Free Tool · Karvonen Formula

Heart Rate Zone Calculator

Get your 5 personalised training zones using the Karvonen Heart Rate Reserve method — the most accurate formula for endurance runners. Know exactly where to run on every workout.

Your Data
Max HR: 185 bpm (estimated: 220 − 35) ·  Resting HR: 60 bpm ·  HRR: 125 bpm
Zone 1
123–135 bpm
Recovery · Very easy, conversational. Used for warm-ups, cool-downs, and recovery runs.
Zone 2
135–148 bpm
Aerobic · Comfortable aerobic effort. The foundation of marathon training — most long runs and easy days.
Zone 3
148–160 bpm
Tempo · Moderate-hard. Marathon race pace typically falls in this zone for recreational runners.
Zone 4
160–173 bpm
Threshold · Hard, sustainable for 20–40 min. Lactate threshold training. The most productive intensity for improving performance.
Zone 5
173–185 bpm
VO₂max · Maximum effort. Short intervals (1–3 min). Raises aerobic ceiling but requires full recovery between sessions.

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:

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.

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:

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:

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.

Frequently Asked Questions

Most recreational marathon runners should do 70–80% of their weekly running in Zones 1–2 (aerobic and recovery effort) and 20–30% in Zones 3–4 (tempo and threshold). Zone 5 work is rarely necessary for marathon training and risks overtraining. This polarised distribution is validated by research on elite endurance athletes.
The Karvonen formula calculates target heart rate zones using your Heart Rate Reserve (HRR): HRR = Max HR − Resting HR. Target HR = (HRR × % intensity) + Resting HR. It's more personalised than simple max-HR percentage methods because it accounts for your individual cardiovascular fitness level.
Measure resting heart rate first thing in the morning before getting out of bed. Lie still for 5 minutes, then count your pulse for 60 seconds, or use a fitness tracker. Most adults have a resting HR of 60–80 bpm; highly trained runners can be 40–55 bpm. A lower resting HR generally indicates better cardiovascular fitness.
220 − age is a rough estimate with a standard deviation of ±10–12 bpm — meaning your actual max HR could be 10–12 bpm higher or lower than predicted. For a more accurate max HR, conduct a properly supervised max effort test (e.g. 4 × 1 mile at full effort with HR recorded at the end of the last rep). The calculator uses 220 − age as a default; update it with your actual max HR for better accuracy.
Zone 2 (60–70% HRR) builds mitochondrial density, increases fat oxidation, and improves cardiovascular efficiency without creating significant recovery debt. It lets you accumulate high training volume — which is the most important variable in marathon performance — without getting injured or chronically fatigued. Polarised training (mostly Zone 2 + some Zone 4) consistently outperforms moderate-intensity training in research.

Train in the right zones, every session

PacecraftAI coaches your daily runs with pace zones derived from your biometric data — including real HRV and resting HR from Health Connect, not just estimated numbers.

Start free — no credit card →