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Metabolic Age Calculator

By Rick Campbell · Updated · Sourced to primary literature · Not medical advice

Metabolic age on a gym scale is a marketing label. This page shows the method rather than pretending it is a biological age. We find the age at which a person of your sex and height, at BMI 22, has a Mifflin-St Jeor BMR closest to yours.

Mifflin and colleagues published that resting-energy equation in 1990. It uses weight, height, age and sex. It does not measure mitochondria, thyroid hormones or 'ageing'. A higher BMR reads as a younger metabolic age on this method, which is why extra weight can look younger. Lean mass is what actually moves BMR.

Enter sex, age, height and weight. You get a metabolic age between 18 and 80, the difference from your chronological age, your BMR, and a table of BMI-22 reference BMRs by decade. Then go to the BMR and TDEE pages for numbers this site will plan from.

In brief

  • Metabolic age here is the age whose BMI-22 Mifflin BMR best matches yours. It is not a biological age.
  • Extra weight raises Mifflin BMR, so a heavier body can print a younger metabolic age. That is the method, not a compliment.
  • Lean mass is the tissue that actually changes resting burn. FFMI and a proper BMR page are the useful next steps.
  • The search exists, so the method is shown in public rather than hidden inside a scale.

Calculator

Fields marked * are required. Results update as you type.

Values you have typed are converted when you switch.

The published formulas were derived separately for male and female physiology. If neither fits you well, run both and read the range.

Enter your age in years.

Your inputs stay in this page's address so you can bookmark or share them; nothing is stored on our servers.

What you'll see here

The age at which a BMI-22 person of your height matches your BMR, a table of reference BMRs by decade, and a plain statement that this is a marketing label.

What gym scales are actually doing

Consumer scales estimate BMR from weight, height, age, sex and sometimes a proprietary impedance lean-mass guess. They then look up the age at which a 'reference' person would have that BMR. The reference is rarely published. This page publishes ours: BMI 22, Mifflin-St Jeor, ages 18 to 80.

BMI 22 sits inside the WHO healthy range of 18.5 to 24.9 for adults. It is a conventional mid-range figure, not a personal ideal and not a medical target. Changing the reference BMI would change the printed age. That alone should tell you this is a label.

If a scale gives a different metabolic age, it used a different equation, a different reference, or an impedance lean-mass estimate you cannot audit. Believe none of them as tissue age.

How Mifflin BMR is computed

Men: 10 × kg + 6.25 × cm − 5 × age + 5. Women: the same, minus 161 instead of plus 5. That is the 1990 equation this site uses everywhere. Your BMR on this page is that number at your current weight and chronological age.

The reference BMR at each candidate age uses the same equation at a weight of 22 × (height in metres) squared. We walk ages from 18 to 80 and pick the closest match. The difference (metabolic age minus chronological age) is printed with a sign.

Mifflin is a population fit with a typical error around 10%. Two people with the same printed BMR can differ by 150 to 200 kcal. Stacking an age label on top of that error does not make it more precise.

Why extra weight can look 'younger'

The equation rises with weight. A heavier body at the same height has a higher estimated BMR. Younger reference ages also have higher BMRs because of the −5 × age term. The closest match for a heavy adult is often a younger reference age, sometimes the 18-year floor.

That is the opposite of the story the word 'metabolic age' sells. People hear 'older metabolism' when they are light and older, and 'younger metabolism' when they are simply larger. The form's warning exists because of that inversion.

Lean mass is the honest driver. A muscular person and a high-fat person at the same weight get the same Mifflin BMR. FFMI and a tape or scan split are how you tell them apart. This label cannot.

What this number is not

It is not epigenetic age, not VO2 max, not thyroid status, not 'adrenal age', and not a reason to buy a supplement. It is not in WHO, CDC or NICE as a clinical vital sign.

It is not your TDEE. TDEE is BMR times an activity factor. A 'young' metabolic age with a sedentary day is still a sedentary burn. Open the TDEE calculator for the daily number.

It is not a children's tool. Adult Mifflin maths does not run under 18 on this site.

How to use the decade table

The table shows reference BMR at BMI 22 for ages 20, 30, 40, 50, 60 and 70 at your height and sex. You can see how many kcal the age term moves, about 50 kcal per decade (5 kcal per year times 10).

If your BMR sits near the 40-year reference and you are 40, the label will land near your real age. That is a tautology about weight versus BMI 22, not a health certificate.

If you want the useful number, read your BMR in kcal and ignore the age. The BMR calculator shows Mifflin next to Harris-Benedict and Owen so you can see the equation spread.

What to do instead of chasing the label

Lift, eat enough protein, and walk. Those change lean mass and daily expenditure, which are real. Re-run BMR and TDEE when weight changes by about 3%. Use the deficit or surplus tools if you have a goal.

If a trainer is managing you by metabolic age, ask for the equation and the reference. If they cannot name them, they are reading a scale marketing line.

The TDEE explained guide is the longer reading on what actually sets daily burn.

How it's calculated

Mifflin-St Jeor BMR (1990)

Men: 10×kg + 6.25×cm − 5×age + 5; women: 10×kg + 6.25×cm − 5×age − 161

Your BMR uses your weight and chronological age. Population error is about plus or minus 10%.

Reference weight

Reference kg = 22 × (height m)²

BMI 22 is a mid healthy-range convention, not a personal ideal.

Metabolic age

Age in 18 to 80 whose BMI-22 Mifflin BMR is closest to yours

A label. Higher BMR matches a younger reference age.

Worked example: 40-year-old man, 180 cm, 70 kg

  1. Mifflin BMR = 10×70 + 6.25×180 − 5×40 + 5 = 700 + 1,125 − 200 + 5 = 1,630 kcal.
  2. BMI-22 reference weight = 22 × 1.80 × 1.80 = 71.28 kg.
  3. Reference BMR at age A = 10×71.28 + 6.25×180 − 5×A + 5 = 1,842.8 − 5A.
  4. Set 1,842.8 − 5A = 1,630, so 5A = 212.8 and A ≈ 43. Closest whole year in the 18 to 80 walk is the printed metabolic age.
  5. Chronological age is 40, so the difference is a few years either way, a tautology about sitting near BMI 22, not a diagnosis.

Where this number is used in the real world

  • Seeing the method behind a gym-scale metabolic age.
  • Checking that extra weight can print a younger label.
  • Reading BMR in kcal and ignoring the age headline.
  • Moving from the label to BMR, TDEE and FFMI.
  • Explaining to a client why the number is not tissue age.

Frequently asked questions

Is metabolic age a real medical measurement?

Not in the sense gym advertising means. Resting energy expenditure is real and can be measured in a metabolic cart. Ageing clocks from DNA methylation are real research tools. This label is neither. It is a lookup: which reference age has a similar estimated BMR. WHO and CDC publish BMI and waist screens, not metabolic age. Treat a scale that prints 28 when you are 41 as entertainment, then open the BMR page.

Why did my metabolic age come out younger after I gained weight?

Because Mifflin BMR rises with kilograms. The reference person is fixed at BMI 22, so a higher BMR matches a younger age on the −5 kcal per year slope. The method rewards mass, not health. That is the inversion this page exists to show. If you gained lean mass, BMR really did rise, and that is useful. The age stamp is still a sticker.

Why BMI 22 and not BMI 21 or 23?

We had to pick one published, typical mid-range adult BMI inside the WHO 18.5 to 24.9 healthy band. 22 is conventional in reference tables. A scale that uses 21 or a proprietary 'ideal weight' will print a different age for the same BMR. The fact that the answer moves when the reference moves is the proof that this is a label, not a biomarker.

Should I try to lower my metabolic age?

You should try to raise or protect lean mass and daily movement if those are low, and to lower central fat if the tape says so. Those goals have pages on this site. Chasing a younger metabolic-age print can mean gaining weight (which the method likes) or losing weight (which the method may call older). That contradiction is why we will not give you a metabolic-age goal.

Does a low metabolic age mean I can eat more?

No. Daily allowance is TDEE, which is BMR times activity. Two people with the same BMR and different jobs do not get the same calories. Open the TDEE calculator, look at all five activity factors, and steer by what the scale does over four weeks. A 'young' label does not add 300 kcal to the budget.

Why is the result capped at 18 and 80?

Adult Mifflin maths on this site does not run under 18, and an 80-year reference is already past the age band the original equation was built on. If your BMR is higher than the 18-year BMI-22 reference, the match pins at 18. If it is lower than the 80-year reference, it pins at 80. Both pins mean 'outside the table', not a paediatric or geriatric diagnosis.

Keep going

A single number rarely tells the whole story. Alongside the metabolic age result, the TDEE calculator, the FFMI calculator, the BMI calculator and the recomp calculator each add a different angle on the same measurements. For the reasoning behind the numbers, read Metabolic age and TDEE explained.

Sources

  1. Mifflin MD, St Jeor ST, Hill LA, Scott BJ, Daugherty SA, Koh YO. A new predictive equation for resting energy expenditure in healthy individuals. Am J Clin Nutr 1990;51:241-7. doi.org/10.1093/ajcn/51.2.241
  2. World Health Organization. Obesity and overweight fact sheet. Healthy adult BMI range 18.5 to 24.9. www.who.int/news-room/fact-sheets/detail/obesity-and-overweight

Cite this page

Quoting a figure from here in an article, a report or a piece of coursework? Use whichever of these your style guide asks for.

APA
Campbell, R. (2026). Metabolic Age Calculator. Body Stats. https://bodystats.co/app/metabolic-age-calculator
Plain text
“Metabolic Age Calculator”, Body Stats, last updated 15 September 2026, https://bodystats.co/app/metabolic-age-calculator

Every formula and threshold on this page is written out with its primary source on our methodology page. These results are informational and educational, not a diagnosis or a substitute for professional advice. See the medical disclaimer.

Last updated . Written by Rick Campbell; not medically reviewed. See review status.