Heart Rate Zones Explained: Karvonen vs %HRmax vs LTHR

Your GPS watch shows five heart rate zones. Your friend's app shows three. A coaching article references Zone 2. Another talks about threshold zone. They all use different numbers, they rarely agree, and none of them explain where the boundaries come from.

This guide traces the two main HR zone systems to their source papers, explains the math behind each, and shows why — for the same runner — Karvonen zones and percentage-of-max zones can differ by 15–20 bpm at the same intensity level.

Key takeaways

The Karvonen method (1957) uses heart rate reserve — the gap between max and resting HR — to personalise zones; the %HRmax method uses only maximum HR
For a runner with low resting HR (45 bpm), Karvonen zones sit significantly higher in absolute bpm than %HRmax zones at the same intensity percentage
The 220-minus-age formula for estimating max HR has a standard deviation of ±10–12 bpm — your actual max could easily be 15 bpm above or below the estimate
Lactate threshold heart rate (LTHR), a third method, ties zones to a physiological event rather than a percentage — but requires a threshold test to establish
All three methods produce correct training zones only when calibrated to accurate input values; the most common error is using a wrong max HR

Why HR zones matter

Heart rate is a proxy for metabolic intensity. At low intensities, the body primarily burns fat and can sustain effort indefinitely. As intensity rises, carbohydrate combustion increases, lactate accumulates faster, and the sustainable duration shortens. Heart rate tracks this curve closely enough to be a practical training guide — even if imperfectly.

Training zone systems formalise this by dividing the HR range into bands, each targeting different physiological adaptations. Spending time in Zone 1–2 builds the aerobic base. Threshold work (Zone 4 in most systems) improves lactate clearance and tempo pace. Interval training at Zone 5 stresses the VO2 max system. Training exclusively at one intensity — whether too hard or too easy — produces lopsided adaptations.

The practical value of HR zones over pace zones: they automatically adjust for conditions. Running at the same pace in heat, on hills, or while fatigued produces higher HR. Running by HR keeps the physiological stress constant even when pace varies.

Method 1: The Karvonen formula (1957)

Martti Karvonen, Erkki Kentala, and Olavi Mustala published a study in 1957 in the Annales Medicinae Experimentalis et Biologiae Fenniae — a Finnish medical journal — examining how heart rate changes during and after different training intensities in Finnish fireman. The paper is famous in the running world for one formula:

Target HR = ((HRmax − HRrest) × intensity%) + HRrest

The key insight is the term (HRmax − HRrest), called heart rate reserve (HRR). By using the functional range of the heart rather than just its maximum, the formula accounts for individual fitness differences at the same relative intensity.

Why heart rate reserve matters

Consider two runners with the same maximum heart rate (190 bpm) but different resting rates:

Runner A: HRrest = 45 bpm (well-trained) → HRR = 145 bpm
Runner B: HRrest = 68 bpm (less trained) → HRR = 122 bpm

At 70% Karvonen intensity:

Runner A: (145 × 0.70) + 45 = 146.5 bpm
Runner B: (122 × 0.70) + 68 = 153.4 bpm

Same percentage, same max HR — but Runner A trains at 146 bpm where Runner B trains at 153. The %HRmax method would give both runners the same number (190 × 0.70 = 133 bpm), which is lower than either Karvonen value. The difference is real: the runner with lower resting HR is cardiovascularly fitter, and their zones should reflect that.

Karvonen zone boundaries

Using standard zone percentages of heart rate reserve:

Zone	% of HRR	Purpose
Zone 1	50–60%	Active recovery, base building
Zone 2	60–70%	Aerobic base, long runs
Zone 3	70–80%	Aerobic development, moderate effort
Zone 4	80–90%	Threshold, tempo work
Zone 5	90–100%	VO2 max effort, intervals

Method 2: Percentage of maximum HR (%HRmax)

The simpler alternative ignores resting heart rate and calculates zones as direct fractions of maximum HR. The ACSM (American College of Sports Medicine) uses this system in its exercise guidelines:

Target HR = HRmax × intensity%

Zone boundaries per ACSM:

Zone 1: 50–60% HRmax
Zone 2: 60–70% HRmax
Zone 3: 70–80% HRmax
Zone 4: 80–90% HRmax
Zone 5: 90–100% HRmax

The %HRmax system is simpler and still useful — but it's less individualised. For runners with average resting heart rates, the difference from Karvonen is moderate. For fit runners with resting HRs in the low-to-mid 40s, the difference can be substantial.

The calculator

Interactive calculator

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Max heart rate (bpm)

Estimate: 220 − age

Resting heart rate (bpm)

Measure first thing in the morning

Calculation method

Zone	Name	BPM range	%
Z1	Recovery Easy recovery, active rest	125–138	50–60%
Z2	Aerobic Base endurance building	138–151	60–70%
Z3	Tempo Moderate aerobic effort	151–164	70–80%
Z4	Threshold Lactate threshold training	164–177	80–90%
Z5	VO2 Max Maximum effort intervals	177–190	90–100%

All calculations are performed locally in your browser. No data is sent to any server.

The calculator computes both Karvonen and %HRmax zones side by side for easy comparison. Enter your age (to estimate max HR via 220-age), your resting heart rate, and optionally a measured max HR to override the estimate.

The maximum heart rate problem

Both zone systems depend critically on accurate maximum heart rate. This is where most runners go wrong.

The most-used formula — 220 − age — was not derived from a rigorous study. It was an approximation fit to existing data by Fox, Naughton, and Haskell in 1971, and they explicitly noted large individual variation. The standard deviation is approximately ±10–12 bpm, meaning roughly two-thirds of runners will have a true max HR within ±12 bpm of the formula, but one-third will fall outside that range.

For a 35-year-old runner, 220 − 35 = 185 bpm. But their actual max could reasonably be anywhere from 163 to 207 bpm. Training zones set from 185 bpm are off by up to 20 bpm for a runner whose actual max is 165.

The Tanaka formula, from a 2001 meta-analysis by Tanaka, Monahan, and Seals, is marginally better for older adults:

HRmax = 208 − 0.7 × age

For a 50-year-old: 208 − 35 = 173 bpm vs. 220 − 50 = 170 bpm. The difference is small for this age but the Tanaka formula outperforms 220-age systematically in populations over 40.

How to measure your actual max HR

The only reliable approach is a maximal effort test. Options:

Race effort: A hard 5K or 10K race usually pushes close to max. Note the peak HR on your watch during the final sprint.
Structured test: Warm up 15 minutes. Run 3 × 1 minute at progressively harder effort with 1-minute jogs between. On the third interval, go all out. Note peak HR.
Hill repeats: Run up a steep hill at full sprint 3–4 times. Heart rate typically peaks 15–20 seconds after reaching the top.

The caveat: most recreational runners don't reach true physiological maximum in training runs. A race context (competitive, well-fuelled, motivated) tends to produce higher peaks than solo tests.

Method 3: Lactate threshold HR (LTHR)

A third approach, popularised by coach Joe Friel, ties zones to a physiological event rather than a percentage. The lactate threshold heart rate (LTHR) is the HR at the transition from sustainable to unsustainable aerobic effort.

Friel's test: run a hard 30-minute time trial solo. Note your average HR for the final 20 minutes. This is your LTHR estimate.

Zones in the Friel system are calculated as percentages of LTHR rather than HRmax. The advantage: it directly anchors zones to the physiological boundary that matters most for distance running. The disadvantage: it requires a proper test to establish, and LTHR drifts with fitness.

The RunPaceLab calculator doesn't implement LTHR zones (since LTHR requires a test result, not just age and resting HR), but understanding the concept helps explain why three different HR zone systems give three different numbers.

Worked example: same runner, three systems

Runner: 32 years old. Resting HR: 48 bpm. Measured max HR: 187 bpm.

220-age max HR estimate: 188 bpm (close to measured in this case)

Zone 2 by %HRmax (60–70%):

187 × 0.60 = 112 bpm
187 × 0.70 = 131 bpm
Zone 2: 112–131 bpm

Zone 2 by Karvonen (60–70% of HRR):

HRR = 187 − 48 = 139
(139 × 0.60) + 48 = 131 bpm
(139 × 0.70) + 48 = 145 bpm
Zone 2: 131–145 bpm

A 14 bpm difference at the top of Zone 2 — not trivial. The Karvonen Zone 2 is harder work. For this runner with a low resting HR, the %HRmax system would be setting Zone 2 training too easy.

Limitations

HR zones are training guidance, not physiology. The zone boundaries — 60%, 70%, 80% — are conventional, not derived from individual physiology. Some runners' lactate thresholds fall at 72% of HRmax; others at 85%. Zone-based training works as a practical approximation, not a precise prescription.

HR can be affected by factors other than intensity: heat, hydration, caffeine, stress, sleep quality, and illness all affect resting and training HR. A 5-bpm jump in a run that "should be" Zone 2 might be dehydration, not excessive effort.

GPS and optical HR watches are noisy: Optical HR sensors on the wrist are accurate enough for zone training in steady efforts but struggle with rapid HR changes during intervals. For interval training, a chest strap gives significantly more accurate readings.

Frequently asked questions

Which method should I use — Karvonen or %HRmax?▾

Karvonen is more individualised and recommended for serious training. It accounts for cardiovascular fitness level via resting heart rate. %HRmax is simpler and works fine if you don't know your resting HR, or for very casual use. If you are a trained runner with a notably low resting HR (under 50 bpm), Karvonen will put your zones significantly higher in absolute terms than %HRmax — and those Karvonen zones are more accurate for your physiology.

How do I find my true maximum heart rate?▾

The most reliable method is to observe peak HR during a genuine race effort — the end of a hard 5K or 10K. For a structured approach: warm up 15 minutes, then run 3 × 1 minute at progressively harder effort (final rep all-out), resting 1 minute between reps. Record the peak HR on the third rep. Do this on a slight uphill if possible. A chest strap HR monitor gives better accuracy than a wrist-based optical sensor for this test.

My watch calculates zones automatically. Why are they different from this calculator?▾

Different devices use different formulas and different numbers of zones. Garmin and Polar use %HRmax by default but with slightly different zone boundaries. Apple Watch uses its own algorithm. Some devices let you switch between methods in settings. The numbers from this calculator and your watch will agree only if they're using the same formula with the same inputs.

What is Zone 2 training and why is everyone talking about it?▾

Zone 2 in the Karvonen or %HRmax systems (60–70% of HR reserve or HRmax) corresponds to comfortable aerobic effort — a pace where you can sustain conversation. Research by Stephen Seiler, Phil Maffetone, and others suggests elite endurance athletes spend roughly 80% of their training volume at or below this intensity. The physiological rationale: Zone 2 primarily uses fat as fuel, stimulates mitochondrial biogenesis, and builds aerobic capacity without the recovery cost of harder efforts. Whether to do the same as elites depends on your goals and total training volume.

My heart rate drifts upward during a long run even at constant pace. Is something wrong?▾

No — this is normal and expected. It's called cardiac drift (or cardiovascular drift). As you run, plasma volume decreases with sweat loss, blood temperature rises, and the cardiovascular system works harder to maintain output at the same mechanical pace. Heart rate can drift 5–15 bpm over a 2-hour run at constant pace. This is why it's impractical to maintain a strict HR target on long easy runs — your pace will need to slow as the run progresses to keep HR constant.

Should I use HR zones or pace zones for training?▾

Both have roles. HR zones are better on variable terrain (hills, heat) because they reflect physiological stress regardless of pace. Pace zones are better for quality workouts (tempo, intervals) where you need to hold a specific intensity and HR lag is an issue. Most structured plans use pace for intervals and HR (or effort) for easy and long runs.

References

[1]
Karvonen, M., Kentala, K., and Mustala, O. (1957). The effects of training on heart rate: a longitudinal study. Annales Medicinae Experimentalis et Biologiae Fenniae. 35(3). pp. 307–315.
[2]
Tanaka, H., Monahan, K.D., and Seals, D.R. (2001). Age-predicted maximal heart rate revisited. Journal of the American College of Cardiology. 37(1). pp. 153–156.
[3]
Fox, S.M., Naughton, J.P., and Haskell, W.L. (1971). Relationship of exercise heart rate to oxygen consumption. Annals of Clinical Research. 3(5). pp. 404–412.
[4]
Friel, J. (2016). Triathlete's Training Bible (4th Edition). VeloPress.
[5]
American College of Sports Medicine (2021). ACSM's Guidelines for Exercise Testing and Prescription (11th Edition). Lippincott Williams and Wilkins.