Body Fat Calculator

The U.S. Navy circumference method

Developed for military fitness assessment, the Navy method estimates body fat using circumference measurements relative to height. For men it uses neck and abdomen; for women it adds the hip. The formula applies logarithms to these measurements to produce an estimate accurate to within 3–4% for most people. It is one of the most practical field methods because it requires only a flexible tape measure.

The BMI method (Deurenberg equation)

When circumference measurements aren't available, the calculator uses the Deurenberg equation: Body Fat % = (1.20 × BMI) + (0.23 × age) − (10.8 × sex) − 5.4. It adjusts for age (fat tends to accumulate at the same BMI as we get older) and sex (women naturally carry more fat at the same BMI). Accuracy is lower than the Navy method — expect a margin of 4–6% — making it useful as a rough guide rather than a precise measurement.

How to take accurate measurements

Use a flexible tape measure in the morning before eating. Keep the tape parallel to the floor throughout, snug but not compressing the skin. Neck: just below the larynx, angling slightly downward. Abdomen (men): at the navel, at the end of a normal exhale. Waist (women): at the narrowest point of the torso. Hips: at the widest point of the buttocks. Repeat each measurement twice and average the results for the most consistent reading. Use the waist-to-hip ratio calculator to check your body shape risk score.

Fat mass and lean mass

Entering your weight splits your total body mass into fat mass (total weight × body fat %) and lean mass (total weight − fat mass). Lean mass includes muscle, bone, organs, water, and connective tissue — not just muscle alone. Lean mass figures are useful for tracking progress when you want to reduce fat while preserving or building muscle, and are used as input for the Katch-McArdle BMR formula.

Worked example

Male, U.S. Navy method: height 178 cm, neck 38 cm, waist 88 cm.

The formula is defined in inches, so the measurements are converted first: waist 34.65 in, neck 14.96 in, height 70.08 in.

Body fat % = 86.010 × log₁₀(34.65 − 14.96) − 70.041 × log₁₀(70.08) + 36.76 = 18.8% (Fitness category). At a total weight of 75 kg: fat mass = 75 × 0.188 = 14.1 kg; lean mass = 75 − 14.1 = 60.9 kg.

Related calculations

Body fat and lean mass are two views of the same measurement — the lean body mass calculator gives the other. Waist measurements matter independently of weight: the waist-to-hip ratio calculator uses them as a separate risk signal.

Body fat categories explained

The five categories: Essential fat (men <6%, women <14%) — the minimum for basic physiological function. Athlete (men 6–14%, women 14–21%) — typical of competitive athletes. Fitness (men 14–18%, women 21–25%) — lean and active. Acceptable (men 18–25%, women 25–32%) — healthy for non-athletes. Obese (above those thresholds) — associated with elevated metabolic health risks. The calculator adjusts these thresholds upward by approximately 1% per decade of age.

Why women have higher healthy ranges

Women naturally carry 6–11% more essential fat than men due to hormonal and reproductive factors — oestrogen promotes fat storage in the breasts, hips, and pelvis. A woman at 25% body fat can be just as lean and healthy as a man at 18%. Comparing body fat percentages between sexes without accounting for these biological differences gives a misleading picture of health and fitness.

Body fat % vs BMI — key difference

BMI uses height and weight and cannot distinguish between muscle and fat. A competitive athlete may have a BMI of 27 (overweight) but a body fat of 12% (athlete category), while someone with a BMI of 23 (healthy) may carry an unhealthy proportion of fat. Body fat percentage is a more direct measure of body composition and a stronger predictor of metabolic disease risk than BMI alone.

Practical next steps by category

Essential fat: If below this threshold unintentionally, speak to a GP — it may indicate an underlying condition. Athlete/Fitness: Maintain through balanced training and nutrition. Acceptable: Modest improvements to diet and activity are beneficial but not urgent. Obese: A 5% reduction in body fat meaningfully reduces blood pressure, cholesterol, and blood sugar. A registered dietitian can create a personalised plan for any significant body composition goal.

Why measurement method matters

Methods vary considerably in accuracy and practicality. DEXA scans are the gold standard (±1–2% error) but cost £50–200 and require a clinic appointment. Hydrostatic weighing is similarly accurate but hard to access. Skinfold calipers, used at 3–7 sites by a trained practitioner, achieve ±3–4%; self-measurement is less consistent. The Navy circumference method used here is a practical estimate at ±3–4% for most people. For tracking change over time, consistency of method matters more than absolute precision.

Essential fat vs storage fat

Body fat divides into two types. Essential fat is the biological minimum needed for nerve insulation, hormone production, organ protection, and cell membrane function — approximately 3–5% in men and 10–13% in women (women carry more due to reproductive and sex-specific fat deposits). Falling below this threshold causes hormonal disruption, bone loss, and organ damage. Storage fat is the energy reserve held subcutaneously (under the skin) and viscerally (around the organs). Visceral fat carries the greater metabolic risk but cannot be measured directly by circumference-based methods.

How sex hormones affect fat distribution

Testosterone promotes lean muscle mass and suppresses abdominal fat accumulation, which largely explains why men at the same body fat percentage as women typically have more muscle. Oestrogen encourages storage in the hips and thighs — subcutaneous fat that is metabolically less harmful than abdominal fat. After menopause, declining oestrogen shifts fat distribution towards the abdomen, raising cardiovascular and metabolic risk even without significant weight gain. This is why waist circumference and waist-to-hip ratio become especially important for women over 50.

Why ranges increase with age

Older adults naturally lose lean muscle mass (sarcopenia) and tend to accumulate more body fat even at the same total body weight — a process that begins in the 30s and accelerates after 50. Health guidelines therefore apply age-adjusted thresholds rather than a single universal range. A body fat % that is athletic at 25 may be average at 55.

Key thresholds to know

Essential fat: the minimum compatible with normal physiology — 3–5% for men, 11–13% for women. Athletic: below 14% for men, below 21% for women. Obesity threshold: above 25% for men, above 32% for women (these differ from BMI obesity thresholds, which often underestimate body fat in older adults).

How does the U.S. Navy body fat method work?

The Navy method uses circumference measurements — neck and abdomen for men, neck, waist, and hips for women — combined with height in a logarithmic formula to estimate body fat percentage. It was developed for US Navy fitness testing and is accurate to within 3–4% for most people when measurements are taken correctly.

How should I take my measurements?

Take measurements in the morning before eating with a flexible tape measure kept parallel to the floor. Neck: just below the larynx. Abdomen/waist: at the navel (men) or narrowest point (women) at the end of a normal exhale. Hips: at the widest point of the buttocks. Repeat each measurement twice and average the two readings.

How accurate is a body fat percentage calculator?

The Navy method is accurate to within 3–4% for most people — more accurate than BMI but less accurate than DEXA scans (±1–2%) or hydrostatic weighing. Accuracy varies with age, ethnicity, and body shape. Use the result as a directional guide rather than a clinical measurement; discuss significant results or changes with a healthcare professional.

What is a healthy body fat percentage?

Healthy ranges depend on sex and age. For men aged 20–39, acceptable body fat is roughly 8–20%; for women in the same age group, roughly 21–33%. These thresholds rise by approximately 1% per decade. Athletes typically sit well below these ranges. The category table in the calculator adjusts cut-offs for your specific age group.