The Karvonen formula, five training zones, and AHA targets. How to estimate your max heart rate and assign each zone to a specific training goal.
Heart rate is an objective proxy for exercise intensity because cardiac output rises in near-linear proportion to oxygen uptake during aerobic work. The American Heart Association (AHA) recommends heart-rate monitoring as a practical way to confirm you are working in the intended intensity band, since perceived exertion alone is notoriously unreliable, especially for beginners who tend to overestimate how hard they are training.
Resting heart rate (RHR), measured on waking before standing, is also a useful fitness marker. The Mayo Clinic places the normal adult RHR between 60 and 100 beats per minute (bpm), while endurance-trained athletes commonly rest between 40 and 60 bpm because a stronger heart ejects more blood per beat. A RHR that trends upward over weeks can signal overtraining, illness, or inadequate recovery.
Maximum heart rate (MHR) is the ceiling your heart can reach during all-out effort, and it declines with age. The oldest and best-known estimate is the Fox formula, MHR = 220 − age. It is simple, but a 2001 meta-analysis by Tanaka and colleagues published in the Journal of the American College of Cardiology showed it overestimates MHR in younger adults and underestimates it in older adults, and proposed a more accurate equation:
MHR = 208 − (0.7 × age)
For a 35-year-old, the Fox formula gives 185 bpm while Tanaka gives 208 − 24.5 = 183.5, roughly 184 bpm. The difference is small but compounds across the training zones. Individual variation is significant, however: the standard deviation around any MHR estimate is about 10 bpm, so two people of the same age can have true maxima 20 bpm apart. A lab or field ramp test always beats an age formula for precision.
Training zones are most accurately set using the Karvonen method, which accounts for your resting heart rate by working from heart rate reserve (HRR), defined as MHR − RHR. Because the reserve already excludes the resting baseline, targets land slightly higher than a naive percentage of MHR and better reflect actual metabolic strain.
Target HR = [ (MHR − RHR) × intensity ] + RHR
Intensity is expressed as a decimal between 0 and 1. For the 35-year-old above with an RHR of 65 bpm, HRR = 184 − 65 = 119 bpm. Training at 70% intensity gives a target of (119 × 0.70) + 65 = 148 bpm. The same 70% expressed as a fraction of MHR alone would be 0.70 × 184 = 129 bpm, a 19 bpm difference that is large enough to move someone between training zones.
Coaches and exercise physiologists typically divide the effort range into five zones. The boundaries below use percentage of heart rate reserve (Karvonen); percentage-of-MHR equivalents run a few points lower. Each zone stresses a different energy system, which is why periodized programs distribute work across zones rather than living in a single one.
| Zone | Intensity (% HRR) — Purpose |
|---|---|
| Zone 1 | 50-60% — Recovery, warm-up, very light |
| Zone 2 | 60-70% — Aerobic base, fat oxidation, endurance |
| Zone 3 | 70-80% — Steady aerobic, improved efficiency |
| Zone 4 | 80-90% — Anaerobic threshold, lactate buffering |
| Zone 5 | 90-100% — VO2 max, short intervals, peak power |
The popular "fat-burning zone" label attached to Zone 2 is misleading. While a higher fraction of calories burned in Zone 2 comes from fat, total calorie burn is lower than in higher zones. For body composition, total energy expenditure matters more than the fuel mix, so the AHA and American College of Sports Medicine (ACSM) emphasize total activity volume over time spent in any single zone.
Using the 35-year-old with MHR 184 and RHR 65, the Karvonen targets span the zones as follows. Zone 2 (60-70%) targets 136-148 bpm; Zone 3 (70-80%) targets 148-160 bpm; Zone 4 (80-90%) targets 160-172 bpm. A recreational runner doing an easy aerobic run would hold roughly 136-148 bpm, while threshold intervals would push into the 160-172 bpm range.
The AHA recommends at least 150 minutes per week of moderate-intensity aerobic activity (roughly Zones 1-3, 50-70% of MHR) or 75 minutes per week of vigorous activity (Zones 4-5, 70-85% of MHR), or an equivalent combination, plus muscle-strengthening work on two or more days. Meeting these targets is associated with lower cardiovascular mortality in multiple large cohort studies. The Metriova heart rate calculator computes your MHR, HRR, and all five zone boundaries from your age and resting heart rate.
Heart-rate training is powerful but has constraints worth knowing before you act on the numbers:
Knowing the zones is half the work; distributing them across a week is the other half. The principle most endurance coaches follow is polarized or "80/20" training: roughly 80% of weekly volume stays in Zones 1–2, and only about 20% lands in Zones 4–5, with Zone 3 used sparingly. The reasoning is physiological — easy volume builds mitochondrial density and aerobic base with minimal recovery cost, while the small hard fraction raises the ceiling — and the most common recreational mistake is inverting it, drifting into Zone 3 on days that should be easy.
A concrete example for a runner with a max heart rate of 180 training four hours per week: three 45-minute Zone 2 runs, one 60-minute long run held in Zone 2 (expect cardiac drift into low Zone 3 late in the run — slow down rather than holding pace), and one 45-minute interval session: a 10-minute warm-up, six 3-minute Zone 4 repeats with 2-minute jog recoveries, and a 10-minute cool-down. That interval session contributes about 18 hard minutes, roughly 8% of weekly volume — which is exactly why the other four sessions must stay genuinely easy for the week to balance.
Progress shows up as pace, not heart rate. Every 4–6 weeks, re-run the same 30-minute flat route at the same Zone 2 heart rate and record the pace: if you are covering more distance at the same heart rate, your aerobic base is improving, even if your race times have not moved yet. The four habits below are what most often derail this process:
Sources: American Heart Association (physical activity guidelines and target heart rate), Mayo Clinic (resting heart rate ranges), Tanaka et al. (Journal of the American College of Cardiology, 2001), American College of Sports Medicine (position stands on exercise intensity), and the Centers for Disease Control and Prevention (physical activity basics). This content is educational and is not medical advice; individuals with cardiovascular conditions should consult a physician before beginning a heart-rate-based training program.
Heart Rate Zone Calculator — Fat Burn & Cardio
Put the numbers from this guide to work. The calculation runs entirely in your browser — nothing is sent to a server.