Fitness age is an assessment of the fitness of the cardiovascular and respiratory systems, expressed in years. It is based on VO2max, maximal oxygen uptake: the main indicator of aerobic endurance. Your VO2max is compared with the average for different age groups.
If your VO2max matches that of younger people, your fitness age is lower than your calendar age — your body is "fitter" than your peers.
What fitness age shows
VO2max reflects how much oxygen the body is able to take up at maximal exertion. It naturally declines with age: in men, median values fall from about 46 mL/kg/min at 20–29 years to 21 mL/kg/min by 70–79 years. The American Heart Association suggests treating cardiorespiratory fitness as a "clinical vital sign" — a low VO2max is associated with higher mortality.
How it is calculated
An accurate VO2max measurement is done in a lab (gas analysis during exercise). But there are also "non-exercise" models (for example, the World Fitness Level calculator from Norway's NTNU) that estimate VO2max from age, sex, waist circumference, resting heart rate and level of physical activity. The resulting VO2max is converted into a fitness age by comparing it with reference norms.
How to lower your fitness age
VO2max is one of the few markers of aging that responds well to training at any age. Regular aerobic and interval exercise raises VO2max, whereas inactivity and muscle loss (sarcopenia) lower it. Fitness age is convenient to track over time alongside your biological age.
Calculate using this marker
Calculators where Fitness age is used directly:
Frequently asked questions
Do you have to run to exhaustion to find out your fitness age?
No. There are non-exercise models that estimate VO2max from a questionnaire (age, sex, waist circumference, resting heart rate, usual activity). A direct lab test is more accurate but not required for a rough estimate.
Can you really “rejuvenate” your fitness age?
Yes. VO2max rises with regular aerobic and interval training even in older adults. Over a few months of training, fitness age can drop by years — it is one of the most controllable markers.