Max heart rate is the highest number of beats per minute your heart can hit during all-out effort. It's mostly set by age and genetics, not by how fit you are, and it drifts down slowly as you get older. Every heart rate training zone you'll ever see is calculated as a percentage of this one number, which is why getting it roughly right matters.
Why it matters
Because the famous formula is shakier than people think. "220 minus age" came from a rough line drawn through mixed studies back in 1971, and its real-world error runs about 10 beats per minute in either direction. Sometimes more. If your true max is 15 beats above the formula's guess, every zone you compute from it is wrong. Your "zone 2" becomes a stroll. Your intervals top out below where they should. Two 40-year-olds can have genuine maxes of 165 and 195, and the formula hands them both 180.
How to use it in training
Start with a better formula. The Tanaka equation (208 minus 0.7 times your age) tracks the population data more closely, especially past 40. Still an estimate, though.
The honest answer is a field test. After a thorough warmup, run or ride a long hill hard for 2 to 3 minutes, recover briefly, then repeat twice more, going all-out on the final effort. The highest number your monitor shows is close to your true max. Fair warning: this is a genuinely brutal effort, so if you're new to training, over 45, or have any heart or health concerns, check with a doctor before attempting one. A chest strap beats a wrist sensor for this job, since wrist optical readings lag badly at high intensity.
Then anchor your zones to what you measured, not to a 55-year-old formula.
Related terms
Go deeper
Want the full story on where the formulas came from and which one to trust? Read our breakdown: Max heart rate formula research.