What metabolic age actually is

The gym-scale definition

Consumer scales that offer metabolic age do not agree on a public formula. Each brand estimates resting metabolism from weight, height, age, sex and, if the scale has handles or footpads, a bioimpedance lean-mass guess. It then compares that estimate with a table of 'typical' rates by age and prints the age that matches. The table is rarely published. The error on the impedance step is large. The headline is precise to a year anyway.

What the headline is really saying is 'your estimated resting burn looks like the average for this age'. If the average table was built on smaller, leaner people than you, you will read young. If you are small, older, or have less lean mass than the table expects, you will read old. None of that required a new vital sign. It required a look-up.

This is why two scales can disagree by a decade on the same morning. They are using different BMR equations, different reference tables and different impedance guesses. Arguing about which year is 'right' is an argument about whose table you prefer.

How this site computes it

We use one equation and one reference, both named. Your BMR is Mifflin-St Jeor, published in 1990: 10 times weight in kilograms, plus 6.25 times height in centimetres, minus 5 times age, plus 5 for men or minus 161 for women. Frankenfield's systematic review found that equation the most reliable of the common resting-metabolism predictions in both non-obese and obese adults, meaning it landed within ten per cent of measured rate more often than the rivals.

The reference person has your sex, your height, and a weight at BMI 22, which sits in the middle of the WHO healthy range. We compute that reference BMR at every age from 18 to 80 and pick the age with the smallest absolute difference from your BMR. We also print a small table of reference BMRs at 20, 30, 40, 50, 60 and 70 so you can see the slope.

Worked through: a man of 178 centimetres has a BMI-22 reference weight of about 69.7 kilograms. At age 30 his reference BMR is roughly 1,690 kilocalories. If your BMR is 1,850, the matcher will walk toward younger ages, because a younger reference at the same weight burns a little more. If you are 90 kilograms, your BMR is higher still, and the matcher can hit the 18-year floor. That floor is the method admitting it has run out of table, not a claim that you have the metabolism of a teenager.

Why extra weight can look younger

Mifflin-St Jeor pays ten kilocalories a day per kilogram. Heavier people have higher predicted BMRs. The reference person is fixed at BMI 22, so the only way the matcher can raise the reference BMR is to lower the reference age. Weight therefore masquerades as youth. A 110 kilogram adult will often out-burn the BMI-22 table at every age, which is why the result pins at 18.

That reading is easy to sell and easy to misunderstand. A higher BMR from a larger body is not a metabolic advantage. It is the cost of carrying the extra mass. Lose 15 kilograms and the same method will age you, because the BMR fell. People then say the diet damaged their metabolism. The equation got smaller because they got smaller.

The honest comparison is BMR per kilogram of lean mass, or a measured resting rate against a prediction that includes body composition. Metabolic age as a single year cannot do that work. It has no lean-mass input on this site, and most gym scales that claim one are guessing it from a foot current.

What actually moves basal metabolic rate

Fat-free mass is the largest determinant of resting energy expenditure. Organs and muscle cost far more per kilogram than adipose tissue. Two adults of the same weight, height, age and sex can differ by a couple of hundred kilocalories a day if one carries more lean mass. Mifflin-St Jeor cannot see that difference. It will print the same BMR for both, and the same metabolic age.

Age enters the equation as minus five kilocalories per year. That is a population slope, not your personal decline. Training, illness, thyroid disease, some medicines and recent weight change move the measured rate off the line. A metabolic age that 'worsened' by four years after a month of bed rest is a BMR that fell. Name the BMR.

The TDEE guide on this site is the planning document. Metabolic age is a headline translation of one input to that document. If you came from a gym printout, use this page to see the trick, then go to the BMR and TDEE calculators for numbers this site will actually plan from.

Why it is not a medical number

No clinical guideline uses metabolic age to diagnose disease, dose a medicine, or decide who has obesity. NICE, CDC and WHO talk in BMI, waist, body-fat distribution, blood pressure, lipids and glucose. A year printed on a scale does not join that list. Treating it as a diagnosis is how people ignore a 0.6 waist-to-height ratio because the scale said they were 28.

If your resting rate is far from what Mifflin-St Jeor predicts, that can be worth a clinical conversation, especially with fatigue, cold intolerance, hair change, or a history of thyroid disease. The useful number in that conversation is the estimated or measured BMR and the symptoms, not a marketing age. Do not start or stop a medicine because a gym scale aged you.

The BMI-22 reference is also a choice, not a law. We picked the middle of the healthy range so the table has one clear person in it. A brand that uses BMI 21 or an unpublished 'athlete' table will print a different year. Disagreement between brands is expected. It is not a mystery about your mitochondria.

What to use instead

For energy planning: TDEE, then a percentage adjustment, then a fortnight of seven-day averages. For risk: waist-to-height ratio and the WHO waist action levels, plus whatever blood work a clinician orders. For composition: a consistent fat-percentage method and a tape, not a year.

If you like the metabolic-age headline as a motivational sticker, you can keep computing it. Just know what moved when it moves. Weight up, age down, on this method, is the coefficient of ten. Lean mass up at the same weight is the change worth wanting, and this label cannot see it.

The metabolic age calculator prints the delta in years, your BMR, the reference weight, and the decade table. Read the BMR column. That is the number with a citation. The year is a pointer at that column.

Frequently asked questions

Put it into practice

Run your own numbers through the metabolic age calculator, the TDEE calculator and the BMI calculator. Related reading: TDEE explained, BMI vs body fat vs waist and Adaptation and plateaus.

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. Frankenfield D, Roth-Yousey L, Compher C. Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults: a systematic review. J Am Diet Assoc 2005;105:775-89. doi.org/10.1016/j.jada.2005.02.005
3. WHO. Obesity and overweight. Fact sheet. www.who.int/news-room/fact-sheets/detail/obesity-and-overweight
4. NICE. Overweight and obesity management. NICE guideline NG246, published 14 January 2025. www.nice.org.uk/guidance/ng246