Every zone calculation starts from an estimate of your maximum heart rate, and this is where most of the error enters. The three formulas in common use disagree, and all three are population averages:
| Formula | Equation | At age 35 |
|---|---|---|
| Fox, 1971 | 220 − age | 185 |
| Tanaka, 2001 | 208 − 0.7 × age | 184 |
| Gellish, 2007 | 207 − 0.7 × age | 183 |
The familiar 220 − age has no original research behind it — it was an informal fit to scattered data and was never validated. More importantly, its standard deviation is about 10 to 12 beats per minute. That means roughly a third of people have a true maximum more than 10 beats away from the estimate, and some are off by 20. Tanaka, derived from a large meta-analysis, is more accurate on average and notably better for people over 40, where 220 − age tends to underestimate.
The only reliable way to know your maximum is to measure it in a supervised graded exercise test, or to observe the highest value you have genuinely reached during an all-out effort. If you have such a number, use it and ignore the formulas.
Two people with the same maximum heart rate but resting rates of 45 and 75 are not working equally hard at 140 beats. The Karvonen method fixes this by working from heart rate reserve — the span between rest and maximum:
Target = resting HR + (intensity % × (max HR − resting HR))
Age 35, resting heart rate 60, using the Tanaka estimate of 184. Heart rate reserve is 184 − 60 = 124 beats.
| Zone | % of reserve | Beats per minute | What it is for |
|---|---|---|---|
| 1 — Recovery | 50 to 60% | 122 to 134 | Warm-up, cool-down, active recovery days |
| 2 — Aerobic base | 60 to 70% | 134 to 147 | Fat oxidation, mitochondrial density, the bulk of endurance volume |
| 3 — Tempo | 70 to 80% | 147 to 159 | Sustained moderate effort, aerobic capacity |
| 4 — Threshold | 80 to 90% | 159 to 172 | Lactate threshold work, race pace intervals |
| 5 — Maximum | 90 to 100% | 172 to 184 | Short intervals, VO2 max development |
This trips up a lot of people comparing numbers with a friend or a watch. The simpler percentage-of-maximum method would put 70% at 0.70 × 184 = 129 beats. Karvonen puts 70% of reserve at 147 beats — eighteen beats higher. Neither is wrong; they are different definitions of the same percentage. If a plan tells you to train at 70%, it matters enormously which method it means. Karvonen is generally preferred because it individualises for fitness, and a well-trained person with a resting rate of 45 gets appropriately different targets from a beginner at 80.
The most common training error is spending nearly all sessions in zone 3 — hard enough to be tiring, not hard enough to drive high-intensity adaptation, and too hard to accumulate real aerobic volume. Endurance athletes across sports converge on roughly 80% of training time in zones 1 and 2 with 20% in zones 4 and 5. Zone 2 should feel almost too easy: you can hold a full conversation. If you are breathing in short phrases, you have drifted into zone 3.
Heart rate is a signal about your whole physiological state, not just your effort. Expect it to move by 5 to 15 beats for reasons that have nothing to do with fitness:
These zones are built on an age-based population estimate with a known error of around 10 beats, then divided into bands whose boundaries are conventions rather than measured physiological thresholds. Your actual lactate threshold might sit anywhere in zone 3 or 4. Chest-strap monitors are reliable; wrist optical sensors lag during rapid changes and can lock onto cadence instead of pulse during running, producing convincingly wrong data. If you take heart medication, have a diagnosed cardiac condition, or are new to intense exercise after a long gap, get clearance from a clinician before training at zones 4 and 5. This page is general information, not medical advice.
Immediately on waking, before getting out of bed or looking at a phone, counted over a full 60 seconds — or read the overnight low from a wearable. Take an average across several mornings rather than one reading.
Check which method your watch uses. Many default to percentage of maximum, and many still use 220 − age. Most let you enter a measured maximum and switch to the reserve method, which is worth doing.
No. Maximum heart rate is largely genetic and declines with age regardless of training. Fitness shows up as a lower resting rate, faster recovery after effort, and more speed at the same heart rate.
No. Your age and resting rate are used only in your browser to compute the zones; nothing is transmitted or saved.
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