"220 minus your age" is a guess. Your resting heart rate makes it better.

The number 220 minus age is probably the most quoted formula in fitness, and it is printed on treadmills, posters, and countless gym walls. It is also a statistical shortcut with a wide scatter: the true maximum heart rate of two 40-year-olds can easily differ by 20 beats per minute. The formula describes an average trend across a population; it was never meant to predict any single person's heart.

What maximum heart rate actually is

Maximum heart rate (HRmax) is the highest your heart can beat in a minute during all-out effort, and it declines with age, but the rate of decline varies. It is also mostly a fixed trait — training does not meaningfully raise it. If you want the real number, it takes a graded exercise test in a lab; if you want an estimate, the age-based formula is the cheap compromise, with all the error that implies.

Why the Karvonen formula is a genuine improvement

Most people's training heart rates are better described not as a percentage of HRmax, but as a percentage of their heart-rate reserve — the gap between resting and maximum heart rate. Two people can share the same HRmax yet have very different resting rates, and a lower resting rate means a wider reserve, which shifts every zone.

The Karvonen formula uses that reserve: it takes the difference between HRmax and resting heart rate, multiplies it by the target intensity, then adds resting heart rate back. This is why your resting rate is an input rather than an afterthought. Measured first thing in the morning, before getting out of bed, it is the single most useful number you can supply to make the zones yours.

The zones are ranges, not walls

Zone charts — warm-up, fat-burn, aerobic, anaerobic, maximum — are convenient labels for different intensities, but the boundaries are not physiological cliffs. Nothing magical happens at exactly 137 beats per minute. The value of zones is planning and pacing: keeping a long, easy run in a lower zone so it stays easy, or pushing an interval into a higher zone on purpose. Treat the numbers as signposts, not checkpoints.

One warning the poster never prints

Several common medications — particularly beta-blockers — lower heart rate and blunt its response to exercise. If you take anything like that, your real zones can look nothing like the formula's output, and "push harder to hit the number" can become actively bad advice. This is the clearest case where a calculator must defer to a clinician.

Heart-rate zones are training estimates, not medical targets. Consult your doctor before using them if you have a heart condition or take medication that affects heart rate.