Heart rate zones only work if the numbers underneath them are honest. Set your max HR wrong, and every zone built on top of it is wrong in the same direction. Get it right, and the five-zone model turns a vague idea like "easy day" into a number your watch can hold you to.
The five zones
Most zone systems split effort into five bands as a percentage of max heart rate:
- Zone 1 (50-60% max) — Recovery. Barely above resting. Shakeout runs, cooldowns, the day after a hard effort.
- Zone 2 (60-70% max) — Easy. Conversational pace. This is where most weekly mileage should live — and where most recreational runners, watch in hand, are actually running 10-15 beats too hot.
- Zone 3 (70-80% max) — Aerobic / steady. Comfortably hard. Marathon-pace territory for many runners.
- Zone 4 (80-90% max) — Threshold. The effort you can sustain for roughly an hour at your absolute limit — the classic "comfortably uncomfortable" tempo zone.
- Zone 5 (90-100% max) — Max / VO2max. Interval and repetition effort. Sustainable for minutes, not miles.
Why most easy runs are too fast
The single most common training zone mistake isn't running the hard days too hard — it's running the easy days too hard. Zone 2 running builds aerobic capacity and mitochondrial density without accumulating the fatigue that hard days require to actually be hard. Runners who blur zones 2 and 3 together end up perpetually moderately tired: never recovered enough to nail the workout, never easy enough to actually recover.
Two ways to set your zones
Straight percentage-of-max zones (used above) are the simplest starting point — see the Heart Rate Zones calculator. But two runners with the same max HR can have very different resting heart rates, and a low resting HR usually signals better aerobic fitness that flat percentage-of-max zones don't account for.
The Karvonen method corrects for this by anchoring zones to heart rate reserve — the gap between resting and max — rather than max alone. It's a better fit for trained runners with a notably low resting heart rate. Try both in the Karvonen Zones calculator and compare; for well-trained runners, Karvonen zones typically sit a few beats higher at the low end than straight percentage-of-max zones for the same intensity.
Where the numbers come from
Both methods depend on an accurate max HR. Formula estimates (220 minus age, or the more modern Tanaka formula) are population averages with real individual spread — some runners' true max sits 15-20 beats from the formula estimate in either direction. A hard-effort field test, or a max HR observed during a genuinely maximal race, beats any formula. Start with the Max Heart Rate Estimator if you don't have a tested number, but treat it as a starting estimate to refine, not a fixed ceiling.