Your birthday says one age. Your body may be keeping a different one.

Two people turn 45 in the same week. One carries the engine of a 38-year-old, the other of a 53-year-old. Nothing mystical separates them: a waist measurement, a resting heart rate, a training pattern, and how they do on two small floor tests.

Those are exactly the inputs this calculator uses. The core of it is a fitness model built on 4,637 adults who did real treadmill tests, so your estimate is anchored to measured physiology, not to a quiz.

What you get

• The live calculator Your metabolic age from the HUNT fitness model, with a breakdown showing exactly which of the four inputs is aging you and which is buying years back.
• The home test sheet Printable Letter handout with the exact protocols for all four measurements, the reference tables, and boxes for today's numbers and the eight-week retest.
• The reference tables Average fitness by age and sex from 3,816 directly tested adults, the push-up bands, and the balance cut-offs, so you can see where you sit without guessing.

The four measurements, and why these four

Waist, resting heart rate, and your training pattern feed a published model that estimates VO2max, your body's engine size, without a treadmill in sight. In meta-analyses, every extra MET of that fitness, about 3.5 points of VO2max, was associated with an 11 to 17 percent lower risk of dying from any cause. A resting heart rate 10 beats higher was associated with about 9 percent higher long-term risk.

Push-ups stand in for strength. In a 10-year study of 1,104 working men, those who could do more than 40 had a 96 percent lower rate of heart attacks and other cardiovascular events than those under 10. Women were not in that study, so the calculator uses scaled bands and tells you so.

The one-leg stand is the quiet one. In adults 51 to 75, failing to hold 10 seconds went with markedly worse survival over the following years, and in older adults shorter times are associated with losing independence. It costs nothing to test and it trains fast.

Where the numbers come from

All of this is association evidence from cohort studies and meta-analyses. It tells you where your numbers sit against large populations. It does not diagnose anything, and it is not the "metabolic age" a bathroom scale prints.

Sources

1. Nes BM, et al. Estimating VO2peak from a nonexercise prediction model: the HUNT Study, Norway. Med Sci Sports Exerc. 2011. PMID 21502897.
2. Loe H, et al. Aerobic capacity reference data in 3816 healthy men and women 20-90 years. PLoS One. 2013. PMID 23691196.
3. Han M, et al. Cardiorespiratory fitness and mortality from all causes, cardiovascular disease and cancer: dose-response meta-analysis of cohort studies. Br J Sports Med. 2022. PMID 35022163.
4. Aune D, et al. Resting heart rate and the risk of cardiovascular disease, total cancer, and all-cause mortality. Nutr Metab Cardiovasc Dis. 2017. PMID 28552551.
5. Yang J, et al. Association Between Push-up Exercise Capacity and Future Cardiovascular Events Among Active Adult Men. JAMA Netw Open. 2019. PMID 30768197.
6. Hirayama Y, et al. Association of Shorter Secondary One-Leg Standing Time with Loss of Independence or Death in Community-Dwelling Older Adults in the Aizu Cohort Study (LOHAS). J Am Med Dir Assoc. 2025. PMID 40409342.
7. Rikkonen T, et al. Long-term effects of functional impairment on fracture risk and mortality in postmenopausal women. Osteoporos Int. 2018. PMID 29860666.
8. Araujo CG, et al. Successful 10-second one-legged stance performance predicts survival in middle-aged and older individuals. Br J Sports Med. 2022. PMID 35728834.
9. Ashwell M, Gunn P, Gibson S. Waist-to-height ratio is a better screening tool than waist circumference and BMI for adult cardiometabolic risk factors: systematic review and meta-analysis. Obes Rev. 2012. PMID 22106927.