Your watch shows a number while you run, 149, and a colored ring shifts. The number is your heart rate and the ring is a training zone. Zones are percentage bands of your maximum heart rate, and the whole system leans on one estimate that has been argued about for years without being replaced.
The most repeated formula in fitness
Maximum heart rate is estimated, rarely measured, by an age-based shortcut:
max HR ≈ 220 − ageFor a 40-year-old, that is 180. Many people land close to this, but plenty do not: some run 10 beats hot, others 15 beats under. The formula survives because it is easy and roughly right, and the exact number matters less than the bands it generates.
What the zones are
The common model splits max HR into five bands, each a percentage of the max:
| Zone | % of max HR | What it does |
|---|---|---|
| 1 | 50-60% | Recovery, easy spinning |
| 2 | 60-70% | Endurance base, conversational |
| 3 | 70-80% | Aerobic tempo |
| 4 | 80-90% | Threshold efforts |
| 5 | 90-100% | Sprints, max work |
The arithmetic for a zone is one multiplication:
zone range = max HR × percent
max HR 180 × 70-80% → 126-144 bpmWatches do not measure the zones at all. They measure beats and multiply by the number you entered. The interface hides the multiplication, which makes it feel like magic until you do one band by hand.
Why the bands matter
Zone 2, 60-70% of max, is where base fitness is built. The effort stays low enough that you can hold a conversation, which is how a consistent runner can spend hours there week after week. Zone 4 and 5 train the top end but ask for real recovery afterward. Picking the zone is the training plan; the percentage is just the dial.
The honest caveat: if 220 minus age is a guess, every displayed beat inherits that guess. Sports scientists have proposed replacement formulas and an actual lab test beats them all, but the age rule survives because it is easy and roughly right, and the zones are forgiving enough to absorb the error.
Where the watch gets it wrong
Wrist sensors read light, not electricity. Sweat, fit, and skin tone produce dropped beats that momentarily bounce into the next zone, the difference between “recovery” and “threshold” if the glitch crosses a band. A chest strap reads the heart directly and stays a steadier partner for hard days.
Your resting rate is the daily check. A normal resting rate for adults sits between 60 and 100 bpm. A sustained jump of 10 beats above your usual across a week is the classic sign of insufficient recovery. Your watch marks it, but you feel it first.
FAQ
What is 220 minus age? The standard method to estimate max heart rate. Subtract your age from 220 to get beats per minute. Real tests are better, but this works for most planning.
Which zone should I train in? Zone 2 for the bulk of conditioning, zones 3-4 for tempo work. For beginners, that means most of the week in 2 and 3.
Why does my watch disagree with the treadmill? Different sensors and different math. Pick one consistent source and stay with it instead of switching whenever the two disagree.
Should I see a doctor first? With a known heart condition, yes: that is a conversation to have before you train to a zone plan, and no calculator replaces it. For otherwise healthy adults the age formula is a reasonable place to begin.
How do I actually measure my max HR? Run a long, hard uphill effort until you cannot hold the pace, and record the highest beats you hit. One honest session beats any age formula for your individual numbers.