How it works: Maximum heart rate is estimated as 220 − age (Fox) or 208 − 0.7×age (Tanaka, often closer for adults). The Karvonen method then uses your heart-rate reserve (max − resting): target = resting + intensity% × reserve, which personalises zones far better than percentages of max alone. Individual max heart rate varies by roughly ±10 bpm, so treat zones as guides and listen to your body.