What a heart rate zone actually is
A heart rate zone is a bracket of beats per minute that stands in for a physiological intensity you cannot see directly. What a coach cares about is where you sit relative to two thresholds: the first ventilatory threshold, above which lactate starts to accumulate faster than it clears at rest, and the second, above which it accumulates without limit and the clock starts running on the effort. Neither threshold announces itself on a watch face. Heart rate is the cheapest available proxy, so zones exist to translate a bpm reading into a training intention.
The translation is only as good as its two anchors. Every zone system needs a top of the scale, and the reserve systems need a bottom as well. Get either anchor wrong and every boundary moves with it. That is why this calculator asks for a measured maximum first and falls back to an age equation only when you do not have one — and why it tells you, in the results, which of the two it used.
The zones themselves are conventions, not constants of nature. The five-band 50/60/70/80/90% split is a coaching convention that has been in general use for decades; the three-band polarised split is a research convention that maps directly onto the two thresholds. Neither is more correct than the other. They answer different questions: the five-zone model gives you a graded intensity dial, and the three-zone model gives you a way to audit how much of your week was genuinely easy.
Percentage of maximum versus percentage of reserve
The two methods differ in where they start counting. The %HRmax method takes a fraction of your maximum measured up from zero: 60% of a 180 bpm maximum is 108 bpm. The Karvonen method takes the same fraction of your reserve — the span between resting and maximum — and adds it to your resting rate: with a resting rate of 60 bpm the reserve is 120 bpm, and 60% of the reserve puts the same boundary at 60 + 72 = 132 bpm.
The gap between the two is not a matter of opinion, and it is worth writing out because it explains every argument people have about zone 2. Subtract one from the other at the same fraction p:
[HRrest + p(HRmax − HRrest)] − p·HRmax = HRrest(1 − p)
So the reserve method sits higher by exactly your resting heart rate multiplied by one minus the fraction. At p = 0.60 with a resting rate of 60 bpm that is 24 bpm; at p = 0.90 it is 6 bpm; at p = 1.00 it is zero and the two methods agree on the maximum itself. Since your resting rate is always above zero, the reserve figure is higher at every boundary below 100% — never lower, never equal until you reach the top of the scale.
That identity has a practical consequence. Zone 2 by %HRmax for the 40-year-old in the example below runs 108–126 bpm, which for most trained people is barely above a brisk walk. Zone 2 by reserve runs 132–144 bpm, which is a recognisable easy run. If a plan says “zone 2” without saying which anchor it uses, those two readings differ by nearly a minute per kilometre of pace. Karvonen's original 1957 work anchored intensity to the reserve precisely so that people with very different resting rates got comparable relative loads, and that is the reason the reserve method is the default here.
Because the reserve method needs a resting rate, it also inherits the error in that reading — but only at the weight the identity gives it. A resting rate that reads 10 bpm high moves your zone 2 floor by (1 − 0.6) × 10 = 4 bpm, not by 10.
Worked example: a 40-year-old with a resting rate of 60 bpm
Take a 40-year-old who has never done a maximal test, with a resting heart rate of 60 bpm measured lying still on waking.
- Estimate the maximum. The Tanaka equation gives HRmax = 208 − 0.7 × 40 = 208 − 28 = 180 bpm. (The older 220 − age gives 180 too — the two equations cross at exactly age 40, because 220 − a = 208 − 0.7a solves to 0.3a = 12, a = 40.)
- Find the reserve. HRR = 180 − 60 = 120 bpm.
- Build the zone 2 floor by reserve. 60 + 0.60 × 120 = 60 + 72 = 132 bpm.
- Build the zone 2 ceiling. 60 + 0.70 × 120 = 60 + 84 = 144 bpm.
- Build the threshold boundary. 60 + 0.80 × 120 = 60 + 96 = 156 bpm, the bottom of zone 4.
- Check against the %HRmax method. 0.60 × 180 = 108 and 0.70 × 180 = 126. The differences are 132 − 108 = 24 and 144 − 126 = 18, which match HRrest(1 − p) = 60 × 0.40 = 24 and 60 × 0.30 = 18 exactly.
Reading it back: easy aerobic work for this athlete is 132–144 bpm by reserve, threshold work starts at 156 bpm, and anything above 168 bpm is in the top band where sessions are counted in minutes. If a later ramp test shows a true maximum of 189 rather than 180, every boundary moves up by 0.6 × 9 = 5.4 bpm at the zone 2 floor and by 0.9 × 9 = 8.1 bpm at the zone 5 floor.
How to read your zones once you have them
Use zone 2 as a volume container, not a target to chase. The physiological case for accumulating easy aerobic time rests on adaptations — capillary density, mitochondrial content, fat oxidation rate — that respond to duration at a sustainable intensity. If you find yourself pushing to stay inside zone 2 on a hilly route, the zone is doing its job badly; the point is that the effort remains conversational for the whole session.
Zone 3 has a reputation problem it partly deserves. In a polarised framework the band between the two thresholds is where hard sessions are not hard enough and easy sessions are not easy enough. That is a claim about training distribution, not about the band being harmful: plenty of good sessions, including marathon-pace work and long tempo efforts, live there deliberately. The three-zone view of the same athlete is the honest way to audit it, because it collapses the question to a single number — what share of your week was below VT1?
Treat the top of the scale sceptically until you have measured it. If your watch has recorded 191 bpm during a hard finish and your age equation says 180, your maximum is at least 191 and every boundary on this page is too low. Enter the measured value. Conversely, an isolated spike of 210 bpm during a cold-water swim start is usually an artefact of a wet strap or an optical sensor losing contact, not a new maximum. Look for a value you have reached and sustained for a few seconds at the end of a genuinely maximal effort.
The heart rate reserve figure is worth watching on its own. It widens as resting rate falls with training, and it narrows with age as maximum falls. It also narrows temporarily with illness, dehydration and heavy training load, which is why a resting rate 8–10 bpm above your own normal is a reasonable prompt to make the day easier. Compare it against the effort-based checks in the Cooper test VO2max calculator if you want a field measure of aerobic fitness that does not depend on heart rate at all.
The same athlete's zones under both methods
| Zone | Fraction p | %HRmax (bpm) | %HRR / Karvonen (bpm) | Difference at the floor |
|---|---|---|---|---|
| Zone 1 | 0.50–0.60 | 90–108 | 120–132 | 30 bpm |
| Zone 2 | 0.60–0.70 | 108–126 | 132–144 | 24 bpm |
| Zone 3 | 0.70–0.80 | 126–144 | 144–156 | 18 bpm |
| Zone 4 | 0.80–0.90 | 144–162 | 156–168 | 12 bpm |
| Zone 5 | 0.90–1.00 | 162–180 | 168–180 | 6 bpm |
Both columns end at 180 bpm because HRrest × (1 − p) is zero when p = 1. The gap shrinks steadily as the zones climb, which is why the two methods argue most about the easy end of the scale and hardly at all about the hard end.
Mistakes that produce wrong zones
- Mixing anchors between a plan and a watch. Most training platforms default to %HRmax; most coaching literature that mentions Karvonen means reserve. Decide which one your plan uses before you copy numbers into a device.
- Using a resting rate you did not measure at rest. A reading taken sitting at a desk is typically well above a true resting value, and under the reserve method it inflates every boundary by HRrest × (1 − p).
- Treating an age-predicted maximum as a measurement. The scatter of individual maximums around any age line is on the order of 10–12 bpm, which is more than a whole zone is wide at the top of the scale. The max heart rate calculator shows how far four published equations disagree for your age.
- Chasing the “fat burning zone”. The fraction of energy coming from fat is highest at low intensity, but the absolute rate of fat oxidation and the total energy cost are not, and body composition responds to total energy balance rather than to substrate mix within a session.
- Forgetting cardiac drift. On a long session in the heat, heart rate climbs at a fixed pace and power. Holding the bpm number constant means slowing down; that is often correct, but it is a decision, not an accident.
- Applying zones across sports without re-testing. Maximum heart rate in cycling is commonly a few beats below running for the same athlete because less muscle mass is recruited. Zones built from a treadmill test read slightly high on the bike — the FTP calculator is the better anchor for cycling intensity.
Where these conventions come from
The reserve method traces to Karvonen, Kentala and Mustala's 1957 training study, which expressed intensity as a fraction of the interval between resting and maximum heart rate. The five-zone percentage bands are a coaching convention rather than a standards-body definition — ACSM's Guidelines for Exercise Testing and Prescription prescribes intensity in terms of %HRR and %VO2R and gives classification bands for light, moderate and vigorous work rather than five numbered zones. The three-zone split used here follows the way training intensity distribution is reported in the endurance literature, with boundaries at the first and second ventilatory thresholds, commonly approximated at about 82% and 87% of maximum heart rate in group data.
When to stop using heart rate and measure something else
Heart rate lags. At the start of an interval it is still climbing while your muscles are already working at target intensity, so short repetitions — anything under about three minutes — cannot be controlled by bpm at all. Use pace, power, or rating of perceived exertion for those, and use heart rate to confirm the session afterwards rather than to steer it.
Heart rate also drifts with heat, hydration, caffeine, altitude, illness and accumulated fatigue. None of those change your zones; they change the relationship between your zones and the work you are doing. If your easy pace suddenly requires zone 3 heart rates, the sensible reading is that today is harder, not that your zones have moved.
Where a direct measure of the work exists, prefer it. Cyclists have power meters, so functional threshold power is a better anchor than heart rate and the FTP calculator handles the zone math on that scale. Runners have pace, and Jack Daniels' VDOT system converts a race result into training paces directly — the VDOT calculator and the running pace calculator cover that route. Swimmers have critical swim speed. Heart rate remains the general-purpose fallback and the only one of these that works across every sport at once, which is exactly why it is worth setting up correctly.
For a single target band rather than the whole set — the case where a rehabilitation programme or a physician has specified an intensity as a percentage of reserve — the Karvonen target heart rate calculator does the same arithmetic for one prescribed percentage.
Key terms
- Heart rate reserve (HRR)
- The difference between maximum and resting heart rate. It represents the range of heart rates available to you for work, and it is the span the Karvonen method divides into zones.
- VT1 and VT2
- The first and second ventilatory thresholds — the two breakpoints in the relationship between ventilation and work rate during a graded test. They mark the boundaries of the three-zone model and are located directly only in a laboratory or by a well-designed field test.
- Cardiac drift
- The gradual rise in heart rate over a prolonged steady effort at constant work rate, driven mainly by rising core temperature and falling plasma volume. It is the reason a long run finishes in a higher zone than it started in.
- Polarised distribution
- A training pattern in which a large majority of sessions sit below VT1 and a small minority sit above VT2, with comparatively little time between them. The three-zone model exists to make that share measurable.