Biological age

What it is and what it is not#

Your calendar age is the number of years since your birth date. Your biological age, as we use the term, is the age whose readings yours resemble. If your resting heart rate, overnight HRV, sleep, activity and fitness all look like the typical person of 36 while you are 40, the card says "36.0 yrs" and "4.0 yrs younger".

Three things to keep in mind:

• It is a distance from your calendar age. Every domain starts from your age and moves up or down. Without a birth date there is nothing to measure from, so there is no number (see What you need to see a number).
• It is built on published studies where they exist. Cardio, Nervous system, Activity, VO₂max and most of Sleep read their input through studies of how it relates to all-cause mortality, against a published reference for your age and sex. Body composition and the stage share of sleep use reference tables. None of it is a blood test, a DNA clock or a diagnosis.
• It is for direction. The card says so itself: "Follow the trend, not one tenth." One decimal is shown so that a slow change is visible, not because a tenth of a year means anything on its own.

MyBodyAI is a wellness app, not a medical device. Biological age compares your device readings with reference data. It does not diagnose anything and it does not say how long you will live. If you feel unwell, see a doctor.

Where you see it#

• Health tab: the card "Biological age" ("Biologický věk") in the right-hand column of the body figure, and below it on a narrower screen. It holds the headline age, the distance from your calendar age, the radar of the six domains, the "Youngest area" and "Room to improve" boxes, the button "What if" and the button "Detail". Further down the tab the card "Biological age trend" draws the line over the period you picked.
• Overview: "Biological age trend" is one of the cards you can pick for the paired card row, over the last 30 days. It is offered once at least two days of the last 30 have a value.
• All graphs (the button "All graphs" on the Trends tab): a summary with the age and its distance from your calendar age.
• Share image: a pill "Biological age" with the same number.
• Weekly report e-mail: a box "Biological age" with the age and the distance, when there is one.
• Watch app: the age and its distance from your calendar age are sent to the watch.

"Detail" opens a short description with the line "Not a medical diagnosis, just a guideline from wearable data."

How the number is built#

Each domain gives an age: your calendar age plus an offset in years. The offset is negative when that area of your body reads younger than typical for your age and sex, positive when it reads older. The six domain ages are then combined with fixed weights.

The weights do not have to add up to one. We divide by the weights of the domains you actually have, so the shares above are for an account with all six. A domain your devices do not deliver is left out and the others take its share. It is never filled with your calendar age, because that would pull the result toward "average" without any evidence.

Years from a hazard ratio#

Cardio (from the resting rate), Nervous system, Sleep (length and regularity), Activity and VO₂max turn a reading into years through a published all-cause mortality study. The idea is the one used for the "risk advancement period" (Brenner, Gefeller and Greenland, 1993): adult all-cause mortality doubles about every 8 years of age. A reading with a hazard ratio of 1.5 against the reference carries the risk of a person who is log2(1.5) × 8 = 4.7 years older.

So the reference of each study is the point that reads as your calendar age, and the study's hazard ratio sets how many years the distance from that point is worth. Studies give small effects for most single readings. That is why the domains seldom stray far from your age unless the reading is far from the typical one.

Hazard ratios come from large groups of people, and a hazard ratio is not a personal forecast. We use them only as a scale that converts a distance into years.

Putting the domains together#

1. Each domain age is held between 18 and 85 years.
2. Each domain's distance from your calendar age is cut to at most 12 years either way, so that one odd reading cannot carry the result.
3. The distances are averaged with the weights of the domains present.
4. We keep 75 % of that weighted distance and pull 25 % back toward your calendar age. The largest distance the model can ever show is therefore 12 × 0.75 = 9 years.
5. The result is rounded to one decimal, half up. The distance on the card is taken from that rounded age, so "36.4 yrs" and "3.6 yrs younger" always sit beside a calendar age of 40.

A domain 8 years older than the rest moves the headline less than you might expect: with five domains at 40.0 and one with weight 0.10 at 48.0 the headline is 40.7.

The six domains#

Domain ages in this section are in years. "Reads younger" always means below your calendar age for your sex. Where the norm depends on sex, we use the sex in your profile ("Sex" in Settings, section "Profile"); with no sex set we use the norms for men. Age is your calendar age in whole years on the day.

Cardio#

The label on the radar is "Cardiovascular system".

With a measured vascular age. A Withings scale measures the speed of the pulse wave through your arteries, and the vascular age is derived from that velocity. The latest such value in the last 30 days (age between 18 and 120, velocity on the same row, written by the same device) is the Cardio domain. It wins over the resting-rate estimate for everyone, including people who also wear a watch. Aortic pulse-wave velocity relates to cardiovascular events and mortality (Vlachopoulos and colleagues, 2010). An Oura ring estimates a vascular age without a velocity, so the ring's number is not used here (vascular age).

With a resting heart rate. When there is no measured vascular age, Cardio reads your resting heart rate averaged over the last 30 days with a value. Mortality rises by 9 % per 10 beats per minute (Zhang, Shen and Qi, 2016, 46 cohorts). The point that reads as your calendar age is the middle of the range that wrist-worn trackers record in adults: 65 bpm for men and 67.5 bpm for women (Quer and colleagues, 2020). The reference does not move with age, because the data give no figures by age. The rate is read between 45 and 100 bpm. One beat is about 0.1 year, so 10 bpm is 8 × log2(1.09) = 1.0 year.

If you have no night on a wearable at all in 30 days but a vascular age from a device, that vascular age stands in for the resting-rate estimate.

Devices define the resting rate differently (the lowest rate of the night, the rate on waking, the average at rest), and no study gives the exact step between them. We take your device's value as it is. A few beats of difference between devices is a few tenths of a year.

Nervous system#

The label on the radar is "Autonomic nervous system".

Cardio reads your heart at rest. Nervous system reads how variable your heartbeat is during the night (HRV, measured as RMSSD, see overnight HRV).

• We take the nights of the last 30 days with an RMSSD value. A night counts only when the device reports RMSSD itself. A night that carries only the other HRV measure (SDNN) is skipped, because the norm below is for RMSSD.
• It needs at least 5 nights. The median of those nights is used, so one extreme night does not decide.
• The median is compared with the median for your age and sex from a study of overnight RMSSD in millions of people wearing a wrist sensor (Natarajan and colleagues, 2019). Between the study's ages the norm is interpolated, and past 60 it continues on the study's power law.
• The distance is taken on the log scale and divided by the spread of that group (from its 25th to 75th percentile). It is held between the 10th and the 90th percentile of the group.
• It is turned into years through the mortality of the lowest quarter of HRV against the other three (hazard ratio 1.56, Jarczok and colleagues, 2022). At the 90th percentile it is 3.9 years younger, at the 10th 3.9 years older, at the median 0.

We do not use the 5-minute peak of the night, the last week against the month, the spread from night to night or a long trend in this domain. No study prices them as years of age, and a rising week is recovery, which Recovery reads. HRV is not the same as your Nervous System index (HRV and the nervous system): that is a daily score against your own normal, this is a distance from a population norm.

Sleep#

The label on the radar is "Sleep". The domain reads the last 14 days and adds up to four parts. It exists when at least one of the stage, duration or regularity parts could be measured.

1. Deep and REM share (half weight). For each night with at least one hour of staged time we compute the deep and the REM share of sleep. Nights with an impossible architecture are skipped (deep over 35 % of staged sleep, deep with no light sleep at all, or stages longer than the night). A stage your device never names is not counted as zero. The average deep share is read against bands for your age and sex, and the result is multiplied by 0.5, because wearables stage sleep much less well than they time it (Chinoy and colleagues, 2021).

A deep share at "superior" or above is -6 years before the half weight; "excellent" -3; "good" -1; "fair" 0; and at the "low" edge +4, rising to +8 below it. In between the offset moves in a straight line. REM at 22 % of sleep or more is a further -2 (before the half weight), under 12 % is +2, between them 0.

2. Length (needs at least 5 nights in the 14 days). The median night, capped at 12 hours. The cohorts asked people how long they slept, a wearable measures it, and people over-report. So we convert: reported hours = 0.6 × measured hours + 3.2 (Lauderdale and colleagues, 2008). Seven to nine reported hours, which is about 6.3 to 9.7 measured hours, read as no risk at all (the adult range of Hirshkowitz and colleagues, 2015). Below it and above it the curve of Shen and colleagues (2016) applies.

A short night costs little in this domain, because the studies find small risk differences. What a short night costs you today is read by Sleep Quality, Recovery and Sleep Debt.

3. Regularity (needs at least 5 pairs of consecutive nights). The Sleep Regularity Index is the chance of being in the same state, asleep or awake, at two moments 24 hours apart, from -100 to 100. We work it out from each night's start time, end time and minutes awake, for every pair of consecutive days in the 14. The reference is the median of 60,977 adults with a week of accelerometer data, 81, and the index is held between 67.3 and 91.1 (Windred and colleagues, 2024). That makes this part between -1.7 and +2.4 years. Naps are not in it, so it can read slightly more regular than a wrist accelerometer.

4. Efficiency (needs at least 7 nights with a measured efficiency). Efficiency is the share of time in bed spent asleep, on the app's one definition (sleep efficiency). An average of 92 % or more is -2 years, under 80 % is +3, between them 0. It is applied only when at least one other part above exists.

The four parts are added to your calendar age and held between 18 and 85.

Body#

The label on the radar is "Body composition". It uses the app's own bands, built on reference tables for body fat (ACSM, NHANES) and on skeletal-muscle shares (Janssen, 2000). No mortality study stands behind these bands.

With a body-fat reading. The latest body-fat reading in the last 90 days is placed on bands for your age and sex. Lower fat is younger.

At or under "excellent" is -6 years, at "good" -3, at "fair" 0, at "poor" +4, and above "poor" +4 rising to +8. In between the offset moves in a straight line.

Then adjustments:

• Muscle. When the scale reports muscle mass, we take its share of your weight. Over 42 % (women 35 %) is -3 years, over 38 % (women 31 %) is -1. Under 35 % (women 28 %) is +1, under 32 % (women 25 %) is +3. The bonus does not apply once body fat is above "poor", and a share above 60 % is ignored, because that is lean mass, not muscle.
• BMI (when height and weight are known): under 20 is -1, from 27 +1, from 30 +2.
• Weight over the year (at least 14 weights): a fall of 0.5 to 3 % from the first to the last quarter of the span is -1, a rise over 3 % is +2. A quarter needs at least 3 readings. With at least 60 weights, a spread under 2 % of the average is -1 and over 5 % is +1.
• Body-fat trend (at least 30 readings): a fall of over 2 points is -2, over 1 point is -1, a rise over 1 point is +1, over 2 points is +2.

Without a body-fat reading but with height and weight, BMI sets the domain: under 18.5 is +3, under 22 is -3, under 25 is 0, under 27 is +2, under 30 is +4, 30 or over is +8. The weight and trend adjustments still apply.

Activity#

The label on the radar is "Physical activity". It averages two readings of the last 30 days. They describe the same habit, so they are averaged, not added.

Minutes. The mean of the day's active minutes over the days that have a value (a rest day with zero counts), times seven. Moderate minutes count once and vigorous minutes twice, as in the WHO guideline (see Active minutes and the WHO guideline). When the device gives only a total, it counts as moderate. The reference is exactly the WHO minimum of 150 minutes a week, which reads as your calendar age. The hazard ratios are from Arem and colleagues (2015, 661,137 adults).

Steps. The mean daily steps over the days with steps (Paluch and colleagues, 2022, 15 cohorts). The benefit stops growing at 9,000 steps for people under 60 and at 7,000 from 60. The reference is the median of the age group: 7,803 steps under 60, 5,649 from 60. For a person of 40: 3,553 steps or fewer is +6.9 years, 5,000 is +3.1, 6,000 is +0.9, 7,803 is 0.0, 9,000 or more is -0.7.

The day still running is left out. Its steps and minutes are only the sums so far, and in the morning they would read as a rest day.

When both readings are already at their ceiling, the radar's text for this domain says so: more movement will not move it. When one has more room, the text names it and how much you do now.

VO₂max#

The label on the radar is "Cardiorespiratory fitness". It needs a VO2max above 10 ml/kg/min. We take the latest value your device reported in the last 90 days, and without one the value in your profile.

The reference is the median VO2max of people of your age and sex who did a maximal treadmill test (the FRIEND registry, Kaminsky and colleagues, 2015, 7,783 tests). Between the decades the median is interpolated, and your value is read between the 5th and the 95th percentile of the registry. Each 3.5 ml/kg/min (one MET) higher fitness carries 13 % lower all-cause mortality (Kodama and colleagues, 2009), which is 1.6 years for one MET, or about 0.46 years for one ml/kg/min.

A watch's VO2max is the maker's estimate from your heart rate and pace, not a treadmill test. The registry is the test, which is what the estimate is built to predict.

This is the domain with the widest reach: for a man of 38, a value at the 95th percentile is about 8 years younger and one at the 5th percentile about 7 years older. Fitness is also the domain where a real change takes months.

What you need to see a number#

• A birth date. Without it the card says: "Biological age is measured as a distance from your real age, so without a birth date there is nothing to measure against. Add it in Settings and this fills in from the data we already have." A button "Add birth date" opens Settings, section "Profile". The age is your calendar age in whole years, from 10 to 120.
• At least 7 days with data in the last 30.
• At least three of the six domains with a value. Below three there is no age. The radar needs the same three, and below that the card says "Not enough data for the biological-age breakdown yet. We collect it from your connected devices."

What each domain needs:

A scale alone gives Body (and Cardio, when it is a Withings scale that measures the pulse wave). A watch alone gives Cardio, Nervous system, Sleep, Activity and, on devices that report it, VO₂max. To see how several devices fit together, see several devices and sources.

The radar, the trend and What if#

The radar#

The radar has one petal for each domain with a value. The dashed ring is your calendar age. A domain inside the ring reads older, outside the ring younger, and each year of difference moves the point by 4 % of the radius. The color follows the same ladder as the headline: "3+ yrs younger", "same or younger", "up to 3 yrs older" and "over 3 yrs older". Tap a petal and you see the domain's full name, its age, its distance from your age and a short text on what it reads. "Youngest area" and "Room to improve" name the lowest and the highest domain.

When your device measures a vascular age as well as Cardio, a small box "Vascular age" shows both: "measured (Withings PWV), the cardio domain is this measurement", or the estimate from resting heart rate next to the device's value.

The trend#

The card "Biological age trend" draws the biological age for every day of the period, each one computed with the data and the calendar age of that day. The line is colored by the same ladder, the dashed line is your calendar age. Three tiles say "Now", "Lowest" and "Highest".

The line is sampled: it has a point for each of the last 14 days, then one every third day up to 90 days, then one every week, for as far back as the period you picked reaches (a year at most). The newest point is always computed fresh. The card needs at least two points to draw a line.

The sentence above the chart says which way the period went, for example "Over the last 27 days your biological age fell by 0.6 yrs." We compare the mean of the last quarter of the period's time with the mean of the first quarter. The direction counts only when:

• the period spans at least 14 days and has at least 6 points, otherwise "... is too short to show a direction; give it a few weeks.";
• the change is at least 0.3 years and larger than what the line's own swing from point to point would produce by chance. Otherwise it says "... your biological age has held steady."

What if#

The button "What if" opens "What if, biological age" with one slider for each domain, from 18 to 85 years in steps of a tenth. When you move one, the age is worked out on our server with the same weights and the same rules as above, and the dialog says "what-if estimate". "Reset" returns to your current values. It is "A hypothesis; it changes no data or score." It is not a forecast of what will happen.

Why it moves#

Each domain reads a window, so it moves when the window moves:

• Sleep: the last 14 days, so a new routine shows within weeks.
• Cardio: the 30-day mean resting rate. A hard season of training or an infection that raises your resting rate for a few weeks moves it by a few tenths.
• Nervous system: the median of the nights of 30 days.
• Activity: the last 30 days, without today.
• VO₂max: the latest value of 90 days, so it moves when your watch re-estimates it.
• Body: the latest body-fat reading of 90 days, with the year's trends.

A single night, one rest day or one long run is one day of 14 or 30. That is why we ask you to follow weeks.

Three other things move it without a change in your body:

• A new device or a new domain. When a domain appears for the first time (a scale with body fat, a watch that reports VO2max) the headline changes, because it is now built from more than before. A thin picture often reads kinder than a full one.
• A birthday. Your calendar age rises by one, and every norm (resting rate does not move with age, but HRV, VO2max, deep sleep and body fat do) is read at the new age. The domains and the distance change only through the norm.
• A change to the model. When we change how a domain is calculated, the whole line is calculated again with the new version, so a trend is never a mix of two versions. The model is now in its eighth version. Since 2 November 2026 HRV and VO2max are read against the median for your age and sex.

When it updates#

• The age is worked out with your other scores from the windows above. The result is stored and reused for up to 6 hours.
• A change in your profile (birth date, sex, height) refreshes it at once, together with the trend.
• The trend's newest point is computed fresh on every load, so it follows the headline.
• Data that arrive late are included the next time the result is worked out, and the trend's past points follow, because they are computed from your stored days.

Younger than you came#

On the Progress tab, the badge family "Younger than you came" ("Mladší než při příchodu") has three tiers, at 1, 2 and 3 years: "Biological age lower than at joining". It counts how many years further below your calendar age you are now than at the start.

• The baseline is your biological age on the day you joined, from the history your device brought with it. If there is no reading for that day, the first reading the account has becomes the baseline.
• The first 30 days after the baseline do not count, because the model needs time to settle.
• Like for like. Only the domains present in both readings are compared, and at least three are needed. A domain that arrives later (a scale, a watch that reports VO2max) cannot make you "younger".
• It measures the distance from your calendar age, so a birthday moves both sides and changes nothing.
• The progress is your best so far, never below zero.
• When the model changes, the baseline day is read again under the new version and your best so far starts again from that reading. A tier is kept only if the new comparison reaches it.

The badge is not offered in the energy pacing mode, and no new tier is earned there. See Progress for classes, tiers and the other badges.

Free and Premium#

The biological age is Premium. Since 1 November 2026 the free plan shows today's Body Status and the safety alerts, and the biological age, its trend, the radar and the "What if" simulator are part of Premium. Premium includes the 7-day trial. Your data keep arriving and nothing is lost (plans).

Energy pacing mode#

In the energy pacing mode we do not show a biological age anywhere: not on the Health tab, the Overview, the Trends tab, the share image, the weekly e-mail or your watch, and the badge is not offered. Steps, active minutes and VO2max count in the model, and a person who has to stay under a heart-rate ceiling would be told they are ageing for keeping to it. The age is still calculated and stored. When you switch the mode off, the card returns with its history (energy pacing).

What you can do#

• Add your birth date and sex in Settings, "Profile". Everything is measured from them.
• Connect what you have, so more domains are filled. A scale adds Body and, for a Withings scale that measures the pulse wave, Cardio; a watch adds the rest. A thin picture reads less reliably, and the card does not hide that.
• Read the weeks, not the day. Look at "Biological age trend" over 30 and 90 days.
• Look at "Room to improve". It names the oldest domain. Then read its text on the radar.
• Sleep at regular times. The regularity part is one of the few that move within a fortnight.
• Move, and give VO2max months. Minutes and steps move Activity within a month. Fitness moves VO₂max over a season.
• Do not chase a decimal. The card is a wellness estimate for direction. Questions about your health belong with a doctor.

Frequently asked questions#

Is this the same as a clinical test of biological age?#

No. Clinical clocks read DNA marks or blood values from a sample. We read what your devices measure daily and compare it with published references and mortality studies. That makes it a fitness-style age that changes over weeks, not a measure of your cells. Two methods can differ by years for the same person, and neither is a diagnosis.

Why is my biological age younger than my real age?#

Either your readings are better than typical for your age and sex, which is common with good fitness and regular sleep, or the picture is thin. Open the radar: if body composition and VO₂max are missing, the age is built from fewer domains and reads kinder. Connecting a scale or a watch that reports VO2max fills it in. The model cannot show a distance beyond 9 years.

Why is there no biological age on my card?#

One of three things is missing: a birth date in Settings ("Profile"), at least 7 days with data in the last 30, or at least three of the six domains with a value. In the energy pacing mode it is hidden on purpose. The card tells you which one applies.

Why did my biological age jump when I connected a scale or a new watch?#

Because a domain appeared. The headline is the weighted mean of the domains present, so adding one with a different distance moves it, though your body did not change. In the example above, the same person reads 39.3 without Body and 39.0 with it. Look at the domain ages on the radar to see which one moved.

Why is my Sleep domain older when I sleep eight hours?#

Length is only one of four parts, and a long night costs nothing in it. The others are the regularity of your bed and wake times over 14 days, the deep and REM share against your age, and efficiency. Irregular bed times alone can add up to 2.4 years, and a low deep share adds up to 4. Open the Sleep tab to see your stages and your regularity (Sleep Quality).

Does my birthday change it?#

Your calendar age rises by one year, and the norms for age are read at the new age. The distance from your age changes only by what those norms change. The trend line is not broken: each point is computed with the age you had on that day.

Is "What if" a forecast?#

No. It is arithmetic: it takes your domain ages, replaces the ones you move and combines them with the same weights. It tells you what the headline would be if a domain read that way. It does not say that it will, or how long it would take.

Why is my cardio age different from my vascular age?#

When a Withings scale measures the pulse wave, the Cardio domain is that measured vascular age. Otherwise Cardio is an estimate from your 30-day resting heart rate, and your device's vascular age is shown beside it as "measured" or "estimate". Both come about differently, and small differences are normal. An Oura ring estimates the vascular age from the pulse wave without a velocity, so it does not replace the resting-rate estimate (vascular age).