Maximum heart rate is estimated with the Tanaka equation (208 − 0.7 × age), and the zones use the Karvonen heart-rate reserve method against ACSM intensity categories. These are population estimates: an individual maximum can differ by 10–12 beats per minute either way. Beta blockers and several heart conditions make heart rate an unreliable guide to effort — if you take one, or have been advised to limit exertion, follow your clinician rather than this tool.