The Karvonen formula sets your training zones from heart-rate reserve: (max HR − resting HR) × intensity% + resting HR. For a 35-year-old resting at 60 bpm, that makes Zone 2 exactly 135–148 bpm. Two inputs, five zones, and a training system that adapts as your resting rate drops. Here's how the numbers work and what each zone is for.
Your body has roughly three engines: phosphocreatine for seconds-long bursts, glycolysis for hard efforts lasting a few minutes, and the aerobic system for everything longer. Heart-rate zones are a proxy for which engine you're loading. Train too hard on an easy day and you blunt recovery without adding fitness; drift too easy on a hard day and you never stress the top end. Zones keep each session honest.
Three steps. Estimate max HR: 220 − age, or Tanaka's 208 − 0.7 × age for a better fit past 40. Subtract your resting HR to get heart-rate reserve, the headroom your heart can actually expand into. Then each zone boundary is reserve × intensity + resting HR.
For the 35-year-old at 60 bpm resting: max 185, reserve 125.
| Zone | % reserve | Calculation | bpm |
|---|---|---|---|
| Z1 Recovery | 50–60% | 125 × 0.50 + 60 → 125 × 0.60 + 60 | 123–135 |
| Z2 Aerobic base | 60–70% | 125 × 0.60 + 60 → 125 × 0.70 + 60 | 135–148 |
| Z3 Tempo | 70–80% | 125 × 0.70 + 60 → 125 × 0.80 + 60 | 148–160 |
| Z4 Threshold | 80–90% | 125 × 0.80 + 60 → 125 × 0.90 + 60 | 160–173 |
| Z5 VO2 max | 90–100% | 125 × 0.90 + 60 → max | 173–185 |
The percentages-of-reserve detail is what makes Karvonen worth the extra input. Consider two 40-year-olds, both with a 180 max. One is new to exercise and rests at 72; the other has trained for years and rests at 48. A %-of-max chart hands them both a Zone 2 of 108–126 bpm. For the veteran that band is 45–59% of reserve, close to right. For the beginner it's just 33–50% of their smaller reserve, barely above a brisk walk. Karvonen splits them properly: Zone 2 lands at 137–148 for the beginner and 127–140 for the veteran, each at the same honest relative effort.
Measure before standing up in the morning, count a full 60 seconds, and average three to five mornings. Adults typically rest between 60 and 100 bpm; regular exercisers in the 50s; endurance athletes in the 40s. Skip mornings after alcohol, illness, or four hours of sleep. And here's the quiet payoff: as your aerobic base builds, resting HR falls, your reserve widens, and every zone shifts. Recalculate monthly and watch the numbers move.
Age and resting heart rate in; all five Karvonen zones in bpm, with either max-HR formula or your measured max.
Heart Rate Zone Calculator →Zone training decides the demand side of your fitness; the supply side is body composition and fuel. If you're training to lose weight, pair the zone plan with a calorie deficit calculator and a macro split calculator so the kitchen matches the work. And if heart-health metrics are on your mind, the water-intake and BMI calculators cover the basics of the daily routine.
Zone 1 (50–60% of heart-rate reserve) is recovery. Zone 2 (60–70%) is the conversational aerobic base. Zone 3 (70–80%) is tempo. Zone 4 (80–90%) is threshold, where lactate accumulates. Zone 5 (90–100%) is VO2 max work. Each zone stresses a different energy system, which is why training programs mix them instead of living in one.
Because it accounts for resting heart rate, a direct marker of aerobic fitness. Two 40-year-olds share a max of 180, but one rests at 72 and the other at 48. Plain %-of-max gives them identical Zone 2 bands (108–126 bpm), which lands at 45–59% of reserve for the veteran but only 33–50% for the beginner, too easy to stimulate change. Karvonen puts Zone 2 at 137–148 for the beginner and 127–140 for the veteran, each at the same relative effort.
As a population average it's fine; as a personal number it carries a standard deviation of roughly 10 to 12 bpm, meaning a third of people sit more than 11 bpm off their formula. Tanaka's 208 − 0.7 × age fits better over 40. The gold standard is a measured max: the highest rate you've seen in a ramp test or race finish. Use a formula to start, then replace it with data.
Most coaches treat 45 minutes as the point where Zone 2 work starts compounding, though shorter sessions still count. Frequency beats duration: four 45-minute Zone 2 sessions in a week outperform one three-hour grind, and the classic endurance split is about 80% of weekly time in Zones 1–2 with 20% hard. Zone 2 adaptations (mitochondria, capillaries, fat oxidation) build slowly, over months, and persist.