Two runners, both 40 years old, both pushing hard up the same hill. Watch A flashes "Zone 4 — threshold." Watch B flashes "Zone 2 — easy." Same effort, same age, opposite verdicts. The difference is not the watch — it is the math behind the zones, and most fitness watches are using the lazy version of it.
The lazy version is plain max-heart-rate percentages. The smarter version is the Karvonen method, which factors in your resting heart rate. If you have ever wondered why your "easy run" feels surprisingly hard, or why your watch keeps yelling "anaerobic" while you can still hold a conversation, this is almost always why.
Let's break down what zones actually train, why Karvonen is more honest about your fitness, and how to set yours so the numbers stop lying to you.
What a Heart Rate Zone Actually Is
A heart rate zone is just a band of beats per minute that corresponds to a specific physiological intensity. Most coaching systems use five zones, roughly:
- Zone 1 — recovery, very easy
- Zone 2 — aerobic base, fat-burning, conversational
- Zone 3 — tempo, comfortably hard, "moderate"
- Zone 4 — threshold, breath gets short
- Zone 5 — VO2 max / anaerobic, all-out intervals
The reason coaches care about zones is that each one trains a different system. Zone 2 builds the mitochondrial density and capillary network that lets you go long. Zone 4 raises your lactate threshold so you can hold a faster pace before your legs give up. Zone 5 stretches the ceiling of how much oxygen you can actually use. Mix them wrong and you spend months training one system you didn't need and ignoring the one you did.
That's the part the watch is trying to help with — but only if the math is right.
The Plain Max-HR Method (And Why It Lies)
The most common formula is the simplest:
target HR = max HR × intensity %So a 40-year-old using the classic 220 − age rule gets a max of 180 bpm. Zone 2 (60–70%) becomes 108–126 bpm. The American Heart Association uses a similar percentage-of-max model in its target heart rate guidance, and most consumer watches default to it because it needs only one input: your age.
The problem is what it ignores: your fitness.
Two 40-year-olds with the same predicted max of 180 bpm can have wildly different resting heart rates. A trained endurance athlete might rest at 45. A sedentary office worker might rest at 75. The fit one has a much wider working range — 135 beats between rest and max. The sedentary one has only 105. When you take 60% of the same max for both, you ignore that the fit person has more "room" between rest and max to work with. The zones end up squashed for one and stretched for the other.
The Karvonen Method (Heart Rate Reserve)
Karvonen fixes this by working off heart rate reserve (HRR) — the gap between your resting and max heart rates — instead of max alone.
HRR = max HR − resting HR
target HR = (HRR × intensity %) + resting HRLet's run both formulas for that 40-year-old endurance athlete (resting HR 45):
Plain max-HR, 60% intensity:
target = 180 × 0.60
target = 108 bpm
Karvonen, 60% intensity:
HRR = 180 − 45 = 135
target = (135 × 0.60) + 45
target = 81 + 45
target = 126 bpmSame person, same intensity label, 18 bpm difference. The plain formula is telling our athlete that 108 bpm is a "60% effort" — but for a fit person, 108 is barely above resting. They'd have to walk slower than a stroll to hit it. Karvonen's 126 is far closer to what 60% of their actual capacity feels like.
The Wikipedia entry on the Karvonen method walks through the original derivation, and the ACSM exercise prescription guidelines treat HRR as the standard for prescribing intensity. Research on HRR (summary on PubMed) confirms it correlates much better with VO2 reserve — the actual metabolic intensity — than plain max-HR percentages do.
If you want to skip the algebra, our Karvonen heart rate zone calculator takes age and resting HR and spits out all five zones for you.
Getting Your Two Inputs Right
Karvonen is only as good as your two numbers. Both are usually wrong by default.
Resting heart rate. Take it first thing in the morning, before you stand up, before coffee. A chest strap or a wrist tracker overnight average is fine. Do it three or four mornings and average them. If your resting HR jumps 5–10 bpm above your normal, that's a sign you're under-recovered or fighting something off — useful signal in itself.
Max heart rate. The 220 − age rule is a population average with a standard deviation around ±10–12 bpm. That means roughly two-thirds of people are within 10 bpm of it, and one-third are further off. Better estimates exist:
Tanaka: max HR = 208 − (0.7 × age)
Gellish: max HR = 207 − (0.7 × age)Both are slightly more accurate than 220 − age for adults over 40. Best of all is a measured max from a recent all-out effort — the last few minutes of a 5K race, a hill repeats session done to exhaustion, or a supervised treadmill test. If you've ever seen a peak in your watch data after a brutal workout, that's a better number than any formula.
Plug that number plus your honest resting HR into Karvonen, and your zones suddenly start lining up with how the work actually feels.
What Each Zone Actually Trains
Now that the bands are honest, here is what to do in each:
Zone 1 (50–60% HRR) — Pure recovery. Walking, easy spinning, the day after a hard workout. Builds nothing on its own; lets you train more often without breaking down.
Zone 2 (60–70% HRR) — The aerobic base. This is the zone you should spend the most time in if you want to get faster long-term, despite it feeling almost too easy. It increases mitochondrial density, capillary growth, and fat oxidation — the engine room. You should be able to nose-breathe or hold a full sentence.
Zone 3 (70–80% HRR) — Tempo, "moderate." Often called the gray zone because it's too hard to recover quickly from but too easy to drive top-end gains. Use sparingly. If your "easy days" drift into Zone 3, your aerobic base stagnates and you stay tired. This is the most common mistake recreational runners make.
Zone 4 (80–90% HRR) — Threshold. The pace you could hold for roughly an hour at race effort. Cruise intervals, tempo runs, 4×8 minute repeats. Pushes your lactate threshold up so a given pace costs less.
Zone 5 (90–100% HRR) — VO2 max and anaerobic. Short, sharp intervals: 30s–4 min hard with full recovery between. Raises your aerobic ceiling. A little goes a long way; once a week is plenty for most people.
A common base-building split is 80% easy (Z1–Z2), 20% hard (Z4–Z5), with Zone 3 mostly avoided. That ratio is not arbitrary — it's roughly what elite endurance athletes have done for decades.
Putting Zones to Work in a Real Plan
Zones become useful when you connect them to pace. Once you know your Zone 2 HR, do an easy run on flat ground for 30 minutes and see what pace settles in. That's your aerobic pace. The same workout six weeks later, if you've trained correctly, should be a faster pace at the same heart rate. That's aerobic fitness improving, in plain numbers.
The pace calculator is handy here for planning out target paces by zone — figure out what min/km or min/mile each effort should land at, then check your watch on the run.
Calorie burn is the other thing zones interact with. Higher zones burn more calories per minute but a smaller fraction from fat. Lower zones burn fewer calories per minute but lean more heavily on fat as fuel. If you're trying to manage body composition, the zones tell you what energy system is being taxed; the TDEE calculator and macro calculator tell you how to feed the rest of the day around it. And if you just want a baseline body composition snapshot before tweaking training, the BMI calculator does that in ten seconds.
Common Mistakes to Avoid
A few things that consistently throw people off:
- Trusting
220 − ageblindly. It has a wide error margin. If your "Zone 5 max" feels like a brisk jog, your formula max is too low — or it's too high if you can't get into Zone 4 on a flat-out effort. - Using max-HR zones from the watch and HRR zones from a coach. They don't match. Pick one system and stay in it.
- Drifting into Zone 3 on easy days. The single most common reason recreational runners plateau. If your watch keeps showing Z3 on what was meant to be Z2, slow down — even if it feels embarrassingly slow.
- Updating zones once and forgetting. As fitness improves, resting HR drops. Recompute every 2–3 months.
- Heat, caffeine, dehydration, illness. All push HR up at a given pace. On a hot day, the same run can read a full zone higher. Use perceived effort as a sanity check, not just the number.
The Bottom Line
Plain max-HR percentages are good enough for the "is this a workout or a walk?" question. They are not good enough for training prescription. Karvonen brings your resting heart rate into the math, which is where individual fitness actually shows up, and produces zones that match how your body is really working.
Two ingredients: an honest resting HR and a measured (or at least better-estimated) max HR. Plug them into the formula, and your "easy day" will finally feel easy, your threshold day will finally hurt the right amount, and the watch will stop yelling at you for no reason.
Want the math done for you? Try the Karvonen heart rate zone calculator — punch in two numbers, get all five zones in beats per minute. Then go run them.
FAQ
Is the 220-age formula really that inaccurate?
Yes — the standard deviation is about ±10–12 bpm across the population, which means roughly one-third of adults have a max HR that's more than 10 bpm off the formula prediction. For a 40-year-old that's anywhere from 168 to 192 bpm in real-world variation. Tanaka (208 - 0.7×age) is slightly better but still a population estimate. A measured max from an all-out 5K finish or supervised treadmill test always beats any formula.
Why do my smartwatch zones differ from my coach's zones?
Almost certainly because your watch uses %max-HR and your coach uses %HRR (Karvonen). The two systems give different numbers for the same intensity label — sometimes 15+ bpm apart at the lower zones. Pick one system and configure your watch to match it; mixing them is the fastest way to over-train Zone 3 and under-train Zone 2.
How often should I retest my zones?
Every 8–12 weeks if you're actively training, especially in a build phase. As fitness improves, resting HR drops (often by 5–10 bpm over 6 months of consistent base training), which shifts your HRR-based zones noticeably. If you set zones once a year, by month six they're describing a less-fit version of you.
Can I train Zone 2 if I don't have a chest strap?
A wrist optical sensor is fine for steady-state efforts where HR doesn't change rapidly — exactly when Zone 2 work happens. The "talk test" is also a valid backup: if you can hold a full sentence comfortably but a song is a stretch, you're roughly in Zone 2 regardless of what the watch says. Optical sensors struggle most during intervals where HR jumps fast; for Zone 5 work, a chest strap is genuinely worth it.
Why does my Zone 2 pace feel embarrassingly slow?
Because your aerobic base probably is — that's the point. Most recreational runners spend too much time in Zone 3 ("comfortably hard") and not enough in true Zone 2 ("conversational"). The first 4–8 weeks of disciplined Zone 2 training are humbling; the payoff is that the same heart rate maps to a 30+ second/km faster pace by week 12. Trust the process and slow down.
Is the 80/20 polarized training rule actually backed by research?
It is — Stephen Seiler's research on elite endurance athletes documents that the highest-volume rowers, runners, and skiers consistently spend roughly 80% of training time below lactate threshold and 20% above, with very little in between. For recreational athletes the science is less clean, but the directional advice (more easy, less moderate) holds up across most studies on aerobic adaptation.
How do heat and altitude affect zone numbers?
Both push HR up at a given pace. In hot weather, expect 5–15 bpm higher HR for the same effort due to extra cardiovascular load from cooling. At altitude (>1500m), HR rises because oxygen is scarcer and the heart works harder to deliver the same amount. On either, use perceived effort or pace as a sanity check rather than locking onto the HR number — you'll under-perform if you cap workouts based on a sea-level zone in 95°F heat.
Should I use Karvonen for cycling and rowing too?
Yes, but the absolute numbers will differ from running. Most people have a max HR 5–10 bpm lower on the bike than running, and lower still on the rowing machine, because non-impact sports recruit different muscle volumes. Either measure your max separately for each sport or use Karvonen with a sport-specific resting HR (which trends similar across sports anyway). The percentages and zone labels stay the same; only the bpm targets shift.