A common formula estimates a person's maximum heart rate as 220 minus their age. For example, the traditional estimate for a 40-year-old is 180 beats per minute. The result is intended as a rough population estimate, not a guaranteed limit for an individual.
The equation is widely associated with William Haskell and Samuel Fox and became popular through exercise guidance and heart-rate monitors. It was not created from a single definitive experiment; historical accounts indicate that it was derived by plotting data from several earlier sources. That origin helps explain why the formula is easy to remember but not especially precise.
Researchers have proposed alternatives, including 208 minus 0.7 times age, because maximum heart rate varies considerably among people of the same age. Fitness level, sex, genetics, medications, and health conditions can all affect the actual value. Age explains only part of the variation.
The most accurate way to determine an individual's maximum heart rate is a supervised maximal exercise test, often using a treadmill or bicycle while an electrocardiogram is monitored. The 220-minus-age rule remains useful for rough training-zone calculations, but it should not be treated as a medical measurement or an absolute ceiling.