Your speed in Zwift is controlled by your power number, the level of effort you’re putting out at any given moment. Power is measured in watts and you can always see the watts you’re putting out by looking up in the top left corner of Zwift.

If you remember back to physics class, it takes more effort (power) to move bigger, heavier masses around than lighter ones. And that’s why we look at power not just as an absolute number, but as one relative to a rider’s weight: watts per kilogram (power-to-weight ratio). To work out your power-to-weight ratio figure, simply divide your power output (in watts) by your weight in kilograms (kg). For example, a 125lb/56kg rider with a power output of 195 watts, is riding at 3.5 w/kg. Don’t feel like doing the math to find your number? Zwift automatically calculates this for you in game.

Why does w/kg matter?

W/kg matters as an indicator of performance and speed. On a smooth, flat road, cyclists with a greater power number will almost always go faster. But go to the hills or even on undulating roads and w/kg really starts to matter. If we have two cyclists both producing 250 watts, on a flat road, they’ll be about even. But when the road turns to hills, the lighter of the two cyclists will be faster.

W/kg in Zwift

In Zwift, you can always see your w/kg by looking at the number right below your name in the Riders list. This number will change as your ride, but should be relatively consistent if you’re putting in an even effort.

We also use w/kg as a means of categorizing group rides and races.

Want to improve your w/kg?

There are basically just three ways in which you can affect change to W/kg.
Increase your power while keeping your weight constant
Keep your power constant and decrease your weight.
Increase your power while also decreasing your weight.

Want to know even more?

https://www.trainingpeaks.com/blog/why-you-should-focus-on-watts-per-kilogram/

https://zwiftinsider.com/rider-categorization-based-on-ftp-how-do-you-rank/