Biological age, heart age, fitness age: how apps calculate it

Ecleptic is our app; this comparison sticks to each brand's public information, linked in the sources below, checked on September 30, 2026.

Three families of "ages"

Fitness age turns your physical fitness into years: that is the idea behind Garmin's Fitness Age, and Ecleptic's starting point with VO₂ max. Cardiovascular age estimates the state of your cardiovascular system from your pulse: that is Oura's approach. Overall physiological age combines several sleep, activity and fitness metrics, like WHOOP Age.

The comparison table

Garmin: Fitness Age

Garmin presents Fitness Age as a way to compare your fitness with that of a person of the same sex. The watch uses your age, body mass index, resting heart rate and vigorous activity history; with an Index scale, body fat percentage replaces BMI. A complete user profile makes it more accurate.

Whoop: WHOOP Age and Pace of Aging

Within its Healthspan feature, Whoop calculates WHOOP Age, a physiological age averaged over six months from nine metrics: hours of sleep and sleep consistency; weekly time in heart rate zones 1-3 and 4-5, strength training time, daily steps; resting heart rate, VO₂ max and, when available, lean body mass. Pace of Aging reads your last 30 days to show whether that age is likely to rise or fall. HRV is left out, considered too individual for age-based comparison. The feature unlocks after 21 recoveries in your first 31 days, calibrates over 90 days and requires being 18 or older and a Peak or Life plan.

Oura: Cardiovascular Age

Oura's Cardiovascular Age estimates the health of your cardiovascular system relative to your actual age. The ring does not measure pulse wave velocity directly: it estimates it, along with age-related changes in the shape of your pulse, from its optical sensor. After 14 nights within 30 days, the result is rated below, aligned with (within 5 years) or above your age. Separately, Oura estimates Cardio Capacity, its age-adjusted VO₂ max. Both require an active membership.

Apple: VO₂ max by age, and an announced Health Age

Apple Watch estimates your VO₂ max, called cardio fitness, during an outdoor walk, run or hike, taking into account your age, sex, weight, height and certain medications. The Health app shows it as a VO₂ max value and places it against the levels for your age and sex. On September 9, 2026, Apple announced Health Age, which will place your VO₂ max, resting heart rate, sleep and HRV against your chronological age, in a redesigned Health app due later this year, starting in U.S. English.

Ecleptic: an age anchored on your VO₂ max

Our biological age is calculated in two steps. First, your VO₂ max comes from the best available source: your watch via Apple Health, your runs (the VDOT formula, which can only raise the estimate), or a no-exercise estimate using the Nes et al. equation from Norway's HUNT cohort. Compared with the reference for your age and sex, it becomes a fitness age. Then seven bounded, signed factors adjust it: heart rate variability, resting heart rate, waist size, activity, sleep, diet and lifestyle, the last of which can never make you younger. No data point is counted twice; the result is smoothed and comes with an uncertainty margin. The method: biological age.

What these numbers are not

None of them is a diagnosis. Whoop states that there is no clinical benchmark for validating WHOOP Age; Oura, that its ring is not a medical device; Ecleptic, that its biological age is an estimate, not a prediction of your lifespan. All three describe these as slow-moving metrics: the trend over weeks or months is what counts.

What moves these ages

The levers overlap: VO₂ max feeds WHOOP Age, Athlytic Age, Apple's Health Age and Ecleptic's biological age; resting heart rate feeds Garmin's, Whoop's, Athlytic's and Ecleptic's. To act on them: zone 2 endurance and regular bedtimes.