduction). That non-linear reality is exactly why experienced drag racers talk about power-to-weight ratio rather than raw horsepower alone.

The 5.825 constant was empirically derived by Roger Huntington from thousands of NHRA time slips in the 1960s and has been validated against modern data repeatedly. The 234 trap-speed constant is mathematically consistent with it, derived from energy principles assuming constant acceleration.

Solve-for-variable mode

Sometimes you know where you want to be and need to work backwards. The “Solve for” selector lets you:

Find the HP needed to achieve a specific target ET at your current weight.
Find the maximum weight you can run to hit a target ET at a fixed HP level.

Both reverse-solve using simple algebraic rearrangement of the same formula. After solving, the calculator verifies the answer by running the forward calculation — so you can confirm the numbers close perfectly.

Worked example

A 3,400 lb rear-wheel-drive coupe produces 450 hp at the flywheel:

ET = 5.825 × (3400 ÷ 450)^(1/3) = 5.825 × (7.556)^(1/3) = 5.825 × 1.964 = 11.44 s
Trap = 234 × (450 ÷ 3400)^(1/3) = 234 × (0.1324)^(1/3) = 234 × 0.509 = 119.2 mph

Now suppose the driver wants to crack the 11-second barrier. Rearranging:

HP = Weight ÷ (Target ET ÷ 5.825)^3 = 3400 ÷ (10.99 ÷ 5.825)^3 = 3400 ÷ 7.499 = 453 hp

Just 3 additional horsepower — or equivalently, shedding about 20 lb — puts the car into the 10s. This shows how close the car already is, and why small gains matter near a class boundary.

Weight (lb)	HP	ET (s)	Trap (mph)	Class
3,400	300	12.57	108.3	Modified street car
3,400	400	11.67	116.3	High-performance build
3,400	500	11.09	122.0	High-performance build
3,400	600	10.65	126.8	High-performance build
2,800	500	10.68	126.8	High-performance build

The last two rows illustrate the classic debate: 100 extra horsepower versus 600 lb of weight reduction produce almost identical ETs.

Conditions and corrections

The Wallace formula assumes ideal conditions: prepared surface, slicks or drag-spec radials, optimal launch, and sea-level atmospheric pressure. Common real-world corrections:

Street radials on cold asphalt: add +0.3 to +0.5 s
Automatic transmission with torque converter: subtract 0.1 to 0.2 s (they launch harder)
Manual transmission with clutch slip: add +0.1 to +0.3 s
Every 1,000 ft of altitude above sea level: add approximately +0.05 s
Hot day (35 °C vs 20 °C): add roughly +0.1 s

For the 60-foot launch time — often the single most important number in drag racing — the calculator shows a rough estimate (ET ÷ 5.5). Real 60-ft numbers depend almost entirely on driver technique and tyre prep, not raw power.