How Heart Rate Zones Work

Heart rate zones turn one measurement — beats per minute — into a training dial. Spend time in the right band and easy days stay easy while hard days actually get hard. The catch is that every zone is anchored to a max heart rate nobody measures precisely, so the interesting math is in how the estimate is built.

Estimating max heart rate

Zones are expressed as percentages of maximum heart rate (HRmax), the highest beats-per-minute your heart can sustain in an all-out effort. Measuring it means a lab test or a truly maximal field effort, so most people estimate it from age. The famous Fox formula — the one on every gym wall since the 1970s — is:

HRmax = 220 − age

At age 35 that's 185 bpm. The problem is accuracy: for any individual, 220 − age can easily be off by 10 bpm or more in either direction, because HRmax is partly genetic and drifts down at slightly different rates in different people. Newer regressions from large dataset studies do better. The two the calculator offers:

Tanaka (2001):  HRmax = 208 − 0.7 × age
Gellish (2007): HRmax = 207 − 0.7 × age

At 35, Tanaka gives 208 − 24.5 = 183.5 → 184 bpm, a touch under Fox's 185; the gap widens with age, which is why Tanaka is the default. If you know your true max — from a race 5k finish sprint, a lab test, or a wearable that caught a peak — the calculator's custom option takes it directly and is always the better input.

The five zones

Most systems divide the range from 50% to 100% of HRmax into five bands. The calculator's labels:

• Zone 1, 50–60% — Very light / recovery. Conversational pace; warm-ups and flush-out days.
• Zone 2, 60–70% — Light / fat burn and endurance base. The "all day" pace most runners do too fast.
• Zone 3, 70–80% — Moderate / aerobic. Comfortably hard; steady tempo work.
• Zone 4, 80–90% — Hard / around the anaerobic threshold. Talking is limited to single words.
• Zone 5, 90–100% — Maximum / VO2 max and sprint efforts. Seconds to a couple of minutes at a time.

Percent-of-max vs the Karvonen method

There are two ways to turn percentages into bpm targets. The simple one multiplies your max directly:

target = HRmax × zone %

For HRmax 184, Zone 2 becomes 110–129 bpm (0.60 and 0.70 of 184). The Karvonen method argues that this undershoots, because exercise intensity really raises your heart rate above its resting level — the heart rate reserve:

target = resting HR + (HRmax − resting HR) × zone %

Same 35-year-old with a resting heart rate of 60: reserve = 184 − 60 = 124, so Zone 2 becomes 60 + 124 × 0.60 = 134 up to 60 + 124 × 0.70 = 147 bpm. That's 15–18 bpm higher than the percent method at the same zone — because a trained heart with a low resting rate has more room between rest and max, and Karvonen accounts for it. Both methods are legitimate; the mistake is mixing them, or comparing your Karvonen zones to a friend's percent-based zones.

Training with the zones

The classic error is spending every run in Zone 3 — too hard to recover from, too easy to develop top-end speed. Most endurance plans therefore concentrate easy volume in Zones 1–2 and keep the genuinely hard work in Zones 4–5 short and deliberate — the "80/20" idea in its rough form. Zones also make the energy side concrete: a 70 kg person burns roughly 245 kcal an hour walking and about 686 kcal an hour running (using MET values), which is why the Calories Burned Calculator pairs naturally with zone work. For turning a target race time into the paces that produce those zones, the Pace Calculator converts distance and goal time into per-km and per-mile splits.

The limits of the math

Every number above is an estimate stacked on an estimate. HRmax formulas carry individual error in both directions, resting heart rate moves with fitness, sleep, heat, and stress, and cardiac drift pushes your bpm upward over a long session even at constant effort. Treat zone boundaries as guide rails, not walls: if Zone 2 feels like Zone 3, it is Zone 3 today. Recompute the zones as fitness changes — resting heart rate falling by 5–10 bpm over a training block is exactly when the Karvonen targets should move too.

Try it yourself

The Heart Rate Zone Calculator computes all five zones from your age, with Fox, Tanaka, Gellish, or a custom max, and switches between percent-of-max and Karvonen. Feed a goal race into the Pace Calculator to translate zones into splits, and the Calories Burned Calculator to price a session's energy cost for that workout's place in your day.