If you own a Fitbit Alta HR, Fitbit Charge 2, Fitbit Blaze, or Fitbit Surge and have entered your birthday into your profile, you’ve probably noticed a colorful bar graph pop up in your workout summaries. This chart tracks how much time you spent in each heart rate zone—ranges that represent a percentage of your max heart rate and correspond to different exercise intensities.

For example, you’re considered in your Fat Burn zone when your heart rate is between 50 and 69 percent of your max heart rate. Targeting specific heart rate zones can help you exercise more efficiently and hit your fitness goals faster—but only if you’re using the right ones.

How can you know whether you’re exercising at the right intensities? You have to verify your max heart rate. Why? Well, that requires a brief math and science lesson. Don’t worry, you won’t be tested on this later.

The most effective way find out your max heart rate is to get a maximal stress test at a cardiologist’s office, health lab, or sports science facility, says Roy Benson, running coach and co-author of Heart Rate Training. The technician will put you on a treadmill and gradually increase the pace and incline until you have to stop.

To spare you the time and money of a stress test, Fitbit estimates your max heart rate using the industry-standard formula of 220 minus your age (hence why you should put your birthday in your profile).

This formula is designed to work for the majority of the population, says Benson. But it’s impossible for it to be one hundred percent accurate for everyone because max heart rate is like height—it varies by individual.

“Max heart rate doesn’t have anything to do with performance or how fit you are,” says Benson. It’s simply a function of genetic factors, such as the size of your heart. Some hearts are larger and need to beat fewer times, while smaller hearts beat more often to push blood around your body.

As a result, true max heart rates are distributed across the population in a bell curve. That means that although two-thirds of people will have max heart rates within 12 beats of the number the formula (220-age) calculates for them, there will be outliers on both ends of the spectrum. “When you’re approaching the far slope of a bell-shaped curve, you can see some pretty extreme numbers,” says Benson. “And there’s nothing wrong with that. That’s just the way you were born.”

If you’re among that two-thirds, you’ll end up with zones that are close enough to help place you at the right exercise intensity. But if you fall within the remaining one-third, the farther your max heart rate is from the average, the less helpful automatically generated heart rate zones will be for you. And that can lead to a lot of ineffective exercise.

While exercising, compare your rate of perceived exertion (RPE)—i.e. how hard you feel like you’re working on a scale of 6 to 20—to the exercise zone your Fitbit tracker tells you you’re in. This guide should help:

FAT BURNING ZONE
Intensity: Moderate (50 to 69 percent of your max hr)
RPE: 11-14
Feels like: Your heart rate and breathing is elevated, but you’re still able to talk in full sentences or sing out loud.

CARDIO ZONE
Intensity: Hard (70 to 84 percent of your max hr)
RPE: 15-16
Feels like: You’re breathing harder and only able to speak in short sentences, but the pace is sustainable—you’re not fantasizing about quitting or worried that you won’t be able finish the workout.

PEAK ZONE
Intensity: Vigorous (85 to 100 percent of your max heart rate)
RPE: 17-19
Feels like: You can only sustain this effort for a few minutes and uttering more than one word at a time is difficult.

If your effort level seems to match the heart rate zones indicated on your Fitbit tracker, you can skip the rest of this article and carry on, comfortable in knowing you fall within average ranges and the automatically calculated zones are working  for you.

If, however, things feel out of sync—maybe every time you start to work a bit hard, your heart rate shoots to the highest zone and stays there (perhaps even exceeding your predicted max), or you find that you can work yourself to exhaustion and still not get your heart rate out of the moderate zone—then your max heart rate may be an outlier and is not adequately gauged using the predictive formula.

If you exercise regularly and are fit and used to doing hard interval workouts, try a max-out test. “Go out and run for 20 to 25 minutes, gradually increasing your effort, with a really hard two minutes at the end,” says Benson. “Or do a hard interval workout, like three to four 800m (2 laps of a track) or 1200m (3 laps) repeats and make sure the last couple of minutes are all-out.” The duration of your rest periods (jogging or walking) should not exceed your hard efforts. In other words, if one 800m interval takes you three minutes and 30 seconds, your rest period should be three minutes and 30 seconds or less.

And all-out means just that: panting and pushing for the full two minutes and topping it off with a sprint. The top reading at the end of this two minutes of all-out work will be close enough to use as the basis for your heart rate zones.

Note: You can’t just run as hard as you can from the start and look at your heart rate when you feel like quitting. “You need to speed it up gradually, then once you get running really fast, increase the effort,” Benson says. A fast 100m won’t give your heart time to get to max, or, if you run a fast 400m or up a steep hill, your muscles and aerobic system will max out before your heart rate does.

Schedule this test on a day when you’re feeling rested, healthy, and motivated. It will serve as a hard workout in your training week.

If the idea of pushing yourself to your limit is unappealing, you can get an estimate of your max heart rate by carefully paying attention to your effort and heart rate during a sub-maximal test.

Go to a track, run on a treadmill, or use a route where you know approximate distances, says Benson. After warming up, run three-quarters of a mile, or three laps of the track, at a gentle pace. “During this phase you should be able to talk easily and barely notice your breathing,” Benson says. “At this effort, you would not feel tired unless you went for many miles or a long time.” Note your heart rate during the last few minutes of this phase.

At the end of the three-quarter mile, do not stop. Simply speed up to a moderate pace for another 1/2 mile (2 laps). This phase should feel like running, not jogging. You can still talk, but in short sentences. Note your heart rate during the last few minutes of this phase.

At the end of your moderate laps, without stopping, speed up again for a final quarter mile (1 lap) at a strong but not maximal effort. This should feel fast—about as fast as you can run without sprinting. By the end you’ll be panting, and happy to stop, but not straining in a heroic effort. You should finish feeling like you could run another lap if you had to. Take note of your heart rate at the end of this phase.

To calculate your max heart rate, divide the easy heart rate by .75, the moderate by .85, and the final heart rate by .90. Use the average of those three numbers as your max heart rate.

Benson admits that this method is trickier, and relies on subjective evaluation of your effort, but it should get you close enough to set effective heart rate zones. And, he says, “It’s a more humane way to find your max than the cruel and unusual punishment of all-out intervals.”

Once you have your new max heart rate number, you can enter it into your Fitbit settings. On the Fitbit app dashboard, tap or click Account and find the option to change your heart rate zones. From there you can create a custom max heart rate. Your Fitbit will automatically calculate your heart rate zones based on this max. Going forward, you should then feel an appropriate level of effort when exercising in the easy (Fat Burn), moderate (Cardio) and hard (Peak) zones.

If you manually set your zones, they won’t automatically adjust as you age. You’ll have to reset them every few years as your max heart rate declines. Remember, this doesn’t mean you’re getting less fit—your zones are just shifting.