Body fat percentage: the metric that actually matters
If you only track one body-composition number, make it body fat percentage. Scale weight tells you almost nothing about how your body is changing — a 5 kg drop can be water, glycogen, muscle, fat, or some mix of all four. Body fat percentage isolates the variable that actually matters for health and aesthetics: how much of you is fat tissue versus everything else.
Methods of measuring body fat
Several methods exist, each with trade-offs between accuracy, cost and convenience. DEXA (dual-energy X-ray absorptiometry) scans are the closest thing to a gold standard for living humans, with errors typically under 2%. They\'re also expensive (£70–£200 per scan) and not available everywhere. Hydrostatic underwater weighing is similarly accurate but logistically painful. Air displacement plethysmography (the BodPod) is accurate and faster but again requires specialised equipment.
For at-home use, the practical methods are skin-fold calipers, bioelectrical impedance analysis (BIA) via smart scales, and the US Navy circumference method used by this calculator. Calipers can be very accurate in trained hands but inconsistent for amateurs — typical home users see 5–10% error. BIA is convenient but heavily affected by hydration, recent meals and exercise, with errors of 4–8% on consumer-grade scales. The Navy method sits in a useful middle ground: roughly 3% accuracy for typical builds, completely free, and trivially repeatable if you measure consistently.
Healthy body fat ranges
For men, essential fat (the minimum required for hormonal function) sits around 3–5%. Athletes typically run 6–13%, fit non-athletes 14–17%, average 18–24%, and 25%+ is the obesity threshold. For women, essential fat is higher at 10–13% due to sex-specific tissues. Athletic women run 14–20%, fit non-athletes 21–24%, average 25–31%, and 32%+ is the obesity threshold. The lower bounds aren\'t goals — sustained body fat below 10% in men or 16% in women causes hormonal disruption (low testosterone, amenorrhoea, sleep problems, mood crashes) that physique competitors only tolerate for brief peak weeks.
Why fat distribution matters more than total fat
Visceral fat — stored inside the abdomen around the liver, intestines and other organs — carries far more metabolic risk than subcutaneous fat stored under the skin. Two people at identical body fat percentages can have very different cardiovascular and diabetes risk depending on distribution. Visceral fat is metabolically active, releasing inflammatory cytokines and free fatty acids directly into the portal circulation; subcutaneous fat is comparatively inert. This is why waist circumference (a proxy for visceral fat) sometimes predicts disease risk better than body-fat percentage alone.
Genetics largely determine where you store fat. Men typically store viscerally and around the trunk ("apple" pattern), giving higher cardiovascular risk per unit body fat. Women typically store subcutaneously around hips and thighs ("pear" pattern) until menopause, when the storage pattern shifts toward visceral. This is one reason cardiovascular risk in women rises sharply post-menopause despite stable body weight.
Losing body fat without losing muscle
The challenge of any cut is reducing fat mass without losing lean mass. Three variables drive muscle retention during fat loss. First, protein intake: aim for 1.6–2.2 g per kilogram of body weight (or 2.3–3.1 g per kg of lean mass if you\'re lean). Higher protein during a deficit consistently shows superior muscle preservation in trials. Second, resistance training: continuing to lift heavy weights signals the body to retain muscle as functionally necessary. Cardio alone during a deficit accelerates muscle loss. Third, the size of the deficit: aggressive deficits (>25% below TDEE) lose more muscle per unit fat than moderate deficits (15–20%). For experienced trainees, slower is better — 0.5–0.7% of body weight per week minimises muscle loss while still producing visible progress.
Tracking body fat over time
For trend tracking, consistency matters more than absolute accuracy. Pick one method, use it under the same conditions each time (same time of day, hydration status, before eating), and track changes rather than absolute numbers. A monthly Navy-method measurement under consistent conditions will reliably show whether you\'re heading the right direction even if the absolute number is off by 2–3%. Combine it with photos under consistent lighting and waist circumference — three data points triangulate change far better than any single number alone.
What this calculator outputs
This tool returns three numbers from your measurements: body fat percentage, fat mass in kilograms, and lean mass in kilograms. Lean mass includes muscle, bone, organs, connective tissue and water. Tracking lean mass during a cut is the single most useful number — if it stays roughly flat (within 1 kg) while fat mass drops, your cut is going well. If lean mass falls rapidly, your deficit is too aggressive, protein is too low, or training stimulus has dropped. Re-run this calculator every 2–4 weeks during an active cut or bulk and watch the trend lines, not single readings.