This post is part of Find Your Fit Tech, Lifehacker’s fitness wearables buying guide. I’m asking the tough questions about whether wearables can really improve your health, how to find the right one for you, and how to make the most of the data wearables can offer.

“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 you could hold a conversation doing it. You stop a zone 2 session because your workout time is up, not because you’re too exhausted to continue.

But what heart rate should you expect to see on your fitness smartwatch 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 I recommend a good $25 one here).

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; and zones 3, 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 that system 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 to do a real-world test, not just calculate it from a formula. For example, one formula calculates my max heart rate as 178 beats per minute, and another says it should be 169; in reality, I’ve seen as high as 207 when I’m running, and 198 on a spin bike. (And yes, your max can differ for different types of cardio. My heart rate while I’m swimming would probably be lower still; when your body is horizontal, your heart has an easier time moving your blood around.)

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 gadgets 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:

• 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: 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 minus 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 percentage of max heart rate, zone 2 is 73-81%. As a percentage of heart rate reserve, it’s 65-75%. And as a percentage of your lactate threshold heart rate (which the watch can automatically detect for you, and which normally falls between zones 4 and 5), it’s 79-88% of that heart rate. Note that these numbers won’t necessarily line up with each other. A heart rate that is in zone 2 on one of these systems may be in zone 3 on another. And, of course, you can set your max and/or your zones manually.

Some other fitness platforms have defined heart rate zones to be used with your training. To name a few:

• Orangetheory gets its name from the “orange” zone it wants you to be in during workouts. Its equivalent of zone 2 would be the “blue” zone, at 61% to 70% of max heart rate. It uses an “industry standard formula” to determine your max, which Self reports is 208 minus 0.7 times your age. After you’ve taken 20 classes, an algorithm will pick out a new max heart rate for you.

• Peloton defines heart rate zone 2 (no relation to Power Zone 2) as 65% to 75% of your max heart rate. Max heart rate is 220 minus your age unless you adjust it manually in your settings.

• The American College of Sports Medicine defines “light” training, arguably its version of zone 2, as 57% to 63% of maximum heart rate. “Moderate” is 64% to 76%.

How do you know which benchmark to use?

Rather than obsessing over numbers, think about the big picture and decide what training effect you are trying to achieve with your workouts.

If you want to build your endurance with low-intensity cardio, or if you want to rack up minutes in this zone to help with weight loss, it doesn’t matter exactly what your heart rate works out to be. What matters is that you can do the exercise for a long time without fatiguing, but that you’re also not slacking off and barely doing any work at all.

In other words, you can use your gadget’s heart rate numbers as a guide, but keep them honest with a reality check based on what fitness professionals call “perceived exertion.” If you want a number to focus on, you can rate your exertion on a scale of 1 to 10—called RPE for “rating of perceived exertion”—and aim for an RPE of about 3 to 4.

Over time, you’ll start to notice what heart rate tends to show on your watch when you’re at that level. I know that if my heart rate is in the 150s, I’m doing a good job of keeping my jogging to a “zone 2″ sort of effort. If it pokes up into the 160s at the beginning of a run, that’s probably harder than I’m going for—but if it hits 160 at the end of a long run on a hot day, that’s fine. (Heart rate changes with the temperature and the length of your workout, a phenomenon called cardiac drift.)

Ultimately, this is probably the most accurate way of using heart rate to determine exercise intensity: figure out the intensity you want first, and use heart rate as a guide to be able to hit that same intensity on a consistent basis. After all, if there were one correct number that was easy to determine, the different gadgets and platforms would have all gotten on board with it by now. So trust your body more than your watch.