What ‘Cardio Load’ Really Means in the Fitbit App

Cardio load calculation is a metric that the Fitbit app uses to suggest how much exercise Pixel Watch and Fitbit users should do, but it can be difficult to understand. It was also recently updated for the new version of the app, and it works a little differently. Here’s how to use this number and what it means to reach your target.

What is cardio load?

Cardio load is a way to understand how much exercise you’ve done, whether the app recorded it as a workout or not. Exercising for longer and exercising at a higher intensity both increase your cardio load.

For example, on a day when you run five miles at a slow pace, you will have a higher cardio load in the Fitbit app than on a day when you ran three miles at a slow pace. If you run three miles at a more intense pace (let’s say you run a 5K), your cardio load will be somewhere in between.

Here are some examples from my own workouts:

• A track workout that had me alternating between moderate and maximum heart rate zones for an hour (five miles total) had a cardio load of 117.

• 20 minutes of detangling my child’s hair was recorded as training, but since my heart rate was in the light zone all the time, I had no cardio load.

• A 53-minute gym workout, which included a mix of heavy movements and lighter continuous work, timed at one hour cardio load of 63.

“Load” here is in the sense of “workload”. If this summer you exercised an hour a day and now you only exercise 30 minutes every other day, your cardio load for the week (and for each day) will be lower than in the summer. It makes sense, right? If you were to spend the entire next week exercising for an hour a day, that would be much higher than your current cardio load. And the Fitbit app would let you know that you’ve suddenly increased your cardio load and might want to relax a bit.

What is your target cardio load?

The Fitbit app automatically calculates a target cardio load based on what you usually do. You can choose whether you want to improve your fitness (in which case this will encourage you to increase your load a little more each week) or maintain your current fitness. You’ll find this setting when you look at your cardio load in the app: just tap “fitness goal” at the bottom.

How the new FItbit app manages cardio load

In the original implementation, your cardio load goal could change from day to day. Fitbit recently released a preview of the new version of its app, and this version now tracks cardio load every week, making a lot more meaning. So instead of telling you that you should hit a certain load today, you’ll be told that you’re, say, 41% of the way toward your target load for the week.

The next version of the Fitbit app (currently in “public preview”)
Credit: Beth Skwarecki/Fitbit

Which devices support cardio load?

Currently, the devices that have a cardio load are:

• Pixel 1, 2, 3 and 4 Watches

• Fitbit Charge 5 and 6

• Fitbit Versa 2, 3 and 4

• Fitbit Sense 1 and 2

• FItbit Luxury

• Fitbit Inspire 2 and 3

Pixel watches can show you your cardio load on the screen, but for others you’ll have to view it in the phone app.

Other apps and platforms have their own versions of cardio charging. For example, some Garmin devices measure training load (as well as acute/chronic load and load concentration), but it is calculated and displayed a little differently than Fitbit. This article only covers the Fitbit/Pixel version.

The difference between cardio load and active zone minutes

Both metrics describe how much exercise you do and give you extra credit for intense exercise compared to moderate exercise. But they have different purposes and are calculated a little differently.

The purpose of Active Zone Minutes is to determine whether you are meeting certain basic health exercise goals. Active zone minutes correspond to the U.S. physical activity guidelines, which recommend that we all do 150 minutes of moderate exercise per weekor 75 minutes of vigorous exercise. In other words, it’s a minute countdown, vigorous exercise (like running) counting double. This is why your 30 minute workout can count as 45 zone minutes, if 15 of those minutes were moderate and 15 vigorous (15 x 2 = 30).

(There is a caveat to this: Fitbit uses your heart rate to estimate whether a given minute of exercise was vigorous or moderate for you. The the original guidelines used METs, not heart rateso it’s not a perfect match. But it’s close enough to be useful.)

Cardio load, on the other hand, is a metric more often used by athletes to ensure their exercise effort is within the optimal range to improve or maintain their fitness. Fitbit uses a modified version of the TRIMP algorithmwhich essentially multiplies your heart rate by the number of minutes you were at that heart rate. Higher heart rates carry a bit more weight than lower ones, as Google explains in this document. If your heart rate is below a certain level, it’s not counted, which is why my hair brushing sessions don’t count as cardio.

With a cardio load, you’re not just trying to beat a minimum to give yourself a passing grade: you’re trying to stay within a specific window, defined by the amount of exercise you’re used to doing. If you exercise a little more each week, you can stay within your target range while pushing the limits of that target range. This is how you get fitter.

On the other hand, if you exercise a lot more or a lot less this week than your body is used to, you might end up losing some fitness (if you do less) or get more tired than usual (if you do more). Depending on where you are in your training, these results aren’t necessarily a bad thing. But with a cardio goal to compare your load to, you at least know where you stand.