How Apps Calculate Your Biological Age (WHOOP, Oura, Garmin & Dalia)

Open almost any health app today and it will happily tell you your "biological age" — often a flattering number a few years below your real one. It is a brilliant hook: a single figure that supposedly captures how well you are aging. But here is what most apps do not spell out: they are all calculating it differently, and most of them are not measuring biological age at all — they are estimating a proxy from whatever signals they can capture.

That does not make the number useless. It just means you should know what is actually under the hood. Below we break down how the major players — WHOOP, Oura, Garmin, and our own app, Dalia — each calculate it, where the real scientific gold standard sits, and how seriously to take the result.

Chronological vs. biological age

Chronological age is simple: the number of years since you were born. Biological age is the more interesting idea — an estimate of how old your body actually *seems* based on how well it is functioning. Two 45-year-olds can have very different biological ages depending on their fitness, cardiovascular health, sleep, and lifestyle.

The catch is that "biological age" has no single agreed-upon definition or measurement. Researchers have built dozens of ways to estimate it — and the method an app picks determines what your number really reflects.

The gold standard: epigenetic clocks

In research, the most respected way to measure biological age is an epigenetic clock. As you age, chemical tags called methylation groups attach to your DNA in predictable patterns. By reading those patterns from a blood sample, scientists can estimate biological age — and even the *pace* at which you are aging.

You will see names like Horvath's clock, GrimAge, PhenoAge, and DunedinPACE (which measures how fast you are aging rather than a fixed age). These are the tools longevity researchers actually trust — and the key point for this article: they require a lab and a blood sample. No wearable or phone can measure DNA methylation.

So when an app shows you a biological age, understand the trade-off it is making: it cannot run an epigenetic clock, so it estimates an age from the physiological signals it *can* measure. Here is how each one does that.

How each app calculates it

Dalia

Dalia Age — a Klemera–Doubal biological-age estimate

Full disclosure, Dalia is our app. Its Dalia Age uses the Klemera–Doubal Method (KDM) — the same validated statistical framework behind clinical biological-age clocks — to combine up to six biomarkers into one estimate anchored to your real age: VO₂max, systolic blood pressure, HRV, fasting glucose, BMI, and resting heart rate. The markers that track aging most reliably (VO₂max and blood pressure) automatically carry the most weight — the math decides, not a hand-set rule. Heart rate and HRV are read from your phone camera; VO₂max, blood pressure, and glucose come from your connected readings or Apple Health (and VO₂max can be estimated from your resting heart rate if it is not directly measured). It reports a confidence level based on how many signals it has, and is upfront that it is an estimate, not a lab test or medical diagnosis.

Pros

• Uses a published biological-age method (Klemera–Doubal), not a black box
• Multi-biomarker: fitness, blood pressure, HRV, metabolic & body composition
• Heart signals come from your phone camera — no wearable required
• Shows each factor's real value and its effect, with a confidence level

Cons

• Still an estimate, not a lab epigenetic clock
• Sharpens once you add blood pressure, glucose & VO₂max data
• Population-level coefficients — read it as a trend, not a precise verdict

WHOOP

WHOOP Age & Pace of Aging — a 9-metric fitness model

WHOOP estimates a "WHOOP Age" from roughly six months of your data across nine longevity-linked metrics: VO₂max, resting heart rate, lean body mass, sleep duration and consistency, daily steps, time in heart-rate zones, and strength activity. Its "Pace of Aging" score (−1x to 3x) then shows whether your recent habits are speeding up or slowing down that age. It is essentially a fitness-and-lifestyle model dressed as an age.

Pros

• Holistic — blends fitness, sleep, and activity
• Pace of Aging gives fast behavioral feedback
• Grounded in well-studied longevity predictors

Cons

• Requires a WHOOP band and subscription
• Reflects fitness habits, not biological aging directly
• Heavily influenced by recent training and sleep

Oura

Cardiovascular Age — vascular stiffness from PPG

Oura shares part of its DNA with how Dalia reads your heart: its ring uses photoplethysmography (PPG) — the same optical blood-flow signal Dalia reads from your phone camera — but Oura focuses it on estimating arterial stiffness, a proxy for pulse wave velocity (PWV), the medical gold standard for vascular aging. From that it estimates how old your cardiovascular system looks versus your real age, typically after about two weeks of wear.

Pros

• Targets vascular aging specifically (PWV proxy)
• Based on a recognized clinical concept
• Continuous overnight measurement

Cons

• Requires an Oura ring (plus membership)
• Estimates stiffness indirectly from PPG, not a direct PWV reading
• Narrowly cardiovascular — not whole-body aging

Garmin

Fitness Age — VO₂max reinterpreted as an age

Garmin's Fitness Age is the most straightforward of the group: it largely takes your estimated VO₂max (cardiorespiratory fitness) and expresses it as the age of a typical person with that fitness level. Newer devices also factor in resting heart rate, activity intensity, and BMI or body-fat percentage. If your VO₂max matches a typical 35-year-old, your Fitness Age is 35 — regardless of your real age.

Pros

• Simple and intuitive to understand
• Anchored to VO₂max, a strong longevity predictor
• Responds clearly to improved fitness

Cons

• Requires a Garmin device
• Really a fitness score, not biological age
• VO₂max estimates carry a few years of error

A side-by-side comparison

So how seriously should you take the number?

Here is the honest answer: no app's biological age is a substitute for a lab epigenetic test, and you should not treat the exact figure as precise. A wearable or camera estimate can easily be off by a few years, and the number will bounce around with your sleep, stress, training, and even how you measured.

But that does not mean these numbers are pointless. Two things make them genuinely useful:

• The trend beats the number. Whether your estimated age is drifting up or down over months tells you more than any single reading. That is the signal worth watching.
• The inputs are real. Resting heart rate, HRV, VO₂max, and vascular stiffness are all legitimately tied to long-term health. Improving them improves your actual cardiovascular fitness — whatever the headline age says.

The bottom line

Every app's "biological age" is really a proxy: WHOOP's is a fitness-and-lifestyle model, Oura's is a vascular estimate, Garmin's is VO₂max in disguise, and Dalia's applies the Klemera–Doubal method to blend VO₂max, blood pressure, HRV, and metabolic signals into one number. The true gold standard — the epigenetic clocks people like Bryan Johnson rely on — still lives in a lab. Pick whichever tool fits how you like to track, watch the trend rather than the digit, and remember that the habits that move the number are what actually matter.