Heart rate zones — why 70% on your watch is not 70% in the coaching book
Two coaches say 'Zone 2 is 70%' and mean different things. %HRmax uses fraction of max heart rate. %HRR (Karvonen) uses fraction of the reserve above resting. For an athlete with 190 max and 50 rest, 70% HRmax is 133 bpm and 70% HRR is 148 bpm — a full training zone apart.
Heart rate zones exist because different intensities train different physiological systems. The metric matters: same 'zone number' from Garmin, Strava, Whoop, TrainingPeaks, and a coaching textbook can differ by 10–20 bpm. Below are the four common systems and where each applies.
The exact formulas
HRmax (estimate): 220 - age (Fox 1971, ±10 bpm real accuracy) > HRmax (better estimate): 208 - 0.7 × age (Tanaka 2001) > HRR = HRmax − HRrest (Heart Rate Reserve) > Karvonen target: HR = HRrest + (%HRR × HRR) > Field HRmax: highest bpm in a maximal 5 min all-out effort + last-30-sec sprint > LTHR (lactate threshold HR): 20 min all-out time trial average − 5 bpm (last min)
Four zone systems compared
For an athlete: HRmax 190, HRrest 50 (fit adult), LTHR 172.
| Zone | %HRmax | Karvonen %HRR | Coggan by LTHR | Purpose | |---|---|---|---|---| | 1 | 50-60% (95-114) | 50-60% (134-146) | <81% LTHR (<139) | Recovery | | 2 | 60-70% (114-133) | 60-70% (146-148) | 81-89% (139-153) | Aerobic base | | 3 | 70-80% (133-152) | 70-80% (148-162) | 90-93% (155-160) | Tempo | | 4 | 80-90% (152-171) | 80-90% (162-176) | 94-99% (161-170) | Threshold | | 5 | 90-100% (171-190) | 90-100% (176-190) | ≥100% (≥172) | VO2 max / anaerobic |
**Read across the same 'Zone 2' row:** the upper bound differs by 15 bpm between systems. An athlete told 'stay in Zone 2, keep it under 133' (by %HRmax) trains 15 bpm easier than one told 'stay under 148' (by Karvonen). One builds aerobic base slowly; the other overreaches into tempo daily.
Which system to use
**Karvonen / %HRR** is more physiologically meaningful — it accounts for resting heart rate, which reflects cardiac fitness. Two athletes with the same HRmax but different HRrest have different aerobic capacities; Karvonen captures this, %HRmax does not.
**%HRmax** is what most consumer watches default to (Garmin, Apple Watch). Simple, but wrong for very fit or very unfit users. Ignore it if you know your HRrest.
**LTHR-based (Friel/Coggan)** is what serious endurance coaches use. Requires a threshold test but zones are more precise for cycling and running. Not appropriate for team sports.
**RPE (perceived exertion)** is the fallback when HR data is unreliable — cold weather, dehydration, and stimulants all shift HR without changing internal workload.
Worked example — Zone 2 training
Aerobic-base training (Zone 2) should feel conversational — able to hold a full sentence. Physiologically this is below lactate threshold, dominantly fat-oxidation. An athlete with HRmax 190 and HRrest 50:
- **%HRmax Zone 2 upper: 133 bpm** (70% of 190)
- **%HRR Zone 2 upper: 148 bpm** (50 + 0.7×140)
- **LTHR Zone 2 upper: ~152 bpm** (89% of LTHR 172)
Following %HRmax leaves 15 bpm of aerobic training range unused — significantly slower base-building. Following LTHR or Karvonen makes fuller use of the fat-burning window.
The 220-minus-age estimate is unreliable
Fox's 1971 formula (220 − age) has SD ~10 bpm. Meaning: for a 40-year-old (estimated HRmax 180), the actual population range is 160–200. Tanaka's 208 − 0.7 × age (SD ~7 bpm) is better but still bad for individual training.
**Better: measure it.** After 15 minutes warm-up, run 3 × 3-minute all-out efforts with 90-second recovery. Peak HR in the last 30 seconds of the last interval is close to true HRmax. Do this once every 2–3 years.
Cardiac drift — why the same pace shows creeping HR
During prolonged exercise (>60 min), HR rises 10–20 bpm at the same pace due to fluid loss, thermal stress, and glycogen depletion. This is 'cardiac drift' and it means single-workout HR-based pacing over-punishes late-workout intensity. Elite training uses power (cycling) or pace (running) as the primary control, with HR as a secondary monitor.
HR variability (HRV) — the newer metric
HRV measures beat-to-beat timing variability, expressed in ms (RMSSD is the common metric). High HRV = parasympathetic dominance = recovered. Low HRV = sympathetic dominance = stressed. Consumer devices (Whoop, Oura, Garmin) track morning HRV to gauge readiness. HRV zones are still device-specific — no universal scale — but trends within one device are meaningful.
Wrist vs chest strap accuracy
Wrist-based optical HR (Apple Watch, Garmin) is accurate at steady state ±3 bpm but lags 20–30 seconds during rapid changes (intervals). Chest straps (Polar H10, Garmin HRM-Pro) read electrical ECG signal, accurate ±1 bpm with no lag. Serious interval training needs a chest strap; steady-state runs are fine with wrist.
The five-second recap
For most people, Karvonen (%HRR) is a better zone system than %HRmax — it accounts for your fitness. Measure your HRmax with a field test, not 220 − age. For serious endurance training, use LTHR-based zones from a 20-min threshold test.
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FAQ
Which zone burns the most fat?+
Zone 2 (60–70% HRR) has the highest fat oxidation rate as a percentage. Zone 4 burns more calories total, most from carbohydrate. Total fat burnt at high intensity can still exceed Zone 2 in absolute grams. Fat-loss depends on caloric deficit, not zone.
Why does my HR spike immediately when I stand up?+
Sympathetic activation. HR jumps 10–20 bpm on standing, then settles. This is why HRmax tests need proper warm-up — cold HR is not maximum HR.
Can I use the same zones for cycling and running?+
No. HRmax on the bike is typically 5–10 bpm lower than running (smaller muscle mass, no impact). Recalculate zones per activity, or measure HRmax separately for each sport.
What is a good resting heart rate?+
Adult population median 60–80 bpm. Fit endurance athletes: 40–55 bpm. Elite cyclists: 30–45 bpm. Track your own trend rather than compare to a chart — a 5 bpm rise over 3 days signals stress or oncoming illness.
Does caffeine change my HR zones?+
Caffeine raises resting and submaximal HR by 3–8 bpm, but does not shift HRmax. If your zones are HR-based and you drink coffee before training, the same physiological effort registers at a slightly higher HR — the workout is not harder, the number just reads higher.
This article was written by The Turbo Unit Converter engineering desk (Applied exercise physiology) and last reviewed on 2026-07-10 against NIST SP 811 and the BIPM SI Brochure. Read our full editorial policy.