Power-to-Weight Ratio Calculator

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?

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!

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

• The weight of the vehicle $W$; and
• The power of the engine $P$.

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

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 $\text{PWR}$ 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.

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.

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 $1,233\ \text{kg}$ (or $2,718\ \text{lb}$), and a power output of $130\ \text{hp}$ ($97\ \text{kW}$), which gives it a power-to-weight ratio of $78.7\ \text{kW}/\text{kg} = 0.0479\ \text{hp}/\text{lb}$.
• The two engines of a Boeing 777, the GE 90-115B supply a power of $23\ \text{MW}$ ($30,844\ \text{hp}$). The maximum weight of the plane is $299,370\ \text{kg}$ or $660,000\ \text{lb}$. The power-to-weight ratio we calculate is... $76.8\ \text{kW}/\text{kg}$: The same as the DeLorean!
• A $70\ \text{kg}$ cyclist in a really good shape can produce easily $200\ \text{W}$ of power. The power-to-weight ratio is $2.6\ \text{W}/\text{kg}$. Lance Armstrong, by comparison, is assumed to have produced more than $7\ \text{W}/\text{kg}$ at his peak performance. Look at the difference between planes and cyclists, but remember: two wheels and pedals are the best way to move!