How this works
Heart-rate training divides effort into five zones, each tied to a specific physiological adaptation: Zone 1 (50–60%) for active recovery, Zone 2 (60–70%) for aerobic base and fat-burning, Zone 3 (70–80%) for tempo work, Zone 4 (80–90%) for lactate-threshold efforts, and Zone 5 (90–100%) for VO₂-max intervals. To use them you need an estimate of your maximum heart rate. The classic Fox formula (220 − age) is simple but tends to overestimate for young people and underestimate for those over 40; the Tanaka formula (208 − 0.7 × age) was derived from a meta-analysis of over 18,000 subjects and is more accurate, especially in older adults. Both are estimates: actual max HR varies by ±10–15 bpm across individuals of the same age. For more personal zones, enable Karvonen mode and supply your resting heart rate — the calculator then bases zones on heart-rate reserve (HRR = max − resting), which accounts for fitness level.
The formula
pct = the lower or upper bound of the chosen zone, expressed as a fraction. Zone 1 = 0.50–0.60, Zone 2 = 0.60–0.70, Zone 3 = 0.70–0.80, Zone 4 = 0.80–0.90, Zone 5 = 0.90–1.00. HRrest is your resting heart rate, ideally measured first thing in the morning before getting out of bed. Karvonen accounts for the fact that fit people have lower resting heart rates and therefore a wider heart-rate reserve to play with.
Example calculation
- Age 35, Tanaka formula: HRmax = 208 − 0.7 × 35 = 208 − 24.5 = 183 bpm.
- Zone 2 (60–70% of HRmax) = 0.60 × 183 to 0.70 × 183 = 110 to 128 bpm. Easy aerobic work.
- Zone 4 (80–90%) = 146 to 165 bpm. Threshold / race-pace work.
- With resting HR 60 (Karvonen): HRR = 183 − 60 = 123. Zone 2 = 0.60 × 123 + 60 to 0.70 × 123 + 60 = 134 to 146 bpm — slightly higher than the plain %max version, because the trained athlete has more reserve.
Frequently asked questions
Tanaka or Fox — which formula should I use?
Default to Tanaka. Fox (220 − age) was published in 1971 from a small sample and has known accuracy problems: it overestimates max HR for under-25s and underestimates it past about 40. Tanaka (208 − 0.7 × age), from a 2001 meta-analysis of over 18,000 subjects, is more accurate across the full adult range. The difference is biggest at the extremes — at age 60, Fox says 160, Tanaka says 166. For a 35-year-old, both come out near 185. Either is fine for ballpark zones; if you want to be picky, Tanaka edges it.
Why does Karvonen give different (usually higher) zones?
Plain %max scales the zones from 0 to your max HR. Karvonen scales them from your resting HR to your max HR — your "heart-rate reserve". Because resting HR is never really 0 (it might be 60), the bottom of the scale is shifted up. Below about 70% the Karvonen values are higher than %max; above about 90% they converge. The benefit: a fit person with a 45 bpm resting HR and an unfit person with a 75 bpm resting HR end up with very different practical zones, even at the same age. If you have a reliable resting HR, prefer Karvonen.
How do I find my actual max heart rate without a lab test?
A field test gets close. Warm up for 15 minutes, then run a 3-minute hard effort, jog 2 minutes, then 3 more minutes all-out — your highest reading on a chest strap (wrist-optical readings spike unreliably) is a good estimate of your real max. Cyclists can do something similar on a hill. Don't try this if you're new to exercise, have any heart condition, or haven't been cleared by a doctor for max-effort work. The estimated value the calculator gives is fine for general training; only chase the real number if you want precision.
What if my watch shows me well above Zone 5 during a hard effort?
Two likely causes. First, the formula underestimates your real max — if you can repeatedly hold 5–10 bpm above the calculated max during all-out intervals, the calculator number is wrong, not your heart. Override it by setting a custom max in your watch (most platforms let you). Second, optical wrist sensors sometimes "lock onto" your cadence (running, cycling) and report cadence-as-heart-rate, which can suddenly jump to 180+ regardless of effort. A chest strap eliminates this. If neither explains it and the high readings come with chest pain, dizziness, or breathlessness, stop and see a doctor.