Why the Karvonen formula accounts for more than just age
Most simple heart rate zone calculators use a flat percentage of maximum heart rate, which ignores an important individual factor: resting heart rate. Two people of the same age can have very different resting heart rates — a well-trained endurance athlete might rest at 50bpm, while a less active person of the same age might rest at 75bpm — and a flat percentage of max heart rate doesn't account for that difference.
The Karvonen formula, developed by Finnish physiologist Martti Karvonen in the 1950s, solves this by working from heart rate reserve — the gap between resting and maximum heart rate — rather than maximum heart rate alone:
Target HR = ((max HR − resting HR) × intensity%) + resting HR
Because it incorporates resting heart rate, the Karvonen method tends to produce a slightly more personalized — and for most people, slightly higher — target range than a flat percentage of max heart rate, particularly for people with a lower resting heart rate from regular cardiovascular training.
Getting a reliable resting heart rate
Measure it in the morning before getting out of bed, when your body is most settled and hasn't yet been affected by caffeine, movement, or stress. A single reading can vary by a few beats due to normal day-to-day fluctuation, so averaging several mornings gives a more stable, reliable number to base your training zones on.