Heart-rate training zones, explained without the mystique

Your watch insists you were "in zone 3" for forty minutes, and most guides treat the zones as if they were gospel. They are useful, approximate, and worth understanding once properly — starting with where the numbers come from.

The two numbers behind every zone

Zone systems are percentages of a ceiling. The classic estimate of that ceiling is 220 minus your age — a formula that is decades old, derived from population averages, and routinely wrong for an individual by ten beats or more in either direction. The better anchor is the Karvonen method, which uses your heart-rate reserve: the span between your resting heart rate and your maximum. Target = resting + (reserve × intensity%). Two people of the same age with different resting rates get different zones — which is exactly the point the 220-minus-age shortcut misses.

The five zones, plainly

Zone 1 (about 50-60%): very easy — warm-ups, recovery days, walking. Zone 2 (60-70%): conversational effort; the endurance base, covered in depth on the desk's zone 2 guide. Zone 3 (70-80%): "comfortably hard" — talking in phrases, not sentences. Zone 4 (80-90%): hard; a few words at a time. Zone 5 (90-100%): near-maximum, sustainable for minutes, not hours. The percentages vary slightly by system — that variance is a feature: the boundaries are conventions, not physiology.

The 'fat-burn zone' honesty

Lower intensities burn a higher proportion of fat — true, and nearly useless as advice. What matters for weight management is total energy expended and the overall training effect; a harder shorter session can burn more total energy than a longer easy one. The zone framing that deserves keeping is the training one: easy days genuinely easy, hard days genuinely hard, and the common beginner error is living permanently in the mushy middle.

What to actually do with zones

Use the talk test to sanity-check the watch: comfortable conversation in zone 2, phrases in zone 3, words in zone 4. If your watch and your breathing disagree, trust your breathing. The CDC's guidance on intensity makes the same move — relative effort, not device theology. And the resting heart rate feeding the Karvonen formula is itself a fitness signal worth watching: it tends to drift down as aerobic fitness improves, which quietly widens your working range. The desk's zone calculator runs both formulas side by side, with the arithmetic shown.

Sources

• WHO — Physical activity (fact sheet)
• CDC — Physical Activity Basics
• CDC — Target Heart Rate and Estimated Maximum Heart Rate

Every link checked 2026-09-25; sources favour primary and public-health bodies.