Heart-rate zones turn "train easy" and "train hard" into numbers you can see on a watch. This calculator builds your five zones from age — and personalizes them with the Karvonen formula the moment you add a resting heart rate.
Why Karvonen beats plain percentages
Plain %-of-max zones ignore fitness. Karvonen anchors intensity to your heart-rate reserve — the span between resting and max. Two 40-year-olds share an estimated max of 180, but with resting rates of 50 vs 75, their Zone 2 differs by more than 10 bpm. The reserve method scales the zones to the engine you actually have.
What each zone is for
Zone 1 flushes fatigue. Zone 2 — the famous one — builds mitochondria and fat-burning efficiency at conversation pace; most endurance volume belongs here. Zone 3 is sustainable tempo. Zone 4 brushes your lactate threshold, the redline of sustainable pace. Zone 5 is short maximal work for VO₂max. A classic weekly shape: ~80% of time in Zones 1–2, ~20% in 4–5.
The honest limits of 220 − age
It's a population regression with a ±10–12 bpm standard deviation — your true max might be 172 or 195 at age 40. If a race or field test has shown you a real max, trust it over the estimate and shift zones accordingly. Resting HR is easy to get right: measure before getting out of bed, several mornings, and average.
Putting zones to work
Runners: pair zones with the pace calculator to connect effort to speed. Strength athletes: the 1RM calculator is your intensity anchor instead. And the calorie cost of all that Zone 2 time shows up in the TDEE calculator's activity multiplier.
General fitness guidance; consult a professional for medical conditions affecting heart rate.