Heart Rate Training Zones Explained: Tanaka, Karvonen, and the 5 Zones

Heart rate zones turn a single number — your max heart rate — into a practical training tool: five bands, each trainable for a different purpose, from easy recovery to all-out effort. Getting the estimate right matters, because every zone underneath it is calculated as a percentage of that number.

Why "220 minus age" needs to go

If you've seen a heart rate formula before, it was probably this one. It first appeared in a 1970 review paper as a passing observation about a small dataset, not a validated equation — and even its own authors later noted it was never meant to be treated as a precise formula. Despite that, it became the default printed on gym equipment and fitness watches for decades, largely because it was simple to remember, not because it was accurate.

The problem is systematic rather than random: research since has shown "220 minus age" tends to overestimate max heart rate in younger people and underestimate it in older adults, which is exactly the pattern you'd expect from a rough linear guess that was never properly fitted to real data across the full adult age range.

The Tanaka formula

Hirofumi Tanaka and colleagues addressed this directly in a 2001 meta-analysis pooling data from over 18,000 individuals across many studies, and derived a better-fitted equation: 208 − (0.7 × age). It produces a shallower decline with age than the old rule, which matches what the underlying data actually shows, and it's now the formula generally recommended in exercise physiology over "220 minus age."

It's still an estimate, though a considerably better one — the study reported a standard deviation of around 10 beats per minute, meaning your true max heart rate could reasonably sit anywhere from about 10bpm below to 10bpm above the formula's answer. That's a meaningful range for setting zones precisely, which is one reason a real field or lab test remains the gold standard for anyone who trains seriously by heart rate.

Karvonen: adding your fitness level to the picture

Age-based formulas treat everyone the same age identically, but two 40-year-olds with very different fitness levels don't have the same cardiovascular profile. The Karvonen method, published by Finnish physiologist Martti Karvonen in 1957, corrects for this by using heart rate reserve — the gap between your max heart rate and your resting heart rate — rather than max heart rate alone.

The calculation is: target HR = ((max HR − resting HR) × intensity%) + resting HR. A fitter person typically has a lower resting heart rate, which widens their heart rate reserve and shifts their zones to genuinely different numbers than someone with the same max HR but a higher resting rate. This is why FitCalc's calculator asks for resting heart rate as an optional field — provide it, and the zones shown switch to the more personalised Karvonen method automatically.

Resting heart rate is easiest to measure first thing in the morning, before getting out of bed, over several days — take the lowest or average reading rather than a single one, since it fluctuates with sleep quality, hydration, and stress.

The five zones, and what each one is for

Zone 2 gets disproportionate attention in endurance circles, and for good reason: at this intensity the body relies heavily on fat rather than glycogen for fuel, and because it's genuinely easy to sustain, it's where most aerobic base fitness is actually built — through accumulated volume over weeks and months, not through any single hard session. Zones 4 and 5, by contrast, deliver a different, sharper stimulus but can only be sustained briefly and tolerated in small, deliberate doses without accumulating excessive fatigue.

Formula estimate vs a real test

Nothing beats an actual measured max heart rate for precision, and it's more accessible than it sounds — a supervised maximal exercise test at a sports science lab is the most rigorous option, but a well-executed field test (several minutes of progressively harder effort finishing in an all-out final push, on a day you're well recovered) gives most trained individuals a reasonable real-world number. Either approach removes the roughly ±10bpm uncertainty that comes with any age-based formula, Tanaka included.

Using zones in practice

You don't need to hit every zone every week. A common and well-supported pattern for endurance training is "polarised" distribution — the large majority of weekly volume in zones 1–2, a small, focused amount in zones 4–5, and comparatively little time in zone 3, which tends to be hard enough to accumulate fatigue but not hard enough to drive the specific adaptations zone 4–5 work provides. For general fitness rather than performance training, simply knowing which zone a given effort falls into is often enough to make sure "easy days" are actually easy and "hard days" are actually hard — a distinction a lot of casual cardio blurs by accident.

This guide is for general education purposes and isn't medical advice. If you have a heart condition, are on medication that affects heart rate, or are new to intense exercise, check with a doctor before training by heart rate zones.