What Are ‘Zone Minutes,’ and Why Does Fitbit Sometimes Double Them?

If you have a Fitbit, or if you use the Fitbit app with a compatible device like a Pixel watch, you've probably seen zone minutes" or "active zone minutes" pop up on your screen as a metric that the app thinks is worth paying attention to. So what exactly are zone minutes, and how do they compare to the other ways we measure exercise, like steps?

Zone minutes relate to the U.S. Physical Activity Guidelines

Before we get into how zone minutes are measured, I think it will be most useful to look at why we're counting up minutes at all. Fitbit's purpose here is actually pretty neat: They're trying to give you an automated way of telling whether you are keeping up with public health guidelines for exercise, like the World Health Organization's recommendations for exercise, and the Physical Activity Guidelines for Americans (which, conveniently, use the same numbers).

As I explain here, these guidelines state that we should all get at least 150 minutes of exercise each week. Moderate" exercise counts for the guidelines, which includes activities like walking and housework.

The guidelines also say that if you're getting vigorous" exercise, like running, you only need 75 minutes to meet the guidelines. Another way of thinking of these numbers is to consider vigorous exercise as counting double. If you've done 100 minutes of moderate exercise (like walking), you only need 25 minutes of vigorous exercise to meet your weekly goal.

And while 150 minutes is a target, it's actually only the entry level target. The U.S. and World Health Organization guidelines both say that if you're hitting 150 minutes just fine, you should set your sights on a 300-minute target. In general, more exercise is better (within reason), so it's handy to keep tabs on how many minutes' worth of exercise you've done this week. Fitbit keeps track of that for you.

Zone minutes are detected based on your heart rate

Now we can talk about where zone minutes come from, and why they're called zone" minutes. Your Fitbit (or compatible gadget) can measure your heart rate pretty easily, so the app simply marks you down as completing a zone minute" any time your heart rate seems to be in a zone that suggests you're exercising.

There are many ways of dividing up heart rates into zones, and I have a rundown of the most common schemes here. Fitbit likes to use a system where, instead of numbers, your heart rate can be at rest, or in one of these zones:

• Moderate or fat burn": 40% to 59% of your heart rate reserve, or 50% to 69% of max heart rate, depending on your device (each minute here earns you one zone minute)

• Vigorous or cardio": 60% to 84% of your heart rate reserve, or 70 to 84% of your max heart rate (each minute here counts double)

• Peak: 85% or more, by either metric (each minute here counts the same as vigorous)

To be totally clear, the Physical Activity Guidelines are not based on heart rate, but on a different measurement called METs. In the MET system, walking is always moderate and jogging is always vigorous. (More about that system here.) On the other hand, in the zone minutes" system, an experienced runner may find that jogging is so easy for them that their heart rate stays in the moderate zone.

So zone minutes are not a perfect match to the guidelines, but they'll get you pretty close without having to ask you what you're doing or for how long. Your gadget counts up the minutes, and at the end of the week you can see whether you beat your 150-minute (or whatever) target. For example, my app is telling me that I logged 349 zone minutes last week. Great! And I didn't have to do anything to count them.

If your zone minutes don't make sense, check your heart rate settings

Here's my main complaint about zone minutes: they only make sense if your zones are set correctly. By default, Fitbit calculates your heart rate zones from what it guesses your maximum heart rate probably is. And as I've explained before, no formula can accurately tell you what your maximum heart rate actually is. It's very common for these calculations to be off by 10 or more beats in either direction, which screws with your zones.

So go into your app, and take a look at the zone minutes you've earned each day. Are you getting moderate" zone minutes when you walk or do light exercise, and vigorous" zone minutes for jogging or anything strenuous? If so, your zones are probably close enough to accurate, and you can assume they're giving you a reasonable estimate of how much exercise you're getting.

But if you're getting vigorous" minutes from walks, or moderate" minutes from hard exercise, you may want to adjust your zones. Tap your profile picture in the FItbit app, then select Fitbit Settings, then Heart Rate, then Heart Rate Zones, and check the box for Custom Max Heart Rate.

That will let you manually enter a max heart rate. If you've done a max heart rate field test, use that. Otherwise, enter the highest number you've seen when doing a hard workout. If you haven't done a hard effort, but your zones seem really off, you can experiment with putting in different numbers and seeing what that does to your zones. As long as you're getting moderate minutes when doing moderate exercise, and vigorous or peak minutes when you're doing harder exercise, you're getting useful metrics to track how active you are.