Heart rate zones explained: how they're calculated and what each one trains

Every fitness watch colours your workout by heart-rate zone, and most people don't know where the numbers came from. They come from an estimate of your maximum heart rate, a set of percentage bands, and a good deal of physiology that is simpler than it looks. The heart rate zones calculator gives your bands; this guide explains them. General information, not medical advice — check with a doctor before starting hard training, especially with a heart condition or on medication.

Estimating maximum heart rate

Maximum heart rate (HRmax) is the fastest your heart can beat, and it falls with age. The famous formula, 220 − age, is a rough line drawn through population data in the 1970s; its standard error is about ±10–12 beats, so a 40-year-old's true max could be anywhere from 168 to 192. Better-fitting formulas exist — Tanaka (208 − 0.7 × age), Gulati for women (206 − 0.88 × age) — and are a few beats different for most ages. The only accurate method is measuring it: a maximal test in a lab, or a hard, well-warmed-up effort like the last minute of a steep hill repeat, which fit, healthy people can do and others should not. If your watch has shown a peak well above the formula, the watch is right.

The five zones

• Zone 1 (50–60% of max): very easy — warm-up, recovery walks.
• Zone 2 (60–70%): easy, conversational endurance. Fat-burning aerobic base.
• Zone 3 (70–80%): moderate — "comfortably hard"; the pace many people default to and the one coaches call no-man's-land.
• Zone 4 (80–90%): hard — around the lactate threshold; tempo runs and race pace for 10 km to half-marathon.
• Zone 5 (90–100%): maximal — short intervals; unsustainable beyond minutes.

The percentages are of HRmax; the Karvonen method uses heart-rate reserve (max minus resting) and gives higher zone floors for fit people with low resting rates. Both are approximations of two physiological thresholds — where breathing first deepens, and where lactate accumulates faster than it clears — that vary between individuals.

Why zone 2 gets the attention

Endurance research on elite athletes found they spend about 80% of training time at easy intensity and 20% hard, with little in between — polarised training. Easy work builds mitochondria, capillaries and fat metabolism with low fatigue, so more of it can be done; the hard sessions then provide the top-end stimulus. Most recreational runners and cyclists do the opposite, living in zone 3: too hard to recover from quickly, too easy to force adaptation. The practical rule is the talk test: in zone 2 you can speak in full sentences. It feels slow. That is the point, and Running pace explained: converting pace, speed and race times, and what pace to train at converts the effort into pace.

Measuring without a lab

Chest-strap monitors measure the heart's electrical signal and are accurate; wrist optical sensors track well at steady effort and lag or drop out during intervals and in cold weather. A rough threshold check: after a warm-up, run or ride hard for 30 minutes as evenly as you can; the average heart rate over the last 20 minutes approximates your lactate threshold (top of zone 4). Resting heart rate, taken before getting up, tracks fitness and fatigue over months — a rising trend often means illness or overtraining. The BPM counter taps out a pulse if you're counting by hand, and the pace calculator pairs a zone with a speed.

Caveats

Zones from a formula can be off by a whole zone for an individual. Heat, dehydration, caffeine, altitude and poor sleep raise heart rate at a given effort; beta-blockers lower it, making zones meaningless for people on them. Perceived effort and breathing are as reliable as the watch for easy days. And the goal of zones is consistency of training stress over months, not hitting a colour on a given day. Tabata: what the 4-minute workout really is covers the zone-5 end; One-rep max, explained (no maxing required) the strength side; anyone with cardiac history should have zones set by a clinician.