February 25, 2026
ByJulie Corliss,
Executive Editor, Harvard Heart Letter
- Reviewed byKieran F. Reid, PhD, MPH,
Contributor
You hit your stride on the elliptical machine and the electronic readout says your heart rate has reached zone 2 – the “fat-burning” zone. Is this the best zone for weight loss? Maybe you’re more focused on <a re working during <a Tracking exercise intensity: How do you feel when you're "in the zone"?). Gym equipment and watches that can monitor heart rate are the most common ways people determine which zone they're in.
But rather than targeting one of the five specific heart rate zones, focusing on how you feel is a simpler and equally effective way to track your workouts and meet your health goals, says Kieran Reid, who directs the laboratory of exercise physiology and physical performance at Harvard-affiliated Brigham and Women’s Hospital. “In both the rehabilitation clinic and in clinical trials, we often use the rate of perceived exertion, or RPE, scale,” he says. It’s practical, personalized, and a good gauge of how hard your heart, lungs, and muscles are working during physical activity, he adds.
However, if you like numbers and paying attention to your pulse, you can compare your rate of perceived exertion to your estimated target heart rate zone. You can use either method (RPE or heart rate zones) to make sure you meet recommended weekly exercise targets: 150 minutes of moderate-intensity exercise, or 75 minutes of vigorous-intensity exercise, or a combination of the two.
Tracking exercise intensity: How do you feel when you’re “in the zone”?
Here’s how the rate of perceived exertion (RPE) scale corresponds to the five heart rate zones (see main story for determining your maximum heart rate).
Intensity or effort (based on the impact of exercise on your breathing or speaking pattern)
Heart rate zone (percentage of maximum heart rate)
Low (extremely easy, able to sing)
Very easy (able to speak in complete sentences)
Easy to moderate (speech becomes broken)
Moderate (breathing becomes heavier)
Moderate (talking is difficult)
Moderate to vigorous (deep, forceful breathing)
Vigorous (labored breathing, cannot talk)
Vigorous (very labored breathing, borderline breathless)
Very vigorous (gasping for air)
Heart rate zone formulas
Heart rate zones are based on your maximum heart rate (MHR) – an estimate of the highest heart rate you can achieve when you’re exercising as hard as you can. The basic formula to compute your MHR is 220 minus your age. But that’s a ballpark figure, since other factors are at play, including genetics and your baseline fitness level. A 65-year-old who jogs or plays tennis nearly every day will have a stronger, more efficient heart (and a lower resting heart rate) than a 65-year-old who only walks a few times a week.
“For a more personalized estimate, you can use the Karvonen formula,” says Reid. That takes into account your resting heart rate (RHR), which reflects your overall fitness. As such, it’s a more accurate way to estimate your target heart rate zones. You can find an
<p>During easy-to-moderate-intensity (zone 2) exercise, a greater proportion of the energy you use comes from fat than when you exercise at higher intensities. ” as the intensity of your exercise increases to zones and body relies more on carbohydrates because they can generate energy rapidly says reid. but for weight loss what matters most is total amount burns over time not whether fuel source during a specific workout fat or carbohydrate.>
Exercising at higher heart rate zones has physiological <a But you don't need to train in zone 4 to reap the cardiovascular benefits of exercise. Consistent easy-to-moderate-intensity exercise is very effective," says Reid. Finding an activity you enjoy and doing it consistently is the main priority.
About the Author
Julie Corliss,
Executive Editor, Harvard Heart Letter
Julie Corliss has been the executive editor of the Harvard Heart Letter since 2013. She also writes for the Harvard Health Letter, Harvard Women’s Health Watch, and Harvard Men’s Health Watch, as well as for Harvard …
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About the Reviewer
Kieran F. Reid, PhD, MPH,
Contributor
Kieran F. Reid is assistant professor of medicine at Harvard Medical School and director of the Laboratory of Exercise Physiology and Physical Performance at Brigham and Women’s Hospital. He also serves as the associate director of …
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View all posts by Kieran F. Reid, PhD, MPH
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