Power-to-Weight Ratio Calculator

Power-to-Weight Ratio Calculator that turns power and weight in any unit into hp/lb, W/kg or kW per tonne, compares two vehicles, and estimates quarter-mile time and best 0-60 mph.

What the Power-to-Weight Ratio Calculator Compares

The Power-to-Weight Ratio Calculator divides a vehicle’s power by its weight to give one number for how hard it can accelerate. It accepts power and weight in any common unit, reports the ratio in eight units at once, and estimates the quarter-mile time and a best-case 0–60 mph run. For riders and heavy trucks it gives the climb rate instead.

The Calculate menu has three modes. Power-to-Weight Ratio is the usual one. Power for a Target Ratio finds how much power a vehicle of a given weight needs to reach a ratio you choose. Weight for a Target Ratio finds the heaviest the vehicle can be at its power and still hit that ratio.

Power can be typed in horsepower, kilowatts, watts or metric horsepower (PS). Weight can be in pounds, kilograms, metric tons or US tons. The default car makes 300 hp and weighs 3,000 lb, and the Power-to-Weight Ratio Calculator shows 0.1 hp/lb, which is 10 lb/hp or 164.4 W/kg.

Power-to-Weight Ratio Formula

The power-to-weight ratio is power divided by weight. The units come along with the numbers, so the same car can read as 0.1, 164.4 or 220.5 depending on whether you use hp per pound, watts per kilogram or hp per metric ton.

$$PWR = \frac{P}{W}$$

For the default car, 300 hp ÷ 3,000 lb = 0.1 hp/lb. Turned upside down, the same figure is the weight-to-power ratio, 10 lb/hp, and a lower number there means a quicker car. The first result card shows both directions, and the Ratio in Every Unit table lists all eight forward units plus three inverse ones.

Use peak power and curb weight for the ratio, since that is what makers publish. The Driver + Payload field under Compare and Units adds the driver and any load to the performance cards only, so the ratio still matches spec sheets. The step list in the Power-to-Weight Ratio Calculator prints the division with your own numbers.

Power to Weight Ratio Calculator in hp/kg, kW and Metric Units

Metric spec sheets give power in kW or PS and mass in kilograms, and the ratio is then quoted in W/kg, kW per tonne or PS per tonne. Pick any of them in the result unit menu. The conversion factors follow NIST Special Publication 811.

W/kg and kW per tonne are always the same number, since both multiply power and weight by 1,000. To calculate bhp per ton for a UK road test, divide brake horsepower by the weight in metric tons. The default car is 300 ÷ 1.361 = 220.5 bhp per tonne.

Which Is Worth More, Weight or Power?

Because weight sits under the line, cutting it raises the ratio just as surely as adding power. The result alert in the Power-to-Weight Ratio Calculator shows the trade for your entry. For the default car, taking 10 percent off the weight, 300 lb, gives the same ratio as adding 33.3 hp.

The If Weight Changes table under the results runs the weight from 80 to 120 percent of your figure. At 2,400 lb the default car reaches 0.125 hp/lb and an estimated 12.54 s quarter mile. At 3,600 lb it drops to 0.083 hp/lb and 14.35 s, with the same engine.

Weight also has costs that the ratio leaves out. A lighter car brakes in less distance and loads its tires less in corners. Extra power gives higher top speed through air resistance, which the ratio does not capture either.

Does 40 horsepower make a difference?

It depends on what the car weighs. On the 3,000 lb default car, going from 300 to 340 hp lifts the ratio from 0.1 to 0.113 hp/lb, a 13 percent gain. The street quarter-mile estimate drops from 13.51 s to 12.95 s, and trap speed climbs from 106.8 to 111.3 mph.

The same 40 hp on a 6,000 lb truck is only half as large a change in ratio. Run both through the Power for a Target Ratio mode to see what a given gain is worth on your own vehicle.

Power-to-Weight Ratio Calculator Comparison

Open Compare and Units to enter a second vehicle. The second result card then turns into a comparison, showing how much higher or lower your ratio is and the power vehicle B would need to match it.

With the default figures, a 200 hp, 3,200 lb vehicle B has a ratio of 0.0625 hp/lb. The first car is 60 percent higher, and vehicle B would need 320 hp at its own weight to draw level. Without a vehicle B, the card shows the ratio in three more units instead.

Comparisons only work when both vehicles use the same weight basis. Curb weight against curb weight is fair, while curb weight against a test weight with the driver aboard tilts the result. Driver + Payload never touches the comparison card, so vehicle B is always judged on the weights you typed.

Quarter-Mile Time From Power to Weight

The third result card of the Power-to-Weight Ratio Calculator estimates the quarter-mile time and trap speed from the ratio alone. The headline uses the 2001 formulas of Geoffrey Fox, a physics professor at Santa Clara who later led Fox Racing USA, with weight in pounds and power in horsepower.

$$ET = 6.269 \times \left(\frac{W}{P}\right)^{1/3}$$

$$v_{trap} = 230 \times \left(\frac{P}{W}\right)^{1/3}$$

The default car gives 13.51 s at 106.8 mph. A comparison against 243 Road and Track road tests found Fox’s time formula fits street cars best, with an R² of 0.787. Patrick Hale’s version, ET = 5.825 × (W/P)^(1/3), scored only 0.623 and predicts much quicker times than street cars run.

Hale’s figure still has a use. The card lists it as With race prep (Hale), 12.55 s for the default car, which is closer to what a drag-prepped car with sticky tires could run. The gap between the two rows shows how much launch, traction and shifting cost a street car.

Older rules like Huntington’s take the weight with the driver aboard. Put a 180 lb driver in Driver + Payload and the default car reads 13.77 s. For a fuller estimate that works from speed and acceleration, use the Quarter Mile Calculator.

Best-Case 0–60 mph

The Power-to-Weight Ratio Calculator also gives the physical floor for acceleration in its fourth result card. It assumes 1 g of grip and no losses. At low speed the tires set the limit, and once the car is fast enough, power takes over.

The switch happens where the power can no longer push at 1 g, at the speed found by dividing power per kilogram by g. Below it the car gains speed at g. Above it, kinetic energy rises at the rate of the power.

$$v_s = \frac{P/m}{g}$$

$$t = \frac{v_s}{g} + \frac{v^2 – v_s^2}{2 \times P/m}$$

At 164.4 W/kg the default car switches at 16.76 m/s, which is 37.5 mph. It takes 1.71 s to get there on grip and 1.33 s more on power, so it could not reach 60 mph in less than 3.043 s, or 100 km/h in less than 3.201 s. Power alone, with unlimited grip, would give 2.19 s.

The card also gives the pull at 60 mph, the acceleration power alone can deliver at that speed, as a multiple of gravity. For the default car it is 0.625 g. Real cars are slower than the card because gearing, drag and rotating parts take a share. The Acceleration Calculator and the Kinetic Energy Calculator cover the same motion from other angles.

What Is a Good Power-to-Weight Ratio?

A good ratio depends on what the vehicle is for. The quarter-mile estimate gives a rough scale. On Fox’s street formula, about 0.05 hp/lb points to a 17 s car, 0.1 hp/lb to 13.5 s, 0.2 hp/lb to 10.7 s, and 1 hp per kilogram, 0.45 hp/lb, to about 8.2 s.

Some licensing systems put a hard number on it. In New South Wales, P-plate drivers may not drive a car with a power to tare mass ratio above 130 kW per tonne. A 1,500 kg car therefore has to stay at or below 195 kW, about 261 hp.

Learner motorcyclists in Victoria are limited to bikes with a power-to-weight ratio of 150 kW per tonne or less, and nothing over 660cc. Set the result unit of the Power-to-Weight Ratio Calculator to kW per metric ton to check a vehicle against either rule.

Which cars have a 1:1 power-to-weight ratio?

A 1:1 ratio means one horsepower for every kilogram. Koenigsegg named its One:1 for reaching it, with one megawatt of power and a 1,360 kg curb weight. One megawatt is 1,360 metric horsepower, so the car makes exactly 1 PS per kilogram, or 735 W/kg.

For comparison, the 300 hp default car makes 0.22 hp/kg. The One:1 has about four and a half times its power for every kilogram, which is the territory of race cars rather than road cars.

Horsepower Units and Conversions

What is 1 horsepower equal to?

One mechanical horsepower is 550 foot-pounds per second, which NIST lists as 745.7 W. In plain terms, 1 hp can lift 550 lb by one foot every second. That is the answer to how much weight 1 hp can move, though in a vehicle most of the power goes into acceleration, drag and rolling resistance rather than lifting.

Metric horsepower, PS, is a little smaller at 735.5 W. Brake horsepower, bhp, is not a different size of unit. It names power measured at the engine’s crankshaft, before the gearbox and drivetrain. Two horsepower is simply 1,491 W.

Is 1 hp equal to 1 kW?

No. One kilowatt is 1.341 hp, and 1 hp is 0.7457 kW. A 5 hp motor is 3.73 kW, and a 200 kW electric car makes 268 hp. Type either unit into the power field and the result card converts the solved power in the target modes.

A ton is not a horsepower either. A ton of refrigeration, used to rate air conditioners, is 3,517 W in NIST’s tables, which is about 4.72 hp. For electric motors, 1 hp draws at least 746 ÷ 120 = 6.2 A at 120 V, and real motors draw more because no motor is 100 percent efficient.

How much is 300 WHP in hp?

WHP is wheel horsepower, measured on a chassis dynamometer after the gearbox, driveshaft and axles have taken their share. Crank horsepower is higher. To estimate it, divide the wheel figure by one minus the drivetrain loss as a decimal.

$$P_{crank} = \frac{P_{wheel}}{1 – loss}$$

For example, if a drivetrain loses 15 percent, 300 WHP is 300 ÷ 0.85 = 353 hp at the crank. On the same assumption, 650 WHP is about 765 hp, 700 WHP is about 824 hp and 850 WHP is 1,000 hp. Real losses vary with the drivetrain and the dyno, so a measured figure always beats an assumed one.

Power to Weight for Cyclists

Cyclists use the same ratio in W/kg, from a power meter reading and body weight. A 70 kg rider holding 250 W rides at 3.57 W/kg. Set the power unit to watts, the weight to kilograms and the result to W/kg to use the Power-to-Weight Ratio Calculator for training numbers.

Below 30 W/kg a quarter-mile time means nothing, so the two performance cards switch to climbing. If all the power went into lifting, 3.57 W/kg raises the rider at 0.364 m/s, which is 1,311 m per hour. Cyclists call this vertical speed VAM. The other card gives the ceiling speed up 5, 8 and 10 percent grades, 26.25, 16.44 and 13.18 km/h for this rider.

$$v_{climb} = \frac{P/m}{g}$$

Put the bike’s weight in Driver + Payload so the climb uses rider and bike together. Rolling resistance and air drag lower the real figures, most of all on gentle grades. The Potential Energy Calculator gives the energy stored by each meter climbed.

Climb Rate for Heavy Trucks

Loaded trucks also fall under the 30 W/kg line, which is about 55 lb per horsepower. A 500 hp truck at 80,000 lb has 10.27 W/kg. The Power-to-Weight Ratio Calculator shows a climb rate of 12,375 ft per hour and a ceiling of 46.93 mph up a 5 percent grade, or 23.55 mph up 10 percent.

Real trucks climb slower than those ceilings, since rolling resistance on a heavy rig is large. The same ratio still explains why a truck slows on a long grade while a car does not. Other energy and power tools sit on the energy, work and power calculators hub.