How the estimate works
Each factor starts from a published hazard ratio: how much higher or lower long-term mortality risk is compared with a typical person. Because that risk roughly doubles every 8 years of age (the Gompertz law), a hazard ratio can be turned into years: years = ln(hazard ratio) × 8 / ln(2).
A typical person for your age and sex scores zero on every factor. Each factor is capped at ±4 years, and the total is reduced by 25% because the factors overlap: fit people also tend to walk more and have lower resting heart rates.
| Factor | Reference point | Effect used |
|---|---|---|
| VO2 max | Median for your age and sex (FRIEND registry) | 13% lower risk per 3.5 ml/kg/min (Kodama 2009) |
| Resting heart rate | 70 bpm | 9% higher risk per 10 bpm (Zhang 2016) |
| Daily steps | About 5,800 steps | Step quartiles from Paluch 2022 |
| Sleep | 6-9 hours | Short sleep 12%, long sleep 30% higher risk (Cappuccio 2010) |
| Strength training | None | 21% lower risk with any (Saeidifard 2019) |
Where to find your numbers
On an Apple Watch, VO2 max appears as Cardio Fitness in the Health app, and resting heart rate, steps and sleep are recorded automatically. Use averages over the last few weeks, not a single day.
If you do not know your VO2 max, leave it empty. The estimate then uses the other four factors.
What lowers your biological age most
Cardio fitness is the strongest lever. Raising VO2 max by 3.5 ml/kg/min, which is achievable in a few months of regular cardio, is worth roughly 1.6 years in this model before the overlap discount.
Adding strength training once or twice a week, walking toward 8,000-10,000 steps and keeping sleep between 7 and 9 hours are the next biggest changes.
Limitations
The studies behind each factor are observational. They show associations across large populations, not guaranteed effects for one person. Smoking, blood pressure, blood sugar, cholesterol and family history matter a lot and are not included here.
Treat the result as a direction to move in, and use a clinician for anything medical.
Worked examples
These profiles run through the calculator exactly as it works on this page. The two 45-year-old men in the table are almost nine years apart: cardio fitness accounts for about 2.5 years of that gap, strength training 2 years and steps about 3 years together, after the overlap discount.
In the first profile, a low step count alone adds 1.6 years. It is also the easiest factor to change.
| Profile | VO2 max | Resting HR | Steps | Sleep | Strength | Biological age |
|---|---|---|---|---|---|---|
| Man, 45 | 38 | 72 bpm | 5,000 | 6.5 h | No | 46.7 (+1.7) |
| Man, 45 | 45 | 58 bpm | 9,500 | 7.5 h | Yes | 38.1 (-6.9) |
| Woman, 35 | 30 | 64 bpm | 7,000 | 7 h | Yes | 32.1 (-2.9) |
How to read your result
- Within about 2 years of your real age: typical for your age. Differences this small are within the estimate's margin.
- More than 2 years younger: your fitness and habits look better than those of a typical person your age.
- More than 2 years older: look at the factor adding the most years. That is where a change pays off most.
- Recalculate every month or two with fresh averages. Fitness changes take weeks to show up in VO2 max and resting heart rate.
This calculator vs biological age in the Bodly app
This page uses five inputs you type in once. The Bodly app builds the same kind of estimate automatically from your Apple Watch, with nine factors: time asleep, sleep consistency, steps, easy and hard workout minutes, strength training, VO2 max, resting heart rate and lean mass.
It uses 60-day averages, so one bad week does not swing it, updates every day, and adds a pace of aging that compares your last two weeks with the last two months to show whether you are currently getting younger or older.