Heart rate
zones.

The Karvonen method uses your Heart Rate Reserve (HRR = Max HR − Resting HR) to set personalised zone boundaries. Because it accounts for your resting HR, it gives more accurate targets than simple max-HR percentages — two runners with the same max HR but different resting HRs will have different zone boundaries.

Measure it first thing in the morning before getting out of bed. Lie still for a minute, then count beats for 60 seconds (or 30 seconds × 2). Take the average of three mornings for accuracy. Typical ranges: well-trained runners 40–55 bpm; untrained adults 60–80 bpm.

Measured is more accurate. The 208 − 0.7 × age formula is a population average with a standard deviation of roughly 7–10 bpm, so it can be significantly off for any individual. If you have a recent all-out effort — a 5K race, a VO₂max test — use that number instead.

Most endurance research supports a polarised split: roughly 75–80% of training in Zone 1–2 (easy/aerobic), 5–10% in Zone 3 (tempo), and 15–20% in Zone 4–5 (threshold/VO₂max). Spending too much time in the moderate middle zone (Z3) is the most common training error.

Zone 2 (60–70% HRR) is the intensity at which your body primarily burns fat and builds mitochondrial density — the foundation of endurance. It should feel conversational: you can speak in full sentences without gasping. Despite feeling almost too easy, consistent Zone 2 work produces substantial long-term aerobic gains.