Heart-rate zones on this site are training guidance, not medical advice. If you enter a known maximum heart rate, zones use that number. If you do not, maximum is estimated with Tanaka’s formula: 208 − 0.7 × age. Age-predicted max HR is a population estimate. Individual maximums vary.
Percentage-of-max zones use that maximum. Karvonen zones use heart-rate reserve: (max − rest) × percent + rest. Karvonen needs a resting heart rate. Neither method knows medication, illness, heat, or how accurate your strap is.
Pair zones with training paces if you want both intensity languages. Nothing here is a diagnosis or a clearance to train. If you have a heart condition, talk to a clinician, not a calculator.