“Zone 2” is the term the fitness world has (mostly) agreed upon to describe the low-intensity cardio most of us should be doing regularly. When you’re in zone 2, you’re working hard enough that you start breathing more heavily, but easy enough that you could hold a conversation while doing it. You stop a zone 2 session because your workout time is up, not because you’re too exhausted to continue. Zone 2 is defined in terms of heart rate, so what heart rate should you expect to see on your watch when you’re in zone 2? That’s where people disagree.

What is zone 2 training?

As I’ve explained before, the name “zone 2” comes from heart rate training. To train by heart rate, you use either a wristwatch with an optical heart rate sensor (that green light on the back) or a chest strap paired to your watch or just to a phone (chest straps are more accurate, and even a $30 one can do an excellent job).

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To train by heart rate, you aim to keep your heart rate in the "zone" that gives you your desired workout. In most of the popular systems, there are five zones. Zone 1 is your resting or recovery zone; zone 2 is low intensity cardio; zone 3 is more or less medium; and zones 4 and 5 are for harder efforts, usually done for only a few minutes with recoveries in zone 1 or 2 in between. (I have a more detailed guide to the zone system here.) While zone 2 is the trendiest at the moment, the other zones still have uses. Personally, I think zone 3 is underrated, and probably most of us would be better off getting a mix of zones 2 and 3 for our steady cardio rather than pure zone 2. But that's a story for another time.

Heart rate zones are usually defined as percentages of your maximum heart rate. So when I set my Apple Watch to keep me in zone 2 during my runs, it wants my heart rate to be between 60% and 70% of maximum. Even at an easy effort, I found I was commonly exceeding that limit. On the other hand, when I hop on a Peloton bike, my heart rate is often still in zone 1 when I could swear I’m riding at a zone 2 effort. It turns out Peloton defines zone 2 as 65% to 75% of my max.

Who is right? Well, everybody. “Zone 2” isn’t a term with scientifically designed boundaries. Anybody can split up heart rate zones any way they like. (Stay tuned for my patented eight-zone system, coming as soon as I can find a way to monetize it!) If you train with more than one gadget, or if you find yourself discussing heart rate training with a friend who uses a different system than you do, it’s worth knowing the differences.

What heart rate percentage counts as zone 2?

Let’s take a tour of some of the more popular wearables and fitness systems that measure heart rate in a five-zone system, or something like it.

First, it’s important to know that most (not all) of these percentages are based on your max heart rate. To know your max heart rate, you need real-world numbers, and you shouldn't trust the default your app gives you. That default number is derived from a formula, and no formula will be accurate for everybody; max heart rate varies from person to person and can't be accurately predicted for individuals. You can do a field test, like getting your heart rate up by running more and more intense hill repeats. Or if you have plenty of experience with intense exercise, just take note of the highest heart rate number you've seen on your device; it will likely be close to your max.

Most zone systems just use a percentage of your max (however that max is calculated). There are other systems to consider, too. “Heart rate reserve” (HRR) means that you take the difference between your max and your resting heart rate (instead of between your max and zero) and calculate from there. Some devices will estimate a different benchmark, like your lactate threshold, and use that as a basis for the zones.

So, here are the zone 2 percentages from a variety of popular wearables, along with what they are percentages of:

What do you think so far?

- Apple Watch: Zone 2 is 60-70% of your heart rate reserve, with your “resting” heart rate set to either 72 or a number the watch has picked up automatically, and your maximum calculated with the 220-age formula. (You can choose to set the zones manually, instead.)

- Fitbit and Pixel: instead of “zone 2,” Fitbit devices have a "moderate" zone (formerly called “fat burn”) set at 40% to 59% of your heart rate reserve. To find your heart rate reserve, your max is calculated according to the 220-age formula, and your resting heart rate is measured by the device. You can set your max and your zones manually if you prefer.

- Garmin: Depends on your device and on how you've chosen to set up your zones. As a per

Nobody Can Agree on What 'Zone 2' Cardio Is