Why train by heart rate at all?
Two runs at the same pace can produce wildly different training effects depending on your heart rate. One might build an aerobic base, the other might push you deep into threshold territory and wipe out your recovery. Pace-based training assumes a constant environment and a constant body — neither is true. Heart rate automatically adjusts for heat, fatigue, sleep, stress, and altitude. That's why coaches across endurance sports increasingly program by zone rather than by pace alone.
What this calculator uses
We use the Tanaka formula to estimate your max heart rate: 208 − 0.7 × age. This has lower error (~7 bpm) than the classic 220 − age formula (~12 bpm). Then we use the Karvonen method, which calculates zones from your heart rate reserve (HRR = max − resting), not max alone. A 30-year-old with resting HR of 48 has completely different zone numbers from a 30-year-old with resting HR of 75, even though their max HRs are almost identical. Karvonen captures that.
Target HR at X% = resting HR + X% × (max HR − resting HR).
The five zones and what they do
Zone 1 — Recovery (50–60% HRR)
Active recovery. Walking, very easy cycling, mobility work. Used between hard sessions and during down weeks. Feels easy. You can hold full conversations.
Zone 2 — Aerobic base (60–70% HRR)
The money zone for endurance. Builds mitochondrial density, capillary density, and fat oxidation. Elite endurance athletes spend 70–80% of their training time here. Feels easy-to-moderate; you can speak in sentences. If you could only do one kind of cardio, this is it. The walking calories calculator at moderate pace often sits in Z2 for most people.
Zone 3 — Tempo (70–80% HRR)
The "gray zone." Comfortable-but-not-easy. Builds lactate shuttle capacity, efficient at endurance speed. Used in marathon-pace runs and sustained tempo work. Abused in most recreational training — it feels productive but creates outsized fatigue for the adaptation.
Zone 4 — Threshold (80–90% HRR)
Lactate threshold. You can hold this for 40–60 minutes at max effort. Training here raises your sustainable speed. Classic threshold workouts: 4 × 8 min @ Z4 with 2 min jogs, or a 20-minute steady state.
Zone 5 — VO2 max (90–100% HRR)
Top-end aerobic capacity. Held for 3–5 minutes at a time. Trains your ceiling — how much oxygen you can use. Intervals of 3 × 5 min with full recovery, or 8 × 1 min with 1 min rest, are the classic prescriptions.
The 80/20 rule of endurance training
A large body of research (and most elite training diaries) points to roughly 80% of weekly training time in zones 1–2 and 20% in zones 4–5, with minimal time in zone 3. Recreational athletes typically flip this: they spend most of their time in zone 3 (too hard to build a base, too easy to drive adaptation). Running easier when easy and harder when hard is how most amateurs unlock a surprising amount of fitness.
How to measure resting heart rate accurately
- Take it in the morning, before coffee or getting out of bed.
- Use a wrist wearable if you have one, or count manually for 60 seconds.
- Average 3–5 days. A single morning can be elevated by stress, illness, or alcohol.
- Values in the 50s–60s are common for moderately fit people. Elite endurance athletes can be 35–45.
When the formula breaks down
208 − 0.7 × age is an average. Individual max HR can vary ±20 bpm from the formula. Lab tests (maximal treadmill or bike test) give the true number. You can also field-test with a maximal effort: after a full warmup, run three progressively harder 3-minute efforts with short recoveries, finishing the last one all-out. The highest reading you see is close to your true max.
Heart-rate drift and heat
Over a 60+ minute session in warm conditions, heart rate can drift up 10–15 bpm even at constant effort (as blood volume diverts to cooling). Don't chase pace as HR drifts — hold the effort, let the pace drop, and trust that you're still training the intended zone.
FAQ
My watch gives different zones — why?
Most watches use %max HR rather than %HRR, and some use the old 220 − age formula. Karvonen (this calculator) is the more defensible approach for age groups and fitness levels that deviate from the population average.
Can I use these zones for cycling?
Sort of. Cycling max HR typically runs ~8–10 bpm lower than running max HR. If you're serious about cycling training, use a sport-specific max test or power-based zones.
What about lifting — do zones apply?
Not really. Resistance training produces HR spikes and valleys that don't map cleanly to endurance zones. Track lifting by sets, reps, and load instead — see the one-rep max calculator.
Do I need a chest strap?
Wrist optical sensors are fine for Z1–Z3, less accurate for Z4–Z5 and during rapid changes. For serious interval work or threshold sessions, a chest strap is worth the $40.