If you’ve ever been curious about heart rate zones, now’s the time to give target heart rate training a shot. Fitbit trackers with PurePulse automatically set your heart rate zones for you, making getting started a snap. Here’s how to use this function to help improve results, prevent overtraining, and even avoid injury.

It sounds complicated, but target heart rate training just means you track your heart rate during workouts and try to keep it within a set intensity range or “zone” that corresponds to your fitness goal.

You’ve probably noticed that after logging a workout, your Fitbit app shows you a colorful graph depicting how much time you spent in three heart rate zones. Knowing a little bit about how these zones were created can help you make the most of them.

Target heart rate zones are calculated using maximum heart rate—the highest number of times your heart can safely beat in one minute. To keep things simple, Fitbit uses the common formula of 220 minus your age to estimate your maximum heart rate. So a 35-year-old would have a max heart rate of 185 (220-35=185).

If your birthday is entered into your Fitbit account profile, then the app will default to this formula to calculate your max heart rate. However if you already know your max heart rate or want to target a specific number, you can click “Account” on your Fitbit app dashboard, choose “HR Zones,” and then turn on “Custom Max Heart Rate,” and fill in a custom max heart rate there.

Once Fitbit knows your max heart rate, it will create three simplified zones for you. Each one represents an intensity level—or percentage of your max heart rate—that taps into a different energy system in the body, producing specific results. Here’s a breakdown:

Peak
Intensity: Vigorous (85 to 100 percent of your max heart rate)
Benefit: Increases performance speed

Cardio
Intensity: Hard (70 to 84 percent of your max hr)
Benefit: Builds cardiovascular fitness and muscle strength

Fat Burn
Intensity: Moderate (50 to 69 percent of your max hr)
Benefit: Builds aerobic endurance and teaches the body to burn fat as fuel

If you want to create a custom zone, you can also do this in the “HR Zones” settings of your account profile. Turn on “Custom Zone” and enter the upper and lower limits you want.

Now that your heart rate zones are set and you know what they mean, you can work out at the right intensity again.

Using your Fitbit tracker, start a workout as you normally would. As you begin moving, no matter which PurePulse-enabled tracker you own, you’ll see a visual representation of your zones.

For instance, in the picture above the heart is in  the cardio zone. Want to work on speed? Pick up the pace. Looking to blast fat? Ease up to drop back into your “Fat Burn” zone. When you adjust your pace on the fly, your Fitbit tracker responds instantly.

If you see the outline of a heart but no dashes, your heart rate is below 50 percent of your max, meaning you’re not in a zone. And that’s totally ok! If you want to get a cardiovascular boost, just increase your intensity. Here are four ways to upgrade your walk to a workout.

Learn more about Heart Rate accuracy here.

At the end of your workout, click on the exercise tile on your Fitbit app dashboard to view a summary of your workout. In addition to seeing exercise duration, calories burned, and average heart rate, you’ll also be able to see how much time you spent in each of your three heart rate zones and how your heart rate fluctuated (or didn’t) over the course of your workout.

If your heart rate information seems off—too high or too low based on how hard you felt like you were working—and you know you were wearing your tracker correctly (snugly, two finger widths below the wrist bone), you may need to set custom heart rate zones.

Heart rate tracking is helpful in the short term—it helps you evaluate your workout and see what, if anything, you should do differently next time—but it’s also valuable over time. The more you work out with heart rate, the more you’ll also be able to track patterns in your fitness program—like if you’re spending too much or too little time at any particular exercise intensity. The result? You hit your goals faster (bye, fat!) and with less risk of injury or overtraining. Go, you.