Heart rate zones explained: zone 2, the Karvonen formula, and honest limits

Heart rate zones promise something appealing: point a number at your effort and know whether you’re building an aerobic base or digging a recovery hole. The promise is mostly real — but the zones most devices hand you are built on an estimated maximum heart rate that can miss by 20 beats, which is enough to relabel your entire week of training. This guide covers the five-zone model, the Karvonen calculation that personalises it, and the situations where the numbers lie to you.

One note up front: zone training is educational fitness content, not medical advice. If you are new to vigorous exercise, over 40 and sedentary, on cardiac medication, or have any heart condition or symptoms like chest pain or unexplained breathlessness, get medical clearance before chasing maximum heart rate.

The five-zone model

Most training plans and watches use five zones, expressed here as percentages of heart-rate reserve (explained below):

• Zone 1 (50–60%): recovery. Very easy movement — warm-ups, cool-downs, the day after a hard session.
• Zone 2 (60–70%): aerobic base. Conversational endurance work. The bulk of most endurance athletes’ volume.
• Zone 3 (70–80%): tempo.“Comfortably hard.” Sustainable for an hour or so; sentences come out clipped.
• Zone 4 (80–90%): threshold. Around the lactate threshold; sustainable for perhaps 20–60 minutes. A few words at a time.
• Zone 5 (90–100%): VO2max. Interval-only territory, minutes at a time. Talking is off the table.

The boundaries are conventions, not physiology carved at the joints — different platforms shade them slightly differently. What matters is the structure: lots of easy work below the first ventilatory threshold, deliberate doses of hard work above it, and not living in the middle by accident.

Is 220 minus age accurate?

Every zone system is anchored to maximum heart rate, and the anchor most people use — 220 − age— is the weakest link. It is a population regression with a large individual spread, and it was never derived with much rigour in the first place. Tanaka, Monahan and Seals’ 2001 meta-analysis of 351 studies found the true relationship is closer to 208 − 0.7 × age: the classic formula overestimates HRmax in the young and underestimates it in the old. More importantly, the scatter around anyage formula has a standard deviation of roughly 10 bpm — so a 40-year-old’s real maximum is “probably somewhere between 165 and 205”, which is the difference between zone 2 and zone 4 at a given heart rate.

If you are healthy and cleared for hard exercise, a measured maximum — the highest number you’ve seen at the end of a hard 5K, a hill-repeat session, or a formal test — beats every formula. If you must use a formula, use Tanaka’s, and hold the resulting zones loosely.

The Karvonen formula: zones from heart-rate reserve

The second fix is to anchor zones to your heart-rate reserve (HRR) — the span between resting and maximum — rather than to maximum alone. This is the Karvonen method, from a 1957 Finnish training study:

target HR = (HRmax − HRrest) × intensity% + HRrest

Why bother? Because resting heart rate carries real information about your fitness. Two 40-year-olds with the same HRmax of 180 but resting rates of 48 and 72 have very different engines; percent-of-max gives them identical zones, while Karvonen correctly places the fitter athlete’s zone 2 higher. The procedure:

1. Estimate or measure HRmax (Tanaka formula or a real observed maximum).
2. Measure resting heart rate: 60 seconds of pulse-counting on waking, before coffee, averaged over several mornings — or your wearable’s overnight figure.
3. Compute HRR = HRmax − HRrest.
4. Apply the zone percentages from the model above to HRR, adding resting HR back to each boundary.
5. Sanity-check with the talk test (next section).

Worked example: HRmax 180, resting 60, so HRR = 120. Zone 2 runs from 120 × 0.60 + 60 = 132 to 120 × 0.70 + 60 = 144 bpm. The same athlete on naive percent-of-max zone 2 (60–70% of 180) would be told 108–126 — a range that barely overlaps.

What zone 2 actually is — and why everyone talks about it

Zone 2 is steady work just below the first ventilatory threshold: breathing elevated, conversation still easy, effort you could hold for hours. Physiologically it is where fat oxidation is high and lactate stays near baseline, which makes it the intensity that drives mitochondrial and capillary adaptations with minimal recovery cost. That last clause is the real reason for its popularity: you can absorb a lot of zone 2 volume week after week without breaking, and that accumulated aerobic base raises the ceiling for everything faster. The common mistake isn’t skipping zone 2 — it’s doing it too hard, drifting into zone 3 on every “easy” day and arriving at the hard sessions too tired to make them hard.

The talk test: free validation

Because every input to the calculation is an estimate, check the output against the oldest intensity gauge there is. In zone 2 you can speak in full sentences; near the top of zone 3 sentences shorten; in zone 4 you get a few words per breath; in zone 5 you don’t. If your calculated zone 2 leaves you gasping, your HRmax estimate is probably too high — lower the boundaries and let the conversation criterion win. The talk test tracks ventilatory thresholds surprisingly well, and it is immune to every measurement problem below.

Wrist optical vs chest strap

Wrist-based optical sensors estimate pulse from blood-flow changes under the skin. On a steady run in mild weather they are usually fine. They degrade exactly where zone precision matters: rapid intensity changes (intervals), cold constricted skin, tattoos, and activities with wrist flexion — cycling, rowing, lifting — where readings can lock onto cadence instead of pulse. A chest strap reads the heart’s electrical signal directly and remains the reference for training. Practical rule: wrist is adequate for zone 2 volume; if you structure sessions around zones 4 and 5, use a strap.

When zones mislead

• Beta blockers (and some other cardiac medications) deliberately cap heart rate. Formula zones are meaningless on them; train by perceived effort and the talk test, with zone guidance from your doctor or cardiac-rehab team.
• Heat and dehydration raise heart rate at a given pace — the same run can sit 10+ bpm higher on a hot day. Holding a rigid zone ceiling in heat means slowing down, which is usually the correct response anyway.
• Cardiac drift: on long steady efforts, heart rate creeps upward at constant pace as you dehydrate and blood is shunted to the skin. Late-session “zone 3” readings during a genuinely easy long run are often drift, not effort.
• Day-to-day noise: caffeine, stress, poor sleep, and lingering illness all add beats. Zones describe your average physiology; any single day can sit off it.

The honest summary: compute your zones with Karvonen and the Tanaka estimate (or better, a measured max), then treat them as a well-informed draft that the talk test is allowed to edit. Zones built this way pair naturally with the rest of the training math — your pace targets, the energy cost of sessions, and the baseline metabolism that all of it sits on top of.