Engineers will find our power-to-weight ratio calculator a handy tool to measure the performance of an engine.

In this article, you will learn:

  • What is the power-to-weight ratio?
  • How to calculate the power-to-weight ratio?
  • How does the power-to-weight ratio affect the performance of a vehicle?

What is the power-to-weight ratio?

The power-to-weight ratio is a vehicle specification which somehow uniforms the comparison of the performance of a vehicle-engine pair.

The ability of an engine to supply a vehicle with acceleration and speed is measured in power. The power of an engine is assimilable to the energy output per unit of time: we can measure it in watts or, traditionally, in horsepowers.

The power, however, is not everything. Put the engine of a Dodge Viper on a Reliant Robin, and you will easily understand why!

How do I calculate the power-to-weight ratio formula?

To calculate the power-to-weight ratio, be sure to know:

  • The weight of the vehicle WW; and
  • The power of the engine PP.

The formula for the power-to-weight ratio is simple:

PWR=PW\text{PWR}=\frac{P}{W}

As you can see, the weight of the vehicle is at the denominator: the heavier the vehicle, the lower the power-to-weight ratio. The same effect comes from a lower power.

A lower PWR\text{PWR} gives the platform worse performances. If you want a car that accelerates quickly and goes fast, check for higher values of power-to-weight ratio.

🙋 You can calculate the power-to-weight ratio also using the engine's weight, though this will not give you all the information about the vehicle's performance.

Not only cars: how to measure the power-to-weight ratio in different situations

The power-to-weight ratio is not exclusive to cars or motorcycles. Since moving from one place to another requires performing work (in the physical sense) — that is energy — in a given time, we can define a power.

You have a power-to-weight ratio. It doesn't really matter when you are walking in the city, but the moment you don a backpack and head for the mountain, it does matter! That's why you want to pack light. The same holds for running: look at how lean is a marathon runner compared to a sprinter.

We can apply the power-to-weight ratio to cycling, too: it's even easier to calculate: To measure the power-to-weight ratio of a cyclist, you only need a power meter and a scale!

Planes, rockets, and helicopters all have a power-to-weight ratio, though it's common to express the performance of flying vehicles using the lift-to-drag ratio or the thrust-to-weight ratio.

Examples of power-to-weight ratio calculations in real life

Now that you know how to calculate the power-to-weight ratio let's see some examples where we can apply the formula for the power-to-weight ratio.

  • A DeLorean DMC-12 (the car of Back to the Future), has a weight of 1,233 kg1,233\ \text{kg} (or 2,718 lb2,718\ \text{lb}), and a power output of 130 hp130\ \text{hp} (97 kW97\ \text{kW}), which gives it a power-to-weight ratio of 78.7 kW/kg=0.0479 hp/lb78.7\ \text{kW}/\text{kg} = 0.0479\ \text{hp}/\text{lb}.
  • The two engines of a Boeing 777, the GE 90-115B supply a power of 23 MW23\ \text{MW} (30,844 hp30,844\ \text{hp}). The maximum weight of the plane is 299,370 kg299,370\ \text{kg} or 660,000 lb660,000\ \text{lb}. The power-to-weight ratio we calculate is... 76.8 kW/kg76.8\ \text{kW}/\text{kg}: The same as the DeLorean!
  • A 70 kg70\ \text{kg} cyclist in a really good shape can produce easily 200 W200\ \text{W} of power. The power-to-weight ratio is 2.6 W/kg2.6\ \text{W}/\text{kg}. Lance Armstrong, by comparison, is assumed to have produced more than 7 W/kg7\ \text{W}/\text{kg} at his peak performance. Look at the difference between planes and cyclists, but remember: two wheels and pedals are the best way to move!
Davide Borchia
Power
hp(l)
Weight
lb
Power-to-weight ratio
hp(I)/lb
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