Most fitness apps calculate your heart rate zones using a simple formula: take a percentage of your estimated maximum heart rate. It’s easy, but it can be dangerously inaccurate. The Karvonen method, developed by Finnish physiologist Martti Karvonen in 1957, uses your heart rate reserve instead — producing training zones that reflect your actual cardiovascular fitness, not just your age.
What Is Wrong With Percentage-of-Max Zones?
The standard approach uses the formula “220 minus your age” to estimate maximum heart rate, then carves that number into five zones. For a 40-year-old, the estimated max is 180 bpm, making Zone 2 roughly 108–126 bpm.
The problem is twofold. First, the 220-minus-age formula has a standard deviation of approximately 10–12 bpm, meaning your true maximum could be 15 beats higher or lower than the estimate. Second, this method ignores resting heart rate entirely. A highly trained runner with a resting heart rate of 48 bpm and a sedentary office worker at 78 bpm get identical zones despite vastly different cardiovascular capacities.
That mismatch means the runner’s Zone 2 may actually be far too easy, while the office worker’s Zone 2 might be too hard — undermining the purpose of zone-based training altogether.
How Does the Karvonen Method Fix This?
The Karvonen formula calculates your heart rate reserve (HRR) — the difference between your maximum and resting heart rate — then applies intensity percentages to that reserve. The formula is:
Target HR = Resting HR + (intensity % x Heart Rate Reserve)
For that same 40-year-old with a resting HR of 48 bpm, Zone 2 (60–70% intensity) would be 127–140 bpm. For the person at 78 bpm resting, Zone 2 becomes 139–149 bpm. The zones shift to match each individual’s actual fitness level.
This matters because heart rate reserve correlates more closely with VO2 reserve — the gold standard for measuring exercise intensity — than simple percentage-of-max calculations. The American College of Sports Medicine recommends the Karvonen method for this reason.
How Does Zone Accuracy Affect Your Training Score?
When your zones are wrong, every workout metric built on top of them is wrong too. Training load calculations like TRIMP rely on the relationship between your exercise heart rate and your heart rate reserve. If Zone 3 is miscalibrated by 10 beats, your session score misrepresents how hard you actually worked.
ScoreVitals uses the Karvonen method for all heart rate zone calculations. It pulls your resting heart rate directly from Apple Health or Google Health Connect, updates it automatically as your fitness changes, and applies it to every session score and daily readiness calculation. No manual zone configuration required.
The result is a training score that reflects your physiology, not a population average. Your zones evolve as you get fitter, ensuring your scores stay accurate over months and years of training.
Accurate zones mean accurate scores — and accurate scores mean better training decisions every single day. Download ScoreVitals and train with zones calibrated to your body.