Endurometry
What does this calculator do?
The heart-rate zone calculator turns a resting heart rate and a maximum heart rate into five running zones — Z1 Recovery through Z5 VO2max — using the Karvonen heart-rate reserve method. You can type a measured max, or let the page estimate max from age: Tanaka (default), Fox (220 − age) or Gellish (quadratic).
Typical uses: setting a bpm cap for easy days so they stay easy, checking that a “tempo” run is not quietly in Z4, and seeing why two 35-year-olds can have zones 10 bpm apart. It is an intensity map in heart-rate language, not a watch algorithm and not a medical test.
How does it work?
First the page needs a max HR. A measured value in the 120–230 bpm range wins. Otherwise it uses the formula you selected and your age. Heart-rate reserve is then max minus rest. Each zone is a slice of that reserve (50–60, 60–70, 70–80, 80–90, 90–100%). Those five bands are a common running split of reserve; Karvonen (1957) described the method, not this named five-zone table.
The bpm column is the Karvonen result. The third column shows the same numbers as a percentage of max HR, so you can see why a watch that uses a flat % of max will disagree — Karvonen Z2 is often nearer 70–80% of max than 60–70%. Zones here are independent of VDOT; matching paces sit on the training pace calculator.
Formula
HRmax (if not measured) Tanaka (default) 208 − 0.7 × age Fox 220 − age Gellish 191.5 − 0.007 × age² HR reserve = HRmax − HRrest HR = reserve × % + HRrest Endurometry zones (% of reserve) Z1 Recovery 50–60% Z2 Aerobic / Easy 60–70% Z3 Tempo 70–80% Z4 Threshold 80–90% Z5 VO2max 90–100%
A measured max in the box replaces every age formula. Heart rates are rounded to a whole beat only when displayed. Resting HR must sit at least 5 bpm below max, or the page rejects the inputs rather than drawing nonsense zones.
Worked example
A 35-year-old runner with a resting heart rate of 55 bpm uses the default Tanaka formula (the calculator’s defaults).
- Tanaka HRmax = 208 − 0.7 × 35 = 183.5 bpm, shown as 184 bpm
- Heart-rate reserve = 183.5 − 55 = 128.5 bpm
- Z1 (50–60% reserve): 119–132 bpm
- Z2 (60–70%): 132–145 bpm — the same Z2 as the optional table on the training-pace page
- Z3 145–158 · Z4 158–171 · Z5 171–184
- As % of max, that Z2 is about 72–79% HRmax, not 60–70% — which is why a %HRmax watch can look “high” on an easy day
Clicking Calculate with the defaults above returns exactly these zones. Fox for the same age gives HRmax 185; Gellish gives about 183.
How to interpret the result
Treat Z2 as the easy-aerobic band you should spend most of the week in if you train by heart rate. Z3 is moderate work — useful in short doses, easy to overuse. Z4 is threshold / hard tempo. Z5 is short VO2max efforts with recovery. If your “easy” runs live in Z3, you are not recovering the way a five-zone plan assumes.
A measured max from a lab test or a hard, well-paced 5K beats any age formula. Heat, hills, dehydration, caffeine, illness and some medicines (including beta blockers) move heart rate at the same pace; when in doubt, ease off by feel. How the zones sit next to easy / tempo / long-run paces is in the heart-rate zones guide.
Assumptions and limitations
Assumptions
- Resting HR is a true morning rest value (lying down, just after waking), not a seated office pulse.
- If you leave the measured-max box empty, max HR is only an age formula — Tanaka, Fox or Gellish.
- The five named zones are a common mapping of heart-rate reserve, not a single official standard.
Limitations
- Age-predicted max HR has a large individual error — often 10 bpm or more either way. Tanaka is usually closer than Fox; neither replaces a measured max.
- Fox’s 220 − age is a widely used rule of thumb, not a tightly derived individual law. It is offered for comparison.
- Karvonen still needs an honest max and an honest rest. A watch “max” from a random interval, or a rest taken after coffee, will shift every zone.
- This is not a medical device and not a diagnosis. It does not account for medication, arrhythmia, pregnancy or cardiac conditions.
- Wrist optical sensors lag and drift; a chest strap is usually more trustworthy for zone work.
- Age outside 10–90, resting HR outside 30–120, a measured max outside 120–230, or rest within 5 bpm of max is rejected as an input error.
Methodology and sources
Method type: Published model
Zones implement Karvonen (1957) heart-rate reserve. Age estimates of max HR use Tanaka (2001), Fox (1971 / 220 − age) and Gellish (2007). The five zone fractions are a common running split of reserve, not a table printed in those papers. A measured max, when entered, replaces every formula.
Sources: Karvonen 1957 · Tanaka 2001 · Fox 1971 · Gellish 2007
Last reviewed: October 5, 2026