Metabolic Age Calculator

Body-fat route. Fat-free mass includes more than muscle. We calculate it as weight in kg × (1 − body fat % ÷ 100), then use the equation labeled Katch–McArdle in this tool:

Resting kcal/day = 370 + 21.6 × fat-free mass (kg)

This exact equation also appears in Cunningham's 1991 review of body composition and energy expenditure. It predicts resting energy, not age. [1]

Age comparison. The simplified Mifflin–St Jeor equation predicts resting kcal/day as 10 × weight (kg) + 6.25 × height (cm) − 5 × age (years) + S, where S is +5 for males and −161 for females. [2] Our rearrangement is:

Formula-based age = (10 × weight + 6.25 × height + S − resting kcal/day) ÷ 5

Imperial inputs are converted to kg and cm before calculation. Intermediate values are not rounded. Your chronological age supplies the displayed comparison; it is not used to estimate resting energy in the body-fat route. Predicting resting energy with Mifflin and reversing the same equation would simply return the age entered.

Mifflin et al. developed their equation using 498 healthy adults aged 19–78, including normal-weight and obese participants. Resting energy was measured by indirect calorimetry. The study evaluated energy prediction, not whether an energy value can reveal someone's age. [2]

A 2005 systematic review found Mifflin–St Jeor more often predicted RMR within 10% of measured values than the other commonly used equations reviewed. That does not promise ±10% accuracy for each person, for our body-fat estimate, or for an age conversion. [3]

Research also cautions against treating metabolism as a simple aging clock. Pontzer et al. found fat-free-mass-adjusted total daily energy expenditure was broadly stable from about age 20 to 60 in their population data. This concerns total energy expenditure and does not validate our resting-energy age comparison. [4]

Illustrative inputs: female coefficient, age 35, weight 65 kg, height 165 cm, body fat 30%, activity factor 1.2.

1. Fat-free mass: 65 × 0.70 = 45.5 kg.
2. Resting energy: 370 + 21.6 × 45.5 = 1,352.8 kcal/day.
3. Age comparison: (650 + 1,031.25 − 161 − 1,352.8) ÷ 5 = 33.49, displayed as 33 years.
4. Maintenance estimate: 1,352.8 × 1.2 = 1,623.36, displayed as 1,623 kcal/day.

These are arithmetic examples, not observed health outcomes. With other inputs fixed, 100 kcal/day more resting energy reduces the unrounded age by 20 years. At 65 kg, changing body fat from 30% to 31% raises it by about 2.8 years in this model. Neither difference proves a change in biological aging.

We do not report an age confidence interval: this combined method has not been validated well enough to justify one.

Use a recent body-fat estimate rather than guessing. If you have a lab result, enter resting energy—not active calories, total daily expenditure, or food intake. A smart scale's BMR is a prediction, not a lab measurement, and may already use chronological age. A measured RMR still does not turn this conversion into a validated age test.

The tool accepts adults aged 19–78 and displays an exact rounded age only when the unrounded result falls within that window. Otherwise it reports Below 19 or Above 78. The window matches the original Mifflin study's age span, but is our display convention—not a validated range for estimating age.

This is an educational tool, not intended for pregnancy, breastfeeding, children, or medical decisions. Do not use the result to diagnose a condition or to set a restrictive diet. For individual energy needs or concerns about metabolism, speak with a qualified healthcare professional.