Nearly all exercise is good. But to be sure you’re getting the most from your workout yet staying at a level that’s safe for you, you can monitor how hard your heart is working.

Aiming for what’s called a “target heart rate” can help you do this, says Johns Hopkins cardiologist Seth Martin, M.D., M.P.H. Think of it as the “sweet spot” between not exercising hard enough and overexerting.

What is Target Heart Rate?

Your target heart rate is a range of numbers that reflect how fast your heart should be beating when you exercise. “A higher heart rate is a good thing that leads to greater fitness,” says Johns Hopkins cardiologist Michael Blaha, M.D., M.P.H. During exercise, you can monitor heart rate and try to reach this target zone. Doctors also use target heart rate to interpret the results of a cardiac stress test.

How to Find Your Target Heart Rate

First, it helps to know your resting heart rate, Martin says. Find your pulse (inside your wrist, on the thumb side, is a good place). Then count the number of beats in a minute—that’s your resting heart rate. (Alternately, you can take your pulse for 30 seconds and double it.) The average resting heart rate is between 60 and 100, he says. The more fit you are, the lower your resting heart rate; for very fit people, it’s in the range of 40 to 50 beats per minute.

Target heart rate is generally expressed as a percentage (usually between 50 percent and 85 percent) of your maximum safe heart rate. The maximum rate is based on your age, as subtracted from 220. So for a 50-year-old, maximum heart rate is 220 minus 50, or 170 beats per minute. At a 50 percent exertion level, your target would be 50 percent of that maximum, or 85 beats per minute. At an 85 percent level of exertion, your target would be 145 beats per minute. Therefore, the target heart rate that a 50-year-old would want to aim for during exercise is 85 to 145 beats per minute.

But there’s an easier way to figure it out if you want to skip the math: Wear a fitness tracking device, or exercise on a treadmill or other machine that calculates target heart rate for you, Blaha suggests.

Heart Rate Tips to Keep in Mind

• Start at your beginning. Before getting overly concerned about your heart rate, Martin says, it’s best to simply get moving. If you haven’t exercised much before, start where you’re comfortable (around 50 percent of maximum heart rate) and gradually exert yourself more over time.
• Listen to your body. Your body provides other indicators of how hard it’s working that you need to consider along with heart rate. Pay attention to how hard you’re breathing or sweating, and stop if you feel very uncomfortable, Martin says. Devices recording your heart rate have been known to malfunction, for example—another reason listening to your body is important.
• Remember that target heart rate is just a guide. “Don’t get overly fixated on numbers,” Martin says. Ideally, they just push you to work a little harder.