Aging rate is an ambiguous term: in longitudinal research it can mean estimated biological change per calendar year, while some apps use it for biological age divided by chronological age. A value near 1.00× therefore has no reliable meaning until you know the formula. Under Metric’s current calculation, 0.90× means the rolling biological-age estimate is 90% of chronological age; it is an age ratio, not a direct measurement of how many biological years elapsed during the last year.
Editorial note
Published 2026-07-16. Last reviewed 2026-07-16. Evidence, interpretation, and Metric’s implementation are identified separately below.
Three meanings that are often mixed together
- Longitudinal pace: estimated biological change per year of chronological time.
- Age ratio: current biological-age estimate divided by current chronological age.
- Trend slope: how quickly a model’s output changed over a selected recent period.
A person can have a biological-age ratio below 1.00 while the estimate has recently moved upward. Conversely, a person can have a ratio above 1.00 while improving. The current level and recent direction answer different questions.
The research pace example
DunedinPACE was developed from longitudinal change across 19 organ-system indicators in the Dunedin cohort and then distilled into a DNA-methylation measure. It was scaled so 1.00 corresponds to one biological year per chronological year in the reference framework. That interpretation comes from the model’s longitudinal design; it cannot be transferred automatically to an app that divides two ages.
Metric’s current formula
- Metric calculates daily biological-age estimates from eligible Apple Health contributors.
- It takes the mean of generated daily series entries in the most recent 90-day window once at least seven entries are available; missing contributors default to chronological age and the separate coverage indicator reflects sparse data.
- It divides that rolling biological age by the user’s chronological age.
- It constrains the displayed ratio to 0.10×–3.00×.
- A result below 1.00 means the rolling estimate is lower than calendar age; above 1.00 means it is higher.
Example: if chronological age is 50 and rolling biological age is 45, Metric’s ratio is 0.90×. That is equivalent to an age gap of −5 years under Metric’s model. It does not establish that only 0.90 years of biological change occurred over the previous calendar year.
What can move the Metric ratio?
- New or expiring data for VO2 max, resting heart rate, HRV, body composition, blood pressure, steps, active energy, or sleep.
- A sustained change in rolling values rather than one isolated sample.
- Improved or reduced data coverage across weighted contributors.
- A device, source, unit, or permission change that alters the underlying HealthKit record.
- A birthday or demographic correction that changes chronological-age context.
- A model update that changes mappings, weights, or quality rules.
How to interpret a trend
- Confirm that the value is an age ratio under the current Metric formula.
- Check data confidence and whether a major contributor appeared, disappeared, or became stale.
- Open the weighted contributors and identify which underlying trend moved.
- Use a multi-week or multi-month view; avoid narrating daily changes as literal aging speed.
- Choose actions based on the contributor itself—such as consistent sleep or an appropriate fitness plan—not on chasing the ratio.
What the evidence says
Longitudinal pace measures can be associated with morbidity, disability, and mortality in research cohorts, but different age and pace measures do not always respond the same way. In the CALERIE analysis, DunedinPACE changed while several methylation-age estimates did not. Reliability work also shows that small movements can be obscured by technical noise. Those findings support precise labels and conservative interpretation.
Limitations
- Metric’s ratio depends on the design and limits of its biological-age estimate.
- Chronological age expressed in whole years can make the ratio change around birthdays.
- Rolling averages reduce noise but delay and blend changes.
- A lower ratio does not quantify extra lifespan or guarantee lower disease risk.
- Comparing a Metric ratio with a DNA pace score is not a like-for-like comparison.
The safest translation
Read Metric’s aging rate as “rolling estimated age relative to calendar age,” then inspect the contributors. Do not translate it into literal biological years per year.
Sources and further reading
- eLife (2022): DunedinPACE, a DNA methylation biomarker of the pace of agingDefines a pace measure scaled around one biological year per chronological year and reports test-retest and outcome analyses.
- Mechanisms of Ageing and Development (2006): A new approach to the concept and computation of biological ageExplains why biological age lacks a single exact definition and proposes a multibiomarker estimation method.
- Nature Aging (2023): Effect of long-term caloric restriction on DNA methylation measures of biological aging in healthy adults from the CALERIE trialA randomized-trial analysis in which one pace measure changed while several biological-age estimates did not, demonstrating model-dependent results.
- Nature Aging (2022): A computational solution for bolstering reliability of epigenetic clocks: Implications for clinical trials and longitudinal trackingShows that technical noise can materially change individual epigenetic-clock estimates and tests a more reliable computational approach.
Continue exploring
- Biological age app
- Biological vs chronological age
- Accuracy of biological-age estimates
- Metric methodology
Health information disclaimer
Metric is a wellness product. This article is educational and does not provide medical advice, diagnosis, or treatment. Wearable measurements and app-generated scores are estimates; discuss symptoms, unusual readings, medication effects, and changes to a care or training plan with an appropriate healthcare professional.