Most Zone 2 calculators use one formula. This runs all three — MAF, Karvonen, and % of Max HR — simultaneously so you see the full picture. Enter your age, resting heart rate, and fitness level. Get your Zone 2 range three ways, a complete 5-zone training table, weekly volume recommendations, and plain-English explanations. Free. No signup. No email. Built for runners, cyclists, and anyone serious about aerobic base training and longevity.