Metabolic Age Calculator.

Metabolic age compares your basal metabolic rate — the calories your body burns at complete rest — against the average BMR for people of different ages. If your resting metabolism matches that of an average 35-year-old, your metabolic age is 35, regardless of your birthday. A metabolic age below your real age means your engine idles hotter than typical; above it means slower.

The term was popularized by body-composition scales (Tanita, Withings, Renpho) as a single, motivating number. It is not a medical measurement and appears in no clinical guideline. Treat it as a fitness-tracker metric — useful for watching a trend over time, not for diagnosing anything.

The single biggest driver of resting metabolism is lean body mass — muscle, organs, and bone burn energy at rest; stored fat burns very little. That is why the accurate way to compute metabolic age uses the Katch-McArdle equation, which is built on lean mass. Enter your body-fat percentage and this calculator switches to that method.

Without body fat, we fall back to the Mifflin-St Jeor equation, which uses only weight, height, age, and sex. It cannot tell muscle from fat — so a heavy, muscular person and a heavy, high-fat person get the same estimate, and it can even flag a lean lightweight person as metabolically "older." The number is still directionally interesting, but add body fat for a result that reflects your actual metabolism. If you do not know it, the US Navy body-fat method gets you a usable estimate from a tape measure.

There is exactly one durable lever on metabolic age: build and keep muscle. Resistance training two to four times a week, enough protein (roughly 1.6 g/kg), and not under-eating all preserve the lean tissue that drives resting metabolism. Crash diets do the opposite — they burn muscle along with fat and can push metabolic age up.

If your real goal is to know how well you are aging, metabolic age is the wrong instrument. Validated tools exist: PhenoAge reads nine standard blood markers through the peer-reviewed Levine clock, and our Biological Age calculator weighs the lifestyle factors with the strongest links to lifespan. Use metabolic age for motivation; use those for signal.