Power-to-Weight Ratio – How Is It Calculated

By Jiri Kaloc

The power-to-weight ratio is a common cycling term because it’s a great predictor of performance, especially for those riding on steep pitches and long mountain passes. Do you know how is it calculated and what the resulting number means? You will learn how to find out your own power-to-weight ratio in the first part of this series.

Cycling is a sport where the amount of power you can generate with your legs heavily influences how fast you will be. But absolute power is not the only thing. Take two cyclists, one can put out 250 watts (W) of power and the other only 240W, who would win a race to the top of Alpe d’Huez? It should be the 250W guy, right? Not if he weighs 100 kg and the other only 65 kg. That’s because carrying a lot of weight uphill is hard, even on a bike. Larger cyclists usually have more muscle and power but they also have to drag more weight uphill. That’s why the power-to-weight ratio (PWR) is useful for estimating how cyclists will perform in real-world conditions.

Romain Bardet during the Tour de France 2016. © Profimedia, AFP

A simple formula to calculate the power-to-weight ratio

The power-to-weight ratio is usually expressed in watts per kilogram. So, all you need to do is divide your maximum power output in watts by your body mass in kilograms.

PWR = max power (W) / body weight (kg)

For example, an 80-kg rider with a maximum power of 280W has a power-to-weight ratio of 3,5 watts per kilo, or 3,5W/kg, sometimes also expressed as 3.5W.kg-1.

The power-to-weight ratio is time specific

It’s important to remember that the power-to-weight ratio is not a static number. It always corresponds to a specific time. For example, if our rider can sustain those 280W for 60 minutes then his PWR of 3,5W/kg corresponds to those 60 minutes. For 30 minutes, his PWR might be 4W/kg and maybe even 4,5W/kg for 15 minutes.

Test your own power output

As you can see, you need two numbers to calculate your own PWR – weight and power output. The weight part is easy, you can use any bathroom scale and the results will probably be accurate enough. The second number requires a power output measurement. To get that number, you will need a bike with a reliable power meter. You can get a power meter for both your pedals, one or both crank arms, in the chainring spider or the rear hub. If you don’t feel like making that investment, try looking for a gym or a cycling store that has a WattBike you can use.

To measure maximum sustainable power, warm up with an easy spin for about 10 minutes and then take a few minutes rest. After that pedal as hard as you possibly can for 20 minutes and record your average power output in watts. This is your 20-minute maximal sustainable power output. From that, you can estimate your 1-hour max power, which will be roughly 5-10 % lower. If you manage to sustain 295W for 20 minutes then your 1-hour max might be close to 280W.

In the next article, we will take a closer look at why cyclists obsess about their power-to-weight ratio so much and in what scenarios is matters more than absolute power.