The Karvonen method sets a target heart rate as resting heart rate plus a percentage of heart rate reserve (HRR), where HRR is max heart rate minus resting heart rate: target = RHR + (percent x HRR). Because it starts from your own resting heart rate rather than max heart rate alone, it produces a more individualized zone than a flat percent-of-max method.
This calculator offers the Tanaka formula as an estimate: max HR = 208 minus (0.7 x age), from Tanaka, Monahan and Seals (2001). It is a population average, not a measurement, so if you have a genuine max HR from a supervised max-effort test or a race finish, enter that directly instead for a more accurate result.
Heart rate reserve is the gap between your max heart rate and your resting heart rate (HRR = max HR minus resting HR). It represents your full working range, and the Karvonen formula calculates every training zone as a percentage of this range added back onto your resting heart rate.
Most easy and long marathon training runs sit in Zone 2 (60 to 70 percent of HRR), the aerobic base zone. Zone 3 to Zone 4 (70 to 90 percent of HRR) covers tempo and threshold work, and Zone 5 (90 to 100 percent of HRR) is reserved for short, hard efforts. A training plan should spend most weekly time in Zone 2, not the higher zones.
The older 220 minus age rule of thumb was never based on a formal regression study. The Tanaka formula (208 minus 0.7 x age) came from a 2001 meta-analysis of over 18,000 subjects and is the estimate this calculator uses, but any age-based formula is a population average: an actual max-effort result for your own body will always be more accurate than either estimate.
Zone 2 is the Karvonen band from 60 to 70 percent of heart rate reserve, a conversational effort that builds aerobic base and fat-burning efficiency without heavy fatigue. It is popular in endurance training because most of a runner's weekly mileage, easy runs and long runs included, is meant to happen here.