Heart rate reserve (HRR) is the difference between your maximum heart rate and your resting heart rate. It represents the total range your heart has available to work harder during exercise. Calculating it takes about 30 seconds, and using it to set training zones gives you a more personalized target than simply using a percentage of your max heart rate.
The Heart Rate Reserve Formula
The calculation has three steps:
- Step 1: Find your maximum heart rate. The simplest formula is 220 minus your age.
- Step 2: Measure your resting heart rate (more on this below).
- Step 3: Subtract your resting heart rate from your maximum heart rate. The result is your heart rate reserve.
For a 40-year-old with a resting heart rate of 65 bpm, the math looks like this: 220 minus 40 = 180 (max heart rate). Then 180 minus 65 = 115 bpm. That person’s heart rate reserve is 115 beats per minute.
Using HRR to Find Your Target Heart Rate
Heart rate reserve becomes useful when you plug it into the Karvonen formula to find a target heart rate for a specific workout intensity. The formula is:
Target heart rate = (HRR × desired intensity percentage) + resting heart rate
Using the same 40-year-old example, if they want to train at 60% intensity: 115 × 0.60 = 69, then 69 + 65 = 134 bpm. At 80% intensity: 115 × 0.80 = 92, then 92 + 65 = 157 bpm. So a moderate-to-vigorous workout for this person would target roughly 134 to 157 bpm.
You can swap in any percentage depending on your goal. Light exercise typically falls around 40% to 50% of HRR. Moderate exercise sits at 50% to 70%. Vigorous training pushes into 70% to 85%. Cardiac rehab programs often use 60% to 80% of heart rate reserve as their target window.
Why HRR Is More Accurate Than Max Heart Rate Alone
A simpler approach to training zones is to just take a percentage of your maximum heart rate. The American Heart Association, for instance, defines moderate-intensity activity as 50% to 70% of max heart rate. That works as a rough guideline, but it ignores one important variable: your fitness level.
Two people who are both 40 years old share the same estimated max heart rate of 180 bpm. But if one has a resting heart rate of 55 and the other has a resting heart rate of 80, their cardiovascular capacity is very different. The person with the lower resting heart rate has a reserve of 125 bpm, while the other has only 100 bpm. Using a flat percentage of max heart rate treats them identically. The Karvonen method accounts for that gap by folding resting heart rate into the equation, producing a target that reflects your individual cardiovascular fitness rather than just your age.
This is also why heart rate reserve can serve as a rough fitness marker over time. As your cardiovascular fitness improves, your resting heart rate tends to drop, which increases your reserve. A growing HRR, without any change in age, signals that your heart is pumping more efficiently.
How to Measure Your Resting Heart Rate
Your resting heart rate is the number of times your heart beats per minute while you’re awake, calm, and not moving. The best time to check it is first thing in the morning, before you get out of bed or have coffee. For most adults, a normal resting heart rate falls between 60 and 100 bpm, though well-trained athletes often sit in the low 50s or even 40s.
To get a reliable number, measure it on three to five consecutive mornings and average the results. A single reading can be skewed by poor sleep, stress, dehydration, or alcohol the night before. A chest strap heart rate monitor or a pulse oximeter will give you the most consistent readings, but you can also count your pulse at the wrist for 60 seconds.
Which Max Heart Rate Formula to Use
The classic formula, 220 minus your age, dates back to the 1970s and is still widely cited by the American Heart Association. A newer formula, 208 minus (0.7 × age), was developed by researcher Hirofumi Tanaka and is considered more accurate for certain populations. For younger adults, the two formulas produce similar numbers. The gap widens with age: a 60-year-old gets a max of 160 from the classic formula but 166 from the Tanaka version.
Research on recreational marathon runners found that the classic formula underestimated max heart rate in men by about 3 bpm, while the Tanaka formula tracked closer to their actual measured values. In women, both formulas overestimated max heart rate by roughly 5 bpm. Neither formula is perfect, because max heart rate varies significantly between individuals of the same age. If precision matters to you, a graded exercise test supervised by a professional will give you an actual measured max heart rate rather than an estimate.
Adjustments for Medications
Beta-blockers and some other heart medications lower both your maximum heart rate and your resting heart rate, which throws off any formula based on age-predicted values. If you take a medication that slows your heart rate, the standard 220-minus-age estimate will overshoot your true max, and any training zone built on it will be too high.
Research in the European Journal of Preventive Cardiology found that people on beta-blockers should target about 60% of heart rate reserve (using the Karvonen method) rather than the 70% typically recommended for people not on these medications. The most reliable approach in this situation is to have your max heart rate measured during a supervised exercise test while you’re on your usual dose, then calculate HRR from that measured value.
Putting It Into Practice
Once you have your target heart rate range, you need a way to monitor it during exercise. A chest strap paired with a watch or phone app gives real-time feedback and is accurate enough for most people. Wrist-based optical sensors on smartwatches have improved but can lag during interval training or read inaccurately if the band is loose.
Start by picking a single intensity zone for your workout and trying to keep your heart rate in that range for the main portion of your session. If you’re new to heart rate training, aim for 50% to 60% of your HRR and build up over several weeks. You’ll likely notice that the same pace or effort produces a lower heart rate as your fitness improves, which is your signal to either increase the intensity or enjoy the fact that your cardiovascular system is adapting.
Recalculate your HRR every few months. Your max heart rate won’t change much year to year, but your resting heart rate can shift meaningfully with consistent training, changes in stress, or shifts in body composition. Even a 5 bpm drop in resting heart rate changes your target zones enough to be worth updating.