Training zones, and why most people train in the wrong one
How Karvonen differs from the percentage method, why 220 minus age is a poor estimate, and what the 80/20 rule actually asks of you.
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In short
- Karvonen works from heart rate reserve, so it adapts to your fitness. The percentage method gives the same zones to everyone of the same age.
- The two disagree substantially. At 30 with a resting rate of 65, Zone 2 is 138–150 bpm under Karvonen and 112–131 under the percentage method.
- 220 minus age has a standard deviation of 10 to 12 beats. Tanaka's 208 − 0.7 × age fits observed data better, especially over forty.
- Most recreational athletes train too hard on easy days and too easy on hard days, ending in an unproductive middle.
- Zone 2 should allow full conversation. If you cannot speak in sentences, you are not in Zone 2.
Heart rate zones divide the range between rest and maximum into bands, each producing a different physiological adaptation. Training in the right band is most of what separates productive training from time spent tired.
Estimating maximum heart rate
220 − age is the formula everyone knows and it is not very good. It was derived from a small set of studies in the 1970s, was never intended as a precise individual predictor, and has a standard deviation of 10 to 12 beats — meaning roughly a third of people are more than ten beats away from it.
208 − 0.7 × age, from Tanaka's 2001 meta-analysis, fits observed data considerably better, particularly over forty where the older formula understates.
At 30: 220 − 30 = 190 against 208 − 21 = 187. At 60: 160 against 166. The gap widens with age and it matters, because every zone is a percentage of this number.
A measured maximum beats both. A supervised graded exercise test is the reference; a hard field test — a thorough warm-up followed by an all-out effort of three to five minutes, repeated — gets close. Neither is advisable without medical clearance if you have cardiac risk factors.
The two methods, and why they disagree
Percentage of maximum takes flat shares of your maximum heart rate. Simple, and it ignores fitness entirely.
Karvonen works from heart rate reserve — the gap between resting and maximum — and adds the resting rate back:
target = resting + (percentage × (max − resting))
For a 30-year-old with a resting rate of 65 and a maximum of 187, the reserve is 122 bpm:
| Zone | Karvonen | Percentage of max |
|---|---|---|
| Zone 2 — Endurance | 138–150 bpm | 112–131 bpm |
| Zone 4 — Threshold | 163–175 bpm | 150–168 bpm |
Twenty-six beats apart at Zone 2. Someone using the percentage method as their easy-run guide would be walking; someone using Karvonen would be running.
Karvonen is generally the better method precisely because it responds to fitness. A trained athlete with a resting rate of 50 gets a Zone 2 of 132–146 against 138–150 for the same person untrained at 65 — lower zones, reflecting the larger reserve.
Neither is precise. The zones are bands, not lines, and a few beats either side of a boundary changes nothing.
The five zones
Using Karvonen for the 30-year-old above:
| Zone | Range | Effort | What it trains |
|---|---|---|---|
| 1 — Recovery | 126–138 | Very easy | Warm-up, cool-down, active recovery |
| 2 — Endurance | 138–150 | Conversational | Aerobic base, fat oxidation, capillary density |
| 3 — Aerobic | 150–163 | Comfortably hard | Aerobic capacity and efficiency |
| 4 — Threshold | 163–175 | Hard | Lactate threshold |
| 5 — Maximum | 175–187 | All out | Neuromuscular power, VO₂ max |
The talk test is a serviceable substitute for a monitor. Zone 2 allows full sentences. Zone 3 allows short phrases. Zone 4 allows a word or two. Zone 5 allows nothing.
Where most people go wrong
The dominant error in recreational training is spending nearly everything in Zone 3.
It happens because Zone 3 feels productive. It is hard enough to feel like training and easy enough to sustain, so an unstructured run drifts into it. The problem is that it is too hard to recover from and too easy to drive adaptation — it accumulates fatigue without the stimulus that either lower or higher intensities provide.
Elite endurance athletes across running, cycling, rowing and cross-country skiing consistently train about 80% of their volume in Zone 1 and 2 and 20% in Zone 4 and 5, with very little in between. This is the polarised or 80/20 model, and studies applying it to recreational runners have found improvements over a threshold-heavy approach at the same total volume.
For most people the practical instruction is uncomfortable and simple: run your easy runs much slower than you currently do. A genuinely easy Zone 2 run feels almost too easy, and that feeling is the point.
What Zone 2 actually builds
Zone 2 is where the adaptations that make you durable happen: mitochondrial density, capillary growth in the working muscles, and the ability to use fat as fuel — which spares glycogen for when the effort gets hard.
Those adaptations respond to duration, not intensity, which is why the volume goes here. It is also why they take months rather than weeks and why Zone 2 training feels like nothing is happening while it is happening.
The "fat burning zone" on gym equipment is a related and misleading label. It is true that a larger proportion of energy comes from fat at lower intensity, and false that this means more fat is lost — total calories dominate, and a harder session burns more of them. Zone 2 is worth doing for the aerobic adaptations, not for the fuel mix.
What shifts the numbers
Heat raises heart rate by 5 to 15 beats at the same effort, through cardiac drift as the body diverts blood to the skin.
Dehydration does the same, for the same reason.
Caffeine adds a few beats.
Altitude raises heart rate at any submaximal effort.
Illness, poor sleep and accumulated fatigue all raise resting and working heart rate — which makes an elevated morning resting rate a useful early warning that a rest day is needed.
Some medications, beta blockers most notably, lower both maximum and working heart rate substantially and make zone calculations meaningless without medical guidance.
On a hot day, effort-based pacing beats heart rate. On a cool day with good sleep, the monitor is reliable.
Measuring resting heart rate properly
Take it immediately on waking, before getting up, lying still, over a full minute. Average three or four mornings — a single reading is noisy.
Typical adult resting rates run 60 to 100 bpm; trained endurance athletes are frequently in the 40s and occasionally the 30s. A falling resting rate over months is one of the clearest signs that aerobic training is working.
A resting rate consistently above 100 at rest, or a sudden unexplained change in either direction, is worth a doctor's opinion rather than a training adjustment.
Building a week around the zones
The 80/20 split is easier to apply than it sounds once it is expressed in sessions rather than percentages.
For someone training four times a week: three easy sessions in Zone 2 and one hard session in Zone 4 or 5. For five sessions: four easy, one hard, or three easy and two hard if the volume is low.
The hard session should be genuinely hard — intervals, a tempo effort, hill repeats — and the easy sessions genuinely easy. Both halves fail together: an easy day run too hard leaves you unable to complete the hard day properly, at which point the whole week collapses into Zone 3.
A practical test after an easy run: you should feel able to do it again immediately. If it took something out of you, it was not easy.
Beginners are an exception worth naming. Someone new to running frequently cannot stay in Zone 2 while running at all — their heart rate exceeds it at the slowest sustainable jog. Walk-run intervals are the correct answer, and the ability to run continuously in Zone 2 arrives within a few months.
Devices and their accuracy
Chest straps measure the electrical signal directly and are accurate to within a beat or two. They remain the reference for training use.
Wrist optical sensors infer heart rate from blood flow under the skin and are good at steady efforts and poor at rapid changes. Lag of ten to thirty seconds at the start of an interval is normal, and cold weather, a loose strap or a tattoo degrade the signal further.
Cadence lock is the failure mode worth knowing about: a wrist sensor can latch onto the rhythm of your arm swing and report that as heart rate, producing an implausibly steady number in the 160s or 170s throughout a run. If the trace looks suspiciously flat, it is probably not measuring your heart.
For Zone 2 work, where the target band is narrow and being twenty beats out defeats the purpose, a chest strap is worth the modest cost.
What this calculator assumes
- Tanaka's 208 − 0.7 × age for maximum heart rate unless you enter a measured value, which always takes precedence.
- Karvonen by default, using heart rate reserve; the percentage method is selectable.
- Zone boundaries at 50/60/70/80/90% of reserve or maximum, which is the common five-zone scheme — some systems use three or seven zones with different boundaries.
- Resting heart rate as you enter it, ideally measured on waking across several days.
- The zones are estimates. Effort, breathing and the ability to talk are better real-time guides than a number.