Nix Index

What it is, how it's calculated, and why you should care about it

written by Brad Wright and Olivia Crozier

When you open the Nix app, you’ll find a large colored hexagon (or hex) presented to you with a number at the center. Think of this as a score of your current environment. Our app pulls six environmental factors from the Weather app based on your phone’s current GPS location and when they are factored together, the output is a single score between 0-100.

What it is:

The Nix Index is based on Wet Bulb Globe Temperature (WBGT). WBGT is a measure of heat stress in direct sunlight. It was originally developed for the U.S. armed forces during the 1950s in response to heat illnesses and casualties experienced during military training.  

At Nix, we use it as an environmental score to help indicate how the environment will impact your sweat rate during a workout. The Nix Index is based on Relative Humidity, Wind Speed, Solar Load and Temperature, while also taking into account Dew Point and Altitude. Unlike WBGT, the Nix Index does not account for the Sun Angle. The Nix Index is also normalized to a scale of 0 to 100, whereas WBGT can go below 0 or above 100. However, because the Nix Index is normalized it is very easy to interpret. The higher the Nix Index, the more heat stress and sweat you are likely to experience during your workout.

How it is calculated: 

The Nix Index is calculated by taking the weighted sum of three environmental derived factors: wet bulb temperature (Tw), globe temperature (Tg) and ambient air temperature (Td). These three factors are derived by plugging in (Temperature, Humidity, Wind Speed, Cloud Cover) into a very large equation that doesn’t fit on this page. 

It’s important to note that for indoor workouts the Nix Index uses fixed values: 70°F, 50% humidity, 0% Solar Load, 0 MPH Wind Speed. At this point in time, you’ll always receive the same Nix Index of 59 when you log an indoor workout in the Nix app.

How it is displayed: 

You’ll see your current Nix Index on the homepage of the app each time you open it. You’ll also see a Nix Index score alongside each of your completed workouts. The color of the hex will change in the app based on whether the Nix Index is low, moderate, high or extreme. 


0-30: Low

31-60: Moderate

61-90: High

91-100: Extreme

Why you should care: 

Your body is going to sweat differently in different environments, however, most athletes don’t know how differently they sweat day to day and in different conditions. Of course the most obvious differences would be how much fluid you should consume during your workout based on how much you sweat in that particular environment. But with your sweat rate differing based on the environment, it will also affect both your electrolyte loss rate and your overall sweat composition. This means that you should be consuming different types of fluids based on the changes to your composition.

How you can use it: 

The Nix Index can be extremely helpful to athletes during training and leading up to a big race. Let’s say you’ve spent the past eight weeks training for the Boston Marathon in the brutal conditions that are icy sidewalks and snowbanked crosswalks. You’ve likely dialed in your hydration strategy in the weeks leading up to the race and are feeling good about what you’ll be drinking and when. Then the weather forecast comes out and in typical New England fashion it’s going to be far warmer than you had planned for. Think Record-Hot New York City Marathon 2022 where temperatures were 75 degrees F and 75% humidity in November. Once you know the predicted weather for your upcoming race, you can go back to your previous workouts within the Nix app, see your sweat profile in those specific conditions, and plan your hydration accordingly. 

Before we conclude, let's answer the question that many of you have wondered but some haven’t asked. Is the Nix Index a score of my workout? No. The Nix Index is a score of the environment in which your workout took place. If your workout has a Nix Index of 40 next to it, does that mean you failed your workout? No. It means that you were training in moderate conditions. The Nix Index helps athletes take their hydration strategy one step further by not only recognizing how you sweat differently day to day but also environment to environment.