How the calculator works
Max heart rate is estimated with the Tanaka formula, 208 − 0.7 × age. It came from a meta-analysis of 351 studies and a lab study of healthy adults, and it is more accurate than the older 220 − age rule, which underestimates max heart rate in older adults (Tanaka, 2001).
Zones are then set as percentages of that maximum. If you enter your resting heart rate, the calculator switches to the Karvonen method, which applies the percentages to your heart rate reserve (max minus resting) and adds resting back on. That gives fitter people, who have lower resting rates, more realistic zones.
What each zone is for
| Zone | Intensity | What it feels like | Use it for |
|---|---|---|---|
| 1 | 50–60% | Very easy, full conversation | Warm-ups, recovery days |
| 2 | 60–70% | Easy, can still talk in sentences | Most endurance volume, aerobic base |
| 3 | 70–80% | Comfortably hard, short phrases | Tempo work |
| 4 | 80–90% | Hard, a few words at a time | Threshold intervals |
| 5 | 90–100% | All out | Short intervals |
Most endurance plans put the bulk of training time in zones 1 and 2, with a small share of hard work in zones 4 and 5.
How accurate is it?
Any age formula is an estimate: individuals can sit 10 or more beats above or below it. If your zones feel wrong, trust the talk test, or measure your max in a supervised test. Beta blockers and some other medications lower heart rate and make these zones unreliable.