What it means
The formula: target heart rate equals (max heart rate minus resting heart rate) times the intensity percent, plus resting heart rate. The gap between max and resting is called heart rate reserve.
Max heart rate is often estimated as 220 minus age. That estimate can be off by 10 or more beats for any one person.
Using resting heart rate makes zones more personal than a simple percent of max. As fitness improves and resting heart rate drops, your zones shift. Some medicines, like beta blockers, change heart rate response, so zones may not fit if you take them.
Most guidelines describe moderate intensity as roughly 40 to 59% of heart rate reserve and vigorous as 60 to 89%. The calculator shows these bands in beats per minute.
Why it matters
The heart-rate zone calculator uses Karvonen so your cardio targets fit your current fitness.