Metabolic age calculator

What is metabolic age?

Short answer: metabolic age is a marketing term, not a medical one. Tanita, Omron and other smart-scale manufacturers popularised it as a friendlier way of communicating your resting metabolic rate (RMR) — the calories your body burns at rest. Instead of telling you “your RMR is 1,510 kcal/day,” the scale says “your metabolism is like a 29-year-old’s,” which lands very differently.

The number only carries useful information if the calculator knows something about your body composition. Most free metabolic age calculators don’t — they just rearrange BMR equations like Mifflin-St Jeor, which are algebraic in weight, height, sex and age. A 20-year-old and a 60-year-old at the same weight and height come out identical, which is obviously wrong. This is why smart scales are worth the money where free calculators aren’t: the scale measures body-fat percentage via bioelectrical impedance, and body-fat percentage is what actually shifts resting metabolism.

This calculator does the same thing, but without the scale. You either enter a body-fat percentage you already know, or we estimate it from a tape measure using the US Navy circumference method⁵. From there, your metabolic age becomes meaningful.

Why body-fat is the key input

Resting metabolic rate is almost entirely driven by lean body mass — the muscle, organs, bone and water that isn’t fat. Fat tissue burns roughly 4 kcal per kg per day at rest; lean tissue burns 13–15 kcal per kg per day. So two people at the same total weight can have RMRs 200–300 kcal apart, purely because of composition.

The Katch-McArdle equation is the standard formula used when body composition is known:

RMR (kcal/day) = 370 + 21.6 × lean body mass (kg)

Lean body mass comes from subtracting fat from total weight:

lean body mass = weight × (1 − body_fat_% / 100)

We further subtract an age- and sex-specific bone mass reference (roughly 3.5–4% of body weight, declining after about 50 in women and more slowly in men) to isolate the lean soft tissue that actually burns calories. These bone-mass constants come from the Wang 1992 five-level model of body composition³.

How the calculator works

Four steps, all transparent.

1. Work out your lean body mass

From your weight and body-fat percentage (direct entry or estimated via the US Navy formula⁵), we subtract the fat mass and a reference bone mass. What’s left is the lean soft tissue that does most of the resting energy expenditure.

2. Calculate your RMR with Katch-McArdle

370 + 21.6 × LBM. This is the gold-standard equation when composition is known — it outperforms Mifflin-St Jeor in populations outside the average because it uses the thing that actually matters⁴.

3. Build a reference RMR curve by age

For each age between 18 and 80, we apply the same formula using a reference body-fat percentage for someone your sex at that age. The reference curve uses the healthy-range midpoints from Gallagher 2000’s body-fat guidelines¹: roughly 22% for a 20-year-old woman rising to 31% at 80; 13% for a 20-year-old man rising to 24%.

4. Find the closest match

Your metabolic age is the age whose reference RMR most closely matches yours. Capped at 18 and 80.

The practical upshot: if your body composition is better than the population average for your age — lower body fat, more lean mass — your metabolic age comes out younger than your real age. The gap is real information about your body composition, not an arithmetic illusion.

What the number means (and what it doesn’t)

The most useful way to think about metabolic age vs real age: the two are reading different things. Your real age is a calendar fact. Metabolic age is a single-number summary of three things:

• How much of your weight is lean tissue vs. fat. This is the biggest lever by far.
• Your height. Taller frames support more lean mass for the same body-fat percentage.
• Your sex. Reference body-fat % is sex-dependent, so the age-match curve is sex-dependent.

What metabolic age is not: a measure of cellular ageing, telomere length, epigenetic age, disease risk or longevity. Real biological-age markers exist — Horvath’s epigenetic clock, for instance — but they require blood or tissue samples and cost significantly more than stepping on a scale or using a tape measure.

Treat your metabolic age as a readable dashboard for body composition. Not a verdict on anything else.

How to lower metabolic age

Since the calculator’s output is driven by lean-mass-to-fat-mass ratio, “how to lower metabolic age” collapses to two levers that work and a long list that doesn’t.

Lower body fat. A calorie deficit does this. Our calorie deficit calculator gives you the daily target; a moderate 500 kcal daily deficit drops roughly 0.5 kg of fat per week. For every 3% drop in body fat, metabolic age typically comes down 2–4 years — depending on starting point and how much of that weight loss is actual fat versus lean.

Build or preserve lean mass. Resistance training two or three times a week adds muscle, which increases lean body mass at the same or lower total weight. Adequate protein intake (1.2–1.6 g per kg of body weight) during a deficit is what stops the weight loss from coming disproportionately out of muscle. The protein calculator gives you the specific target; our high-protein breakfasts make it easy to hit without thinking about it.

The levers that don’t move the number: cold plunges, “metabolism booster” supplements, green tea extract at commercial doses, HIIT-only programmes. Their effects on resting metabolism are small enough to sit inside the calculator’s measurement error.

Metabolic age by decade — what’s typical

The folk wisdom says metabolism slows in your 30s and it’s downhill from there. Pontzer et al.’s 2021 paper in Science, which measured total daily energy expenditure across 6,400 people aged 8 days to 95 years, told a more interesting story:

• Ages 20–60: fat-free-mass-adjusted metabolism is remarkably stable. The apparent “slow-down in your 30s” is almost entirely about losing muscle and gaining fat — not about the metabolism itself².
• After 60: fat-free-mass-adjusted energy expenditure starts declining at roughly 0.7% per year².

Which is why a “metabolic age by age” chart tracks body composition, not a real metabolic change. Below is a rough metabolic age chart for what the Gallagher 2000 reference curve says about average body-fat by age¹:

• 20s: women ~22% body fat; men ~13%
• 30s: women ~24%; men ~15%
• 40s: women ~25%; men ~17%
• 50s: women ~27%; men ~19%
• 60s: women ~28%; men ~21%
• 70s: women ~30%; men ~23%

If your body-fat sits below the line for your age, your metabolic age comes out younger. The mechanism is that simple.

Metabolic age vs biological age

These are often confused. They’re different things:

• Metabolic age is a body-composition comparison derived from a prediction equation. Free, fast, rough. What this page gives you.
• Biological age (or “epigenetic age”) measures molecular markers like DNA methylation patterns and telomere length. Requires a blood or saliva sample, costs around £150–£400, and maps far more directly to actual cellular ageing.

If you care about longevity markers, biological-age tests (Horvath or GrimAge epigenetic clocks) are the right tool. If you want a useful proxy for how your body composition compares to a population average — and want the number to motivate you toward a meal plan that actually moves it — this calculator and the calorie deficit calculator together tell you most of what you’d learn from a smart scale without the hardware cost. The desk-job weight gain guide on our blog covers the habit changes that actually move the number.

Don’t want to think about it? The free 2-minute quiz builds a meal plan around the real levers — body fat down, lean mass up — so the number moves in the direction you want.