Training by heart rate is one of the oldest principles in endurance sport — Haskell and Fox formalised the maximum heart rate formula in 1970, and Matti Karvonen published his Heart Rate Reserve method a decade earlier in 1957. Despite their age, these methods remain the backbone of every running app, cycling computer and treadmill display you encounter. Understanding what the zones actually mean — not just what numbers they contain — transforms how you train.
The Five Training Zones
Zone 1 — Recovery (50–60% max HR) Very easy; nose breathing; walking or extremely light jogging Purpose: Active recovery between hard sessions; warm-up/cool-down Zone 2 — Aerobic Base (60–70% max HR) Conversational; could maintain for hours Purpose: Fat oxidation, mitochondrial density, capillary development Zone 3 — Aerobic (70–80% max HR) Moderately challenging; can speak in sentences Purpose: General aerobic conditioning; tempo endurance Zone 4 — Lactate Threshold (80–90% max HR) Hard; 3–4 word sentences only; sustainable for 20–45 min Purpose: Raise lactate threshold; improve race pace Zone 5 — VO₂ Max (90–100% max HR) Maximal effort; near-breathless; sustainable for only minutes Purpose: Peak power, VO₂ max increase; short intervals only
Percentage Method vs Karvonen — Which to Choose
The simple percentage method multiplies your estimated max HR by the zone percentage. It is quick and widely used, but it ignores your fitness level. The Karvonen formula — developed by Finnish physiologist Matti Karvonen in 1957 — incorporates your resting heart rate (RHR) to reflect cardiovascular fitness more accurately. A well-trained athlete with an RHR of 45 bpm will have their Zone 2 starting at a higher absolute heart rate than a sedentary person with an RHR of 75 bpm, even if both have the same max HR. For anyone with a measurably low RHR (athletes, regular exercisers), the Karvonen method produces more meaningful and individualised zones.
Simple percentage method: Zone HR = Max HR × Zone % Karvonen (Heart Rate Reserve) method: HRR = Max HR − Resting HR Zone HR = Resting HR + (HRR × Zone %) Example: Age 32, Max HR 188 bpm, Resting HR 52 bpm HRR = 188 − 52 = 136 bpm Zone 2 (60–70%): Simple: 188 × 0.60 = 113 bpm to 188 × 0.70 = 132 bpm Karvonen: 52 + 136×0.60 = 134 bpm to 52 + 136×0.70 = 147 bpm The Karvonen zones are ~15 bpm higher — appropriate for a fit individual.
The 80/20 Principle — Why Most Training Should Be Easy
Exercise physiologist Stephen Seiler's research (2009) on elite endurance athletes consistently found that the most successful athletes spent approximately 80% of their training time in Zones 1–2 and only about 20% in Zones 4–5. This polarised distribution produces better long-term adaptation than a moderate-intensity approach (spending most time in Zone 3). Zone 2 builds the aerobic base — increasing mitochondrial density, capillary networks and fat oxidation capacity — that sustains high Zone 4–5 efforts. Many recreational athletes make the mistake of doing most training in Zone 3, which is hard enough to cause fatigue but not intense enough to maximise adaptation.
Tip
The single most important thing you can do to improve your heart rate zone accuracy is to measure your true resting heart rate properly. Do it on three consecutive mornings — before getting out of bed, after lying quietly for 5 minutes — and average the three readings. A wrist-based fitness tracker can also provide a reliable overnight average. Many people over-estimate their RHR because they measure it after they have already been up and moving for 20 minutes.
Getting Your Max HR Right
The 220 − age formula has a standard deviation of ±10–12 bpm. For a 40-year-old, the predicted max HR of 180 bpm has a genuine range of 156–204 bpm across the population. If you are using HR zones seriously for training, the most reliable max HR comes from a monitored graded exercise test or a field test: after a 15-minute warm-up, run 3 minutes hard uphill twice with 3 minutes recovery between. Your highest recorded value in the second interval is a reasonable approximation of true max HR. The custom max HR field in the calculator lets you use this measured value.