How the zones are calculated
Your estimated maximum heart rate uses the classic 220 − age formula. Each zone is a percentage band of that maximum:
- Zone 1 (50–60%) — warm-up and recovery. Conversational pace.
- Zone 2 (60–70%) — fat-burning and aerobic base. Still conversational; where most easy mileage should sit.
- Zone 3 (70–80%) — aerobic / cardio. Talking in short sentences only.
- Zone 4 (80–90%) — anaerobic threshold. Race-pace effort, sustainable for roughly 20–60 minutes.
- Zone 5 (90–100%) — maximum effort. Sustainable for seconds to a few minutes.
If you enter a resting heart rate, the calculator switches to the Karvonen method: target = (max HR − resting HR) × intensity + resting HR. This heart-rate-reserve approach fits zones to your actual fitness level and is generally more accurate than plain percentages.
Max-HR formulas carry an error margin of roughly ±10–12 bpm. The Tanaka formula (208 − 0.7 × age) is a common alternative for older adults. If you have a chest strap or wrist HRM, use its measured max instead.