Body Composition Calculator — Beyond BMI

This calculator gives you a complete picture of your body and energy needs in seconds. Enter your measurements and get four key metrics: BMI, waist-to-height ratio, body fat percentage, and daily calorie target with macro breakdown.

What each metric means

BMI (Body Mass Index) is the classic weight-for-height ratio. It’s fast and widely used, but it cannot distinguish between fat and muscle. A lean athlete and a sedentary person of the same weight and height get the same BMI — which is why this calculator shows it alongside better indicators.

Waist-to-Height Ratio (WHtR) divides your waist circumference by your height. Research consistently shows it predicts cardiovascular and metabolic risk better than BMI. A value below 0.5 is the widely-cited “healthy” threshold — meaning your waist should be less than half your height.

Body fat percentage (Advanced mode) is estimated using the US Navy method: waist, neck, and hips measurements fed into a validated formula. It’s not as precise as DEXA or hydrostatic weighing, but it’s far more informative than BMI alone. If you know your body fat from an InBody scan, you can enter it directly.

Lean Body Mass (LBM) — weight minus fat — tells you how much of your body is muscle, bone, and organs. This is the mass you want to preserve during a cut and build during a bulk.

Calorie formulas used

BMR (Basal Metabolic Rate) is calculated with the Mifflin-St Jeor equation — the most accurate formula for the general population:

Men: 10 × weight (kg) + 6.25 × height (cm) − 5 × age + 5
Women: 10 × weight (kg) + 6.25 × height (cm) − 5 × age − 161

TDEE (Total Daily Energy Expenditure) multiplies BMR by your activity level: from ×1.2 for a desk job with no exercise to ×1.9 for heavy physical labour combined with daily training.

Target calories depend on your goal:

Maintain: equal to TDEE
Lose weight: TDEE minus 500 kcal — a safe deficit for roughly 0.5 kg per week. The target never drops below BMR.
Gain muscle: TDEE plus 300 kcal — a modest surplus that minimises fat gain

Protein, fat and carbs

Protein is set at 1.8 g per kg of bodyweight (2.0 g when losing, 1.6 g when gaining). Fat covers about 27.5% of target calories. Carbohydrates fill the remainder. These are evidence-based starting points — not rigid rules.

How to measure correctly

Waist: stand relaxed, measure at navel level, after a normal exhale, without pulling in your stomach
Neck (Advanced): measure at the widest point below the larynx (Adam’s apple), tape parallel to the floor
Hips (women, Advanced): widest point around the buttocks and hips

Frequently asked questions

Why is my BMI “overweight” but I look fit?

BMI does not account for body composition. Muscle is denser than fat, so muscular people often score in the “overweight” or even “obese” BMI range while having perfectly healthy body fat levels. The WHtR and body fat percentage are more reliable indicators in this case.

How accurate is the US Navy body fat formula?

Within about ±3–4% for most people. It tends to underestimate body fat in very lean individuals and overestimate in people with unusual fat distribution. For reference, DEXA scans (the gold standard) cost money and require a clinic visit — the Navy formula is a useful free alternative.

What is TDEE and why does it matter?

TDEE is the total number of calories your body burns in a day, accounting for all activity. Eating at TDEE maintains your weight. Eating below TDEE creates a deficit for fat loss; eating above creates a surplus for muscle gain.

Is my data sent anywhere?

No. None of your measurements are transmitted to any server.