Power to Weight Ratio Calculator

Why hp per ton means three different things

There are three tons in daily use and they are not the same weight. NIST Handbook 44, the American reference on units of measurement, defines the US short ton as 2,000 pounds, or 907.18474 kg; the long ton, the one historically used in Britain, as 2,240 pounds, or 1,016.0469 kg; and the metric tonne as 1,000 kg, which is 2,204.623 pounds. The long ton is exactly 1.12 short tons, so any car quoted in hp per ton is quoted 12 percent higher in the British sense than in the American one, and both figures are correct. A 300 hp car weighing 3,000 lb is 200 hp per ton, or 224 hp per ton, depending on nothing but which side of the Atlantic the sentence was written on.

That is the whole reason this page exists. A single figure with the word "ton" in it carries no information unless the ton is named, and most of the time it is not. The metric tonne is the one unambiguous unit of the three, which is why it heads the result here, and W per kg is the same statement with the units stripped down: it is identical to kW per tonne, digit for digit, because the thousands cancel. If you only ever quote one number, quote one of those two.

Which convention is used where

The habits are regional rather than standardised, so the following is what you will typically meet rather than a rule. The British car press has long written power to weight as bhp per ton, and where the figure is old enough it is a long ton; more recent writing often means a tonne while keeping the older phrasing, which is exactly the ambiguity above. European manufacturer spec sheets typically quote kW per tonne, sometimes PS per tonne, both being what the metric system makes natural. American writing typically inverts the fraction and quotes pounds per horsepower, where a lower number is the quicker car, and drag racers think almost entirely in those terms because the classic bracket formulas take weight over power as their input. Cycling, electric aircraft and the battery world use W per kg, since it scales from a hub motor to a locomotive without changing units. All of them are the same division, and this page runs it in all of them at once.

Crank, wheel, curb and as raced

A power to weight ratio is only as honest as the two figures in it, and both have conventions of their own. A manufacturer rating is measured at the crankshaft; a chassis dyno measures at the wheels, after the gearbox, driveshafts and differential have taken their cut. The size of that cut is quoted as folklore rather than measured, commonly 10 to 15 percent, and it is why the horsepower and torque calculator and the trap speed tool both treat any wheel-to-crank conversion as a stated assumption rather than a fact. Weight is worse: curb, kerb, dry, shipping, as-raced and with-driver are all in circulation, and the spread between the lightest and heaviest of them on the same car can reach 10 to 20 percent once a driver, a full tank and a spare wheel are argued about.

Ten percent on the power and fifteen on the weight is a quarter of the answer, which is more than most of the differences people use these numbers to settle. So the tool asks you to label both, echoes those labels in the output, and says plainly when two cars have been entered on different bases. It does not convert between them. Turning a wheel figure into a crank figure would mean picking a loss percentage and presenting a guess as a result, and that is the one thing this section refuses to do.

What this tool will not do

It will not predict a time. Power to weight is the single most quoted performance number and one of the weakest predictors on its own, because it says nothing about gearing, traction, aerodynamic drag, the shape of the torque curve or whether a driver can use any of it. Two cars at 200 hp per tonne, one rear-drive on street tires and one all-wheel-drive on slicks, do not run remotely similar quarter miles. If a time is what you want, the quarter mile calculator takes weight and power and states its own error band, and the top speed calculator solves the road-load equation where drag, not weight, is what decides the answer.

It also does not judge. There is no good or bad ratio in here, no class boundaries and no verdict on whether a number is quick, because that depends on the car, the surface and what it is being compared against. The torque rows are offered for the same reason and with the same caution: torque per ton is meaningful for a tow vehicle or a tractor, and close to meaningless as a performance figure on a car, because gearing multiplies torque and only power survives the gearbox unchanged.