The flat-percentage method
The simpler, more common approach (used for the standard five-zone model) calculates a target heart rate as a straight percentage of estimated maximum heart rate: target = max HR × intensity%. It's simple to compute but has an odd implication at the low end — at 0% intensity, it implies a target of 0 bpm, which is obviously not what "resting" actually means for a real person.
The Karvonen formula
The Karvonen method fixes this by first computing "heart rate reserve" — the gap between max heart rate and resting heart rate — and applying the intensity percentage to that reserve instead, then adding resting heart rate back on top: target = (max HR − resting HR) × intensity% + resting HR. At 0% intensity, this correctly gives back exactly the resting heart rate, not zero — a physiologically sensible result the flat method doesn't provide.
A worked comparison
For a 30-year-old (max HR 190) with a resting heart rate of 60, targeting 50-85% intensity: the flat-percentage method gives roughly 95-162 bpm. The Karvonen method gives 125-171 bpm for the identical person and identical intensity range — a meaningfully higher low end (125 vs 95, a 30 bpm gap) and a somewhat higher top end too, because Karvonen's reserve-based calculation starts counting up from resting heart rate rather than from zero.
Why resting heart rate changes the picture
The same 30-year-old with a higher resting heart rate of 80 (perhaps reflecting lower cardiovascular fitness) gets a Karvonen range of 135-174 bpm instead of 125-171 — both bounds shift upward, but by different amounts, since a smaller heart rate reserve (110 instead of 130) changes how much each percentage point of intensity actually adds. The flat-percentage method would give the exact same 95-162 bpm range regardless of resting heart rate, since it never factors resting heart rate into the calculation at all.
Which one to use
Karvonen is generally considered more personalized and physiologically accurate, since it accounts for individual fitness level via resting heart rate — someone with a naturally low resting heart rate (often reflecting better cardiovascular fitness) gets a correspondingly adjusted target range, rather than the same generic percentage-of-max figure everyone else of the same age gets. The flat-percentage method remains widely used for its simplicity — it requires only age, with no need to first measure resting heart rate — and is a reasonable starting point when a quick, simple estimate is all that's needed.