How Body Age is calculated
Body Age starts from your actual age and adds or subtracts years according to how each of seven signals compares with an age-referenced value — every term is individually capped, and the result is their sum. It is a reference index, not a measurement of biological age.
Inputs and weights
- VO₂max — the largest term, capped at ±5 years
- Resting heart rate — ±3 years
- Heart rate variability — ±3 years
- Daily steps — ±2 years
- Sleep duration — ±2 years
- Sleep performance (WHOOP users only) — ±2 years
- Recovery score (WHOOP users only) — ±2 years
Why VO₂max weighs the most
In a cohort of 122,007 adults, cardiorespiratory fitness tracked all-cause mortality in a dose-response manner with no observed ceiling, and the risk associated with low fitness was comparable to traditional factors such as coronary disease, diabetes and smoking. Of everything a wearable can give us, its link to long-term outcomes has the strongest evidence.
It is also why an Apple Watch owner does not need a second band: VO₂max is precisely the metric Apple Health provides and WHOOP's public API does not.
Reference values are age- and sex-specific
The same VO₂max means very different things at 25 and at 55, so every term is compared against an age- and sex-specific range rather than one universal threshold. This is why onboarding asks for date of birth and sex — without them Body Age cannot be computed.
The steps term is likewise age-referenced, because the mortality benefit of steps plateaus at different levels by age: older adults reach the plateau at fewer steps.
Limitations
- 🚨 This is not a biological-age test. Real biological-age clocks rest on blood or epigenetic markers — for example a blood-panel deep-learning clock for older Chinese adults reports a mean absolute error of 5.68 years. Our Body Age simply translates wearable data onto a scale people can read.
- It cannot be computed without a date of birth, and confidence drops noticeably without VO₂max (an Apple Watch only estimates VO₂max from outdoor walks or runs).
- The fewer inputs, the coarser the result. We show the confidence level alongside it.
- It is not used to judge disease risk and does not constitute medical advice.
References
What it supports：In 122,007 adults, cardiorespiratory fitness tracked all-cause mortality in a dose-response manner with no observed upper limit — why VO₂max carries the largest weight in Body Age.
What it supports：Dose-response between daily steps and mortality, with the plateau varying by age — why the steps term is age-referenced.
Jensen MT, Suadicani P, Hein HO, Gyntelberg F. Elevated resting heart rate, physical fitness and all-cause mortality: a 16-year follow-up in the Copenhagen Male Study. Heart. 2013;99(12):882-7.
What it supports：Elevated resting heart rate independently tracks mortality after adjusting for fitness.
What it supports：Normative sleep duration and stage proportions by age — the reference for what stage shares are typical.
What it supports：Definitions, measurement windows and population norms for HRV metrics.
What it supports：What an actual biological-age clock looks like (blood panel + deep learning, mean absolute error 5.68 years) — cited to make clear our Body Age is a reference index of a different kind.
Related metrics
Every metric on this page is a reference index for observing your own trends. It is not medical advice and cannot replace a professional diagnosis. If you have health concerns, consult a qualified clinician.
Source module: lib/health/body-age.ts
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