Biological age: the real formula, not the questionnaire

Calculate your biological age

The calculator has its own page: calculate your biological age. You need a recent blood result. All nine appear on a standard panel, so if you have results from your doctor or a workplace check, they are probably already there. Free, no account, no email address.

Below is where that number comes from and what it is worth: what biological age means, why almost every other free test measures nothing, and what actually lowers it.

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What is biological age? The meaning

Your chronological age counts how many times the earth has gone round the sun since you were born. There is nothing to be done about it and it says little.

Your biological age tries to capture something else: how old your body really is, compared with what your years would suggest. Two fifty-year-olds can sit years apart in the state of their vessels, kidneys, liver and immune system ^[3].

That is more sober than the advertising suggests. It is not a scan of your cells. It is a risk estimate, converted into a unit people understand.

Why do free biological age tests measure nothing?

Search for a biological age test and you will find questionnaires. How often do you exercise, do you smoke, how much do you sleep, how would you rate your stress.

Out comes a number. That number has measured nothing about you.

You told it yourself that you train four times a week and do not smoke. The output is those answers, added up and converted into years. It is arithmetic performed on your self-image. Someone who honestly reports sleeping badly gets a worse number, and they already knew that.

A real measure measures something you cannot type in.

How do you measure biological age?

PhenoAge, developed by the group of dr. Morgan Levine ^[1].

Nine values from an ordinary blood panel plus your chronological age: albumin, creatinine, glucose, CRP, lymphocyte percentage, mean cell volume, red cell distribution width, alkaline phosphatase and white blood cell count. Between them they cover liver, kidneys, metabolism, inflammation and immune function.

They were not chosen because they sound sensible. They fell out of a statistical model searching a much larger set of variables for whichever combination best predicts death.

The measure was then tested independently in a separate dataset: 11,432 adults, 1,012 deaths over 12.6 years of follow-up ^[2]. PhenoAge predicted all-cause and most cause-specific mortality, and held up when split by age, ethnicity, education, disease count and health behaviour.

Can your smartwatch or scale measure this?

It is the most asked question around this subject, so the short answer goes first: no, not this number.

Your Apple Watch, your Garmin, your smart scale and the machines at the gym all hand back something that looks like an age. They are measuring something else.

• A watch estimates your fitness, usually from something derived from VO2max, and converts that into years
• A scale measures body composition by passing a current through your feet and turns that into a number
• Gym equipment measures how strong you are compared with your age group

That does not make them worthless. A fitness number that climbs genuinely means your fitness is climbing, and fitness is among the strongest predictors there is. It is simply not a biological age in the sense the research uses, which is the mortality risk that corresponds to a given age.

If your watch says something different from the calculator above, nothing is broken. They are measuring different things.

What does biological age mean in youth sport?

Search this and you will also find a great deal about football and about children. That is another subject entirely.

In youth sport, biological age means how far along a child is in growing, independent of their birth year. Two twelve-year-olds can sit years apart in skeletal maturity, and clubs use that to group players more fairly so the early developers are not simply selected by default.

Same words, different field. If that is what you came for, this article is not it.

How do you lower your biological age?

If you come out above your chronological age, the question is which of the nine is pulling. That is more useful than the total, because the nine do not move equally.

What often moves:

• Glucose. Responds to weight, to what you eat, and above all to walking after meals.
• CRP. This is inflammation, and it almost always has a source: smoking, alcohol, too little sleep, excess body fat, and surprisingly often your teeth.
• RDW. Rises with a shortage of iron, B12 or folate. Findable with blood work and correctable.

What is harder to shift:

• Albumin and alkaline phosphatase report on your liver. If they already sit inside the normal range there is little to gain.
• Creatinine tracks your muscle mass as well as your kidneys. More muscle pushes it up, which counts against you in this calculation while being good for you in every other way.

That last point is exactly why this number should not be the target.

If you want to know what is actually established about staying well for longer, that is in what longevity actually is. The short answer is dull: move, sleep, do not smoke, eat enough protein, keep your muscle.

Taking creatine? Read this first

This affects a fair share of our readers, and it can shift your result by a couple of years.

Creatine is converted in your muscles into creatinine, which is exactly the substance your blood result reports. So if you supplement creatine, your creatinine rises without your kidneys doing any less work.

A 2025 review of twenty-one studies found precisely that: creatine produces a modest rise in blood creatinine while kidney function itself, measured as filtration rate, does not change ^[4]. The authors attribute it to ordinary metabolic turnover rather than damage.

For this calculator that matters. A rise of ten to twenty micromoles per litre, which is in that range, adds roughly one to two years of biological age here.

Enter your value as your result reports it. But if you come out a few years above your chronological age and you take creatine, you now know where at least part of that is coming from.

Which choices did we make in the formula?

This belongs in the article, because otherwise you would have to take our word for it.

The two papers print different coefficients. For glucose one gives 0.1953 ^[1] and the other 0.0195 ^[2]. A factor of ten. Unit conversion does not explain it: going from mmol/L to mg/dL would give 0.0108, not 0.0195.

We use 0.1953, and that is no longer a judgement call but a check: the full text of Levine carries Table 1, "Phenotypic aging measures and Gompertz coefficients", and glucose sits there at 0.1953. All ten weights in our calculator match that table exactly, and so do the units.

The reasoning that led us to that choice before we had the table:

PhenoAge is built so that someone with population-average blood values lands roughly on their own chronological age. That calibration is what makes it a measure at all. With 0.1953 it holds: average values at 40, 50, 60 and 70 return results close to chronological age. With 0.0195 everyone comes out about ten years young, at every age. That is not a measure, it is a systematic bias.

On top of that, the low coefficient would effectively remove glucose from the calculation, and glucose is among the strongest mortality predictors there is.

We read it as a typo in the later paper. If you compute this elsewhere and get a different number, that is probably why.

What does your result mean, and what does it not?

It is: an estimate of your current mortality risk, expressed in years, based on nine things actually measured in your blood.

It is not: how long you have left. Not a measurement of your cells or your DNA. Not a diagnosis. And not fixed, because most of those nine values move.

If you come out above your chronological age, the useful question is which of the nine is pulling. A raised CRP points at something. A fasting glucose sitting against the upper limit points at something else. Those are conversations for your doctor, not for a calculator on the internet.

Why is our biological age test free?

Clinics charge hundreds for a biological age, often from the same blood values already sitting in your file.

The formula is public. Both papers are above and free to read. There is nothing about this number that belongs behind a paywall.

If you want to read what sits underneath the number, start with what longevity actually is.