Heart Rate Training Zones: What Each Zone Does and How to Use Them

Heart rate is a convenient, continuously measurable proxy for how hard you are working. Training zones divide that range into bands, each associated with different physiological adaptations, so you can aim your effort deliberately rather than defaulting to whatever feels normal.

The most common training error zones help fix is a familiar one: doing everything at a moderate, uncomfortable intensity — too hard to build an aerobic base efficiently, too easy to develop top-end capacity. Zones make that pattern visible.

Zones are calculated as percentages of maximum heart rate, so the estimate of max matters. The familiar 220 − age formula is simple, widely known, and not very accurate — it was never derived as a rigorous prediction equation and its error margin is large.

The Tanaka formula, 208 − 0.7 × age, comes from a 2001 meta-analysis and predicts measured maximum more accurately across the adult age range. The Target Heart Rate Calculator uses it as the default and shows 220 − age alongside as a familiar reference.

Both remain estimates. Individual maximum heart rate varies by roughly ±10 bpm around any prediction, and it is not related to fitness — a very fit person does not have a higher max, they reach a higher workload at any given heart rate.

Simple percentage-of-max zones ignore where your heart rate actually starts. The Karvonen method fixes this by working from heart rate reserve — the span between resting and maximum heart rate.

Target HR = (max HR − resting HR) × intensity% + resting HR. For a 30-year-old with a resting rate of 70 bpm, Tanaka max is 187, reserve is 117, and 70% intensity gives about 152 bpm.

Because it anchors to your resting rate, Karvonen produces zones that reflect your actual conditioning. As fitness improves and resting heart rate falls, the zones adjust accordingly — which is exactly what you want.

Each zone develops something different. None is superior; a good training week uses several.

Zone 2 is frequently labelled the fat-burning zone, and the label causes more confusion than any other idea in this area. It is based on something true: at lower intensities a higher proportion of energy comes from fat rather than carbohydrate.

But proportion is not total. Higher intensities burn considerably more calories in the same time, and total energy expenditure — along with diet — is what drives fat loss. Thirty minutes in zone 4 will generally cost you more energy, and more fat over the day, than thirty minutes in zone 2.

Zone 2 is genuinely valuable, just not for the reason the label implies. It builds the aerobic base that supports everything else, it is sustainable for long durations, and it is easy to recover from. Do it because it makes you fitter, not because of the fuel mix.

Endurance athletes across many sports converge on a broadly polarised distribution: roughly 80% of training time at low intensity, with the remaining 20% genuinely hard. The trap is the middle — accumulating large volumes at moderate intensity, which is tiring enough to compromise recovery without being hard enough to drive top-end adaptation.

For general fitness rather than competition, a simpler version works: mostly easy aerobic work, one or two genuinely hard sessions a week, and enough recovery between them.

Whatever the distribution, easy days need to be genuinely easy. The most common failure is letting them creep up into zone 3, which quietly turns a polarised plan into a uniformly moderate one.

Heart rate is a useful signal, not a perfect one, and several things distort it.

Cardiac drift raises heart rate over the course of a long session at constant effort, driven by heat and fluid loss — so a steady pace shows a rising rate. Heat and humidity raise it independently. Caffeine, illness, poor sleep and stress all shift it. Optical wrist sensors lag during interval work and can lock onto cadence instead of pulse.

Most importantly, beta blockers and some other medications lower heart rate directly, which invalidates percentage-based zones entirely. Anyone taking them should use perceived exertion and talk to their doctor about appropriate intensity.

Heart rate is not the only way to judge intensity, and it is not always the best. Rating of perceived exertion — a simple 1-to-10 scale of how hard the effort feels — correlates well with physiological intensity and needs no equipment at all.

The talk test is an even simpler version and surprisingly robust. If you can hold a full conversation, you are in the easy range. If you can manage short sentences, you are moderate. If you can only get a few words out, you are hard.

Perceived effort also has an advantage heart rate lacks: it responds immediately. Heart rate lags at the start of intervals and drifts upward over long sessions, so on interval work in particular, effort is often the more honest guide.

Since Karvonen zones depend on it, take resting heart rate carefully: first thing in the morning, before getting out of bed, counted over a full sixty seconds. Average three mornings for a stable figure.

Most fitness trackers report an overnight resting rate that is close enough for this purpose, and has the advantage of being measured every day without effort.

Watching it over time is informative in its own right. A resting heart rate that drifts down over months usually indicates improving aerobic fitness; a sudden rise of several beats often precedes illness or signals accumulated fatigue.