Strap on any modern watch and it will happily colour your run into zones — grey, blue, green, orange, red. Those bands aren't decoration. They're a map of the different physiological systems you tax at different intensities, and training by zone is one of the most reliable ways to make sure your easy days are easy and your hard days are hard. The trouble is that most people never learn what the zones mean or how their watch decided where the lines fall.
This guide explains the five-zone model, the two common ways to calculate the boundaries, and how to use zones to structure a week that actually improves your fitness instead of leaving you tired and stagnant.
Why intensity is the variable that matters
Endurance fitness isn't built by a single mechanism. Easy efforts develop your aerobic base — capillaries, mitochondria, fat-burning enzymes. Hard efforts raise your ceiling — maximal oxygen uptake and the ability to tolerate and clear lactate. Train only in the middle and you get a bit of both and a mastery of neither, the classic "grey zone" trap where every run feels moderately hard and progress stalls.
Heart rate is a convenient window onto intensity because it rises predictably with effort. It isn't perfect — more on that below — but it's cheap, continuous, and personal, which makes it a practical way to keep each session in its intended lane.
The five zones
The standard model splits the range between rest and maximum into five bands, each with a distinct purpose:
- Zone 1 — Recovery (50–60% of max). Very light. Warm-ups, cool-downs, and recovery walks or jogs. It promotes blood flow without adding fatigue.
- Zone 2 — Aerobic base (60–70%). Easy, conversational effort. This is where the bulk of endurance training should happen; it builds the aerobic engine while keeping recovery cost low.
- Zone 3 — Tempo (70–80%). "Comfortably hard." Useful in moderation, but the zone where over-eager runners spend too much time.
- Zone 4 — Threshold (80–90%). Hard, sustainable for tens of minutes at most. This trains your lactate threshold — the pace you can hold before fatigue spikes.
- Zone 5 — VO₂ max (90–100%). Maximal, sustainable only in short bursts. It sharpens top-end power and oxygen uptake.
The bands aren't equally sized in importance. For most endurance athletes, the majority of weekly time belongs in zones 1–2, with a deliberate minority in zones 4–5.
Two ways to draw the lines: %HRmax and Karvonen
Where exactly do the boundaries fall? That depends on the method.
The simplest is percentage of maximum heart rate. Estimate your max as 220 minus your age, then take each zone's percentage of that. For a 30-year-old, max is estimated at 190, so zone 2 (60–70%) runs from 114 to 133 beats per minute. It's quick and needs only your age.
The Karvonen method is more personal. It works from your heart-rate reserve — the gap between your maximum and resting heart rates — rather than max alone. The formula is:
target = intensity × (HRmax − HRrest) + HRrest
Because it factors in resting heart rate, Karvonen accounts for fitness. A well-trained athlete with a resting rate of 45 gets meaningfully different, usually higher, zone boundaries than the plain percentage method would suggest. For the same 30-year-old with a resting rate of 60, zone 2 shifts up to roughly 138–151 bpm.
Neither method is "correct" in an absolute sense — they're different lenses. If you know your resting heart rate (measure it first thing in the morning before you get up), Karvonen gives you zones that reflect your conditioning. If you only have your age, the percentage method is a reasonable start.
The problem with 220 minus age
Both methods usually lean on an estimated maximum, and that estimate is shakier than the tidy formula implies. The 220-minus-age rule is a population average with a standard deviation of around 10 to 12 beats per minute. That means a substantial fraction of people have a true max more than a full zone away from the estimate.
The consequence is real: if your actual max is 10 beats higher than estimated, every zone boundary is set too low, and you'll train harder than you intend on "easy" days. The fix is to anchor the calculation to a measured maximum — from a hard field test (a maximal effort at the end of a thorough warm-up, ideally supervised) or a lab test — and enter that number directly instead of relying on your age.
Pace can act as a useful cross-check. If your heart rate says zone 2 but the pace feels absurdly slow or implausibly fast for an easy run, one of your anchors is off.
Building a week around zones
The most robust approach for endurance athletes is often summarised as roughly 80% easy, 20% hard — a polarised distribution that keeps most volume in zones 1–2 and reserves a smaller share for zones 4–5, with relatively little time in zone 3.
A simple week might look like this:
- Two or three easy runs entirely in zones 1–2. These should feel almost too easy; the discipline is in holding back.
- One threshold session in zone 4 — for example, a few repeats of several minutes at a comfortably hard effort with short recoveries.
- One VO₂-style session in zone 5, if you're experienced — short, sharp intervals with full recovery.
- One long run, kept in zone 2, that builds durability.
The reason this works is that the easy volume builds the base that lets you absorb and benefit from the hard sessions. Skip the easy discipline and the hard days simply dig a fatigue hole.
Heart rate versus power and pace
Heart rate is a measure of physiological cost, not of output. That distinction explains both its strengths and its quirks. On a hot day, up a hill, or when you're under-slept, the same pace or power will drive your heart rate higher — because your body is working harder to produce the same result. That's genuinely useful information: it tells you the true cost of the effort on that day.
But the lag and drift can also frustrate. Heart rate takes a minute or two to respond to a change in effort, so on short intervals it may never fully catch up to the work you're doing. This is why many athletes pace short, hard efforts by pace or power and reserve heart rate for governing longer, steadier work. The two approaches complement each other: pace and power tell you what you produced, heart rate tells you what it cost.
For most recreational athletes, the practical answer is to let heart rate govern easy and long sessions — where keeping the cost low is the whole point — and lean on pace or feel for short, sharp intervals where heart rate simply can't keep up.
Reading the numbers with judgement
Heart rate lags effort by a few seconds and drifts upward over a long session as you fatigue and dehydrate — a phenomenon called cardiac drift. It also responds to heat, caffeine, stress, and poor sleep, so a given pace can produce a higher heart rate on a bad day. Treat the zones as a guide, not a tyrant: if you're clearly unwell or exhausted, the number will tell you to back off, and that's information worth heeding.
One important caveat: if you take medication that alters heart rate — beta-blockers are the classic example — or you have a cardiac condition, the standard formulas don't apply. Set your training intensities with a clinician instead.
Used sensibly, zones turn a vague sense of effort into a repeatable plan. They keep the easy days honest, give the hard days a target, and make it obvious when you've been living in the grey zone. That clarity, more than any single workout, is what turns consistent training into steady improvement.