The Science of Heart Rate Training Zones

“Keep your heart rate at 70% of max” is common advice, but it’s built on a simplification that quietly ignores one of the most important individual differences between two people of the same age: how fit they already are. The more precise version — the Karvonen formula — fixes exactly that gap, and it’s worth understanding both methods to see why.

The simple method, and its blind spot

The simplest way to estimate a target heart rate starts with an age-based estimate of maximum heart rate:

Max HR ≈ 220 − age

A target zone is then just a percentage of that number. A 30-year-old’s estimated max HR is 190, so “70% intensity” on this method is 0.70 × 190 = 133 bpm.

The blind spot: this treats a sedentary 30-year-old and an elite-athlete 30-year-old identically, even though their resting heart rates might differ by 30+ beats per minute — a highly-trained heart pumps more blood per beat, so it doesn’t need to beat as often at rest. Ignoring resting heart rate means ignoring a real, measurable signal of current fitness level.

The Karvonen formula: intensity relative to your own range

Developed by Finnish physiologist Martti Karvonen in 1957, the Karvonen formula computes target heart rate as a percentage not of your maximum, but of your heart rate reserve — the gap between your maximum and your resting heart rate:

Target HR = ((Max HR − Resting HR) × Intensity%) + Resting HR

For that same 30-year-old with max HR 190, at a resting heart rate of 60 bpm: heart rate reserve is 190 − 60 = 130 bpm. At 70% intensity: (130 × 0.70) + 60 = 151 bpm — noticeably higher than the simple method’s 133 bpm for the same “70%” label, because Karvonen is measuring 70% of your range, not 70% of a generic maximum.

This matters most for people whose resting heart rate is well below or above average — the fitter someone is, the bigger the gap between what the two formulas predict for the same intensity label.

The five zones, and what each one trains

Sports science typically divides heart rate reserve into five zones, each targeting a different training adaptation:

• Zone 1 (50–60%) — very light, active recovery; barely elevates breathing.
• Zone 2 (60–70%) — light aerobic base-building; sustainable for long durations, the zone most endurance-training volume happens in.
• Zone 3 (70–80%) — moderate, “tempo” effort; harder to sustain a full conversation.
• Zone 4 (80–90%) — hard, lactate-threshold work; short, structured intervals.
• Zone 5 (90–100%) — maximum effort; sustainable only briefly.

For our example (max 190, resting 60), Zone 3’s upper bound (80% intensity) works out to (130 × 0.80) + 60 = 164 bpm, and Zone 4’s upper bound (90%) is (130 × 0.90) + 60 = 177 bpm — each zone boundary is just the same Karvonen formula run at a different intensity percentage.

A practical limitation worth knowing

The 220-minus-age estimate for maximum heart rate is itself a population average with a real spread — the actual standard deviation across individuals is roughly ±10–12 bpm, meaning a formula-estimated max HR can meaningfully overstate or understate any one person’s true maximum. For casual training, this level of precision is fine. For serious athletic programming, a lab-measured or field-tested max heart rate (from a supervised maximal effort test) is more reliable than any age-based formula, this one included.

Using this without overthinking it

You don’t need a heart rate monitor and a spreadsheet to benefit from this. Two practical takeaways: first, if you’ve been training consistently and your resting heart rate has dropped, your target heart rates for the same subjective effort should also shift — recalculate periodically rather than using a number from a year ago. Second, when comparing your effort to someone else’s on a workout, “same zone” is a much fairer comparison than “same absolute heart rate,” since it accounts for the fitness differences between you. The Target Heart Rate Calculator and Heart Rate Zone Calculator on this site both use the Karvonen method directly, so your resting heart rate is part of the calculation, not ignored by it.