Understanding Heart Rate Training Zones
Heart rate zones divide your effort into bands based on percentage of your maximum heart rate — or, more accurately in this calculator, your Heart Rate Reserve (HRR). Training in different zones produces different physiological adaptations, which is why structured runners don't just "go for a run" — they deliberately target specific zones for specific sessions.
The five-zone system used here is the most common model used by coaches and sports scientists. Different organisations label the zones differently (Garmin uses zones 1–5, Polar uses a similar five-zone system, but the underlying physiology is the same).
The Karvonen Formula Explained
Simple max-HR percentage zones (e.g. "Zone 2 = 60–70% of max HR") ignore your resting heart rate. Two runners with the same max HR but resting HRs of 45 and 70 bpm have completely different cardiac outputs at any given pace — their zones should be different.
The Karvonen formula accounts for this by working from Heart Rate Reserve:
- HRR = Max HR − Resting HR
- Target HR = (HRR × % intensity) + Resting HR
A runner aged 35 with a resting HR of 45 bpm will have Zone 2 starting at a notably higher absolute heart rate than a runner the same age with a resting HR of 75 bpm. This matters in practice.
HRR Calculator: Heart Rate Reserve Explained
Heart Rate Reserve (HRR) is the gap between your maximum and resting heart rate: HRR = Max HR − Resting HR. This heart rate reserve calculator applies the Karvonen method, which sets each zone as a percentage of that reserve added back to your resting heart rate, so fitter runners with a lower resting HR get zones that fit them rather than a generic age formula.
Example: a 40-year-old with an estimated max HR of 180 bpm and a resting HR of 55 bpm has an HRR of 125 bpm. Zone 2 at 60–70% HRR is (125 × 0.60) + 55 = 130 bpm to (125 × 0.70) + 55 = 142 bpm. Enter your age and resting heart rate above to get your own zones.
| Zone | % of HRR | Purpose |
|---|---|---|
| Zone 1 | 50–60% | Recovery and warm-up |
| Zone 2 | 60–70% | Aerobic base, easy long runs |
| Zone 3 | 70–80% | Steady marathon-effort running |
| Zone 4 | 80–90% | Threshold and tempo |
| Zone 5 | 90–100% | VO2 max intervals |
Why Zone 2 Is the Foundation of Marathon Training
Zone 2 (60–70% HRR, aerobic effort) should account for 70–80% of all running for most marathon runners. At this intensity:
- Mitochondrial biogenesis: Your body builds more mitochondria, the cellular engines that produce energy aerobically.
- Fat oxidation: You become more efficient at burning fat as fuel, sparing glycogen for hard efforts.
- Recovery: Zone 2 doesn't create significant lactate or muscle damage, letting you run high volumes without chronic fatigue.
- Cardiac adaptations: Stroke volume (blood pumped per heartbeat) increases, directly improving VO₂max over time.
Many runners make the mistake of running too hard on easy days and too easy on hard days — a phenomenon coaches call "the grey zone." The result is that they never fully adapt to either stimulus. Keep your easy runs genuinely easy (Zone 1–2) and your hard runs genuinely hard (Zone 4–5).
Zone 4: The Performance Zone
Zone 4 (80–90% HRR, lactate threshold) is where the most direct improvements in running economy and race performance come from. Lactate threshold training:
- Raises the pace at which your body shifts from aerobic to anaerobic energy production
- Improves your ability to sustain a fast pace for longer
- Is typically done as 20–40 minute tempo runs, or 3–5 × 8 minute intervals at threshold pace
For a typical marathon runner, one Zone 4 session per week is sufficient. More than two threshold sessions per week without adequate recovery is a primary cause of overtraining.
For a fuller walkthrough of which zone to use for which type of run, read our guide: What Heart Rate Zone Should You Run In?