Target Heart Rate Zones Explained: The Karvonen Method
"Keep your heart rate in the fat burning zone" is common advice, but the zone itself is often calculated in a way that quietly ignores one of the most personal numbers involved: how fast your heart beats when you are doing nothing at all. The Target Heart Rate Calculator on this site uses the Karvonen method, which factors that resting rate in. Here is why that distinction matters.
The simple method, and what it misses
The most common shortcut estimates maximum heart rate as 220 minus your age, then defines each training zone as a straight percentage of that number, for example 70% of max for a moderate effort. It is easy to calculate, and it is the version most fitness trackers default to. The problem is that it treats two very different people identically as long as they share an age. A sedentary person and a trained endurance athlete of the same age get the same target heart rate at "70% effort", despite the athlete's heart working far more efficiently at rest and having a much larger gap between resting and maximum heart rate to work with.
The Karvonen method
The Karvonen method, sometimes called the heart rate reserve method, starts from the same 220 minus age estimate of maximum heart rate, but then subtracts resting heart rate from it to get what is called heart rate reserve, the actual range your heart has available to climb through during exercise. Each zone is a percentage of that reserve, added back on top of resting heart rate: target heart rate equals (heart rate reserve times intensity percentage) plus resting heart rate. A fitter person with a lower resting heart rate has a larger reserve to work with, so the same effort level produces a higher target number than it would for someone with a higher resting heart rate, a more personalized result than the flat percentage method gives.
A worked comparison
Take two 40 year olds, both with an estimated maximum heart rate of 180 (220 minus 40). One has a resting heart rate of 50, the other 75. At 70% intensity, the simple method gives both of them the identical target of 126 beats per minute. The Karvonen method gives the fitter person (resting 50) a target of 141, and the other person (resting 75) a target of 148.5, different numbers that better reflect how much room each heart actually has between its resting state and its estimated maximum.
The five standard zones
| Intensity | Typical use |
|---|---|
| 50% | Very light activity, warm up and cool down |
| 60% | Light aerobic effort, easy conversational pace |
| 70% | Moderate aerobic effort, a typical steady cardio session |
| 80% | Vigorous effort, harder cardio and tempo work |
| 85% | Near maximal effort, short intervals only |
Lower percentages burn a higher proportion of fat relative to carbohydrate for fuel, which is where the "fat burning zone" label comes from, but higher intensity effort burns more total calories per minute even though a smaller share of them come from fat. Neither is universally better. It depends on the goal, whether that is a long, easy aerobic base or a shorter, harder conditioning session.
The limits of 220 minus age
Both methods share the same starting point, 220 minus age, and it is worth being honest about what that number is. It is a population average regression line fitted decades ago across a wide range of ages, not a measurement of any individual's actual maximum heart rate. Real maximum heart rate varies enough between two people of the same age that the formula's typical error runs plus or minus ten to twelve beats per minute, which shifts every zone calculated from it by a similar margin. A supervised maximal exercise stress test is the only way to get an individual's actual maximum heart rate directly. For most people exercising for general fitness rather than training for elite competition, the estimate is a reasonable starting point, useful for judging relative effort across a workout rather than treated as an exact ceiling.
A note on resting heart rate
Resting heart rate is best measured first thing in the morning, before getting out of bed, since sitting up, caffeine, and even a stressful thought can raise it by several beats per minute. Measuring over a few mornings and averaging the results gives a steadier number than a single reading, which matters here since resting heart rate feeds directly into every zone the Karvonen method calculates.