Calculate your body fat % using the US Navy formula or BMI method
The US Navy formula uses circumference measurements (waist, neck, and hips for women) to estimate body fat percentage. It's significantly more accurate than BMI-based estimates because it accounts for body shape, not just weight and height.
The BMI method uses the Deurenberg formula to estimate body fat from BMI, age, and sex. It's less accurate for athletes or very muscular individuals, but useful as a quick estimate when tape measurements aren't available.
For best accuracy, also consider DEXA scans (gold standard) or hydrostatic weighing at a health/fitness center.
Healthy ranges vary by sex and age. For men, 6–24% is generally considered healthy, with athletes typically at 6–13% and fitness levels at 14–17%. For women, 16–30% is healthy, with athletes at 14–20% and fitness levels at 21–24%. Body fat above 25% (men) or 32% (women) is classified as obese by most health organisations.
The US Navy method has a margin of error of approximately 3–4% compared to DEXA scans, which is acceptable for most tracking purposes. It's more accurate than BMI-based estimates for most people, especially those who are muscular. However, it can underestimate body fat in people with large abdomens and overestimate it in very lean individuals.
Use a flexible tape measure and measure each site twice, averaging the results. For the waist, measure at the narrowest point (usually around the navel). For the neck, measure just below the larynx. For hips (women only), measure at the widest point. Stand up straight, breathe normally, and don't pull the tape too tight.
Yes — this is called body recomposition. The key is eating at a slight calorie deficit (200–300 calories below maintenance), consuming adequate protein (0.7–1g per pound of bodyweight), and maintaining a resistance training programme. Progress is slower than pure fat loss, but you preserve or even gain muscle while losing fat.
BMI only uses height and weight — it has no way to distinguish between muscle mass and fat mass. A muscular athlete and an overweight sedentary person can have the same BMI. Body fat percentage is a much better indicator of health because it directly measures the proportion of fat in your body.
This calculator uses the U.S. Navy circumference method, developed by Hodgdon and Beckett (1984) and validated for use across large populations. For men, it uses neck and waist circumference alongside height. For women, hip circumference is added to account for the different fat distribution pattern. The formula uses logarithms because the relationship between circumference and fat percentage is not linear.
The key limitation is that circumference measurements cannot distinguish between subcutaneous fat (under the skin) and visceral fat (around the organs). Visceral fat carries higher health risk and tends to accumulate at the abdomen. Two people with the same body fat percentage can have very different metabolic profiles. For clinical accuracy, DEXA scans are the gold standard — but the Navy method is a reliable and free alternative for most practical purposes.
For men: essential fat (2–5%); athletic (6–13%); fitness (14–17%); acceptable (18–24%); obese (25%+).
For women: essential fat (10–13%); athletic (14–20%); fitness (21–24%); acceptable (25–31%); obese (32%+). Women naturally carry more essential fat due to hormonal and reproductive physiology. Very low body fat — particularly below essential levels — carries serious health risks including hormonal disruption, bone density loss, and immune suppression.
The Navy formula is sourced from US DoD body composition guidelines. Healthy body fat ranges follow WHO standards.
Figures are estimates for guidance only. See about this site — how we source data and what these tools can and cannot do.
Body weight tells you how much you weigh in total — it says nothing about composition. Two people can be identical in weight and height but have very different health profiles: one with 15% body fat and substantial muscle mass, another with 35% body fat and little muscle. BMI cannot distinguish between these cases, which is why it frequently misclassifies muscular athletes as overweight and can miss obesity in people with low muscle mass (a phenomenon sometimes called "skinny fat" or normal-weight obesity).
Body fat percentage directly relates to metabolic health risk. Excess fat — particularly visceral fat stored around the organs — is associated with insulin resistance, elevated triglycerides, cardiovascular disease risk, and inflammation. Tracking body fat percentage over time gives a clearer picture of whether your body composition is improving than weight alone, especially during periods of concurrent fat loss and muscle gain when the scale may barely move.
The US Navy circumference method estimates body fat using waist, neck, and (for women) hip measurements. The formula works because fat storage patterns follow predictable anatomical distributions: people with higher body fat tend to carry proportionally more mass at the waist relative to the neck. The mathematics convert these ratios into an estimated fat percentage using regression equations derived from dual-energy X-ray absorptiometry (DEXA) validation studies.
For men: body fat % = 86.010 × log10(waist − neck) − 70.041 × log10(height) + 36.76. For women, hips are included in the formula because female fat distribution differs from male. Accuracy is approximately ±3–4 percentage points compared to DEXA, making it a reliable screening tool though not a clinical measurement.
Measurement technique matters significantly. Waist should be measured at the navel (not the narrowest point), neck just below the larynx without compressing the skin, and hips at the widest point. Consistent technique across measurements matters more than perfect absolute accuracy.
The gold standard for body fat measurement is DEXA (dual-energy X-ray absorptiometry), which uses low-dose X-rays to differentiate fat, muscle, and bone. DEXA is accurate to within 1–2%, distinguishes regional fat distribution, and also measures bone mineral density. It is available at hospitals, specialist clinics, and some gyms, typically costing £50–£150 per scan in the UK.
Hydrostatic (underwater) weighing uses Archimedes' principle — fat floats, muscle sinks — to estimate body density and calculate fat percentage. It is similarly accurate to DEXA but less accessible. Air displacement plethysmography (the Bod Pod) is a more accessible alternative using the same density principle.
Consumer bioelectrical impedance devices (bathroom scales, handheld monitors) send a weak electrical current through the body and estimate fat from the resistance. These vary widely in accuracy — results can swing 5+ percentage points based on hydration status, time of day, and recent exercise. They are useful for tracking trends but not for reliable single measurements.
The US Navy body fat formula follows Hodgdon and Beckett (1984) as published by the US Navy Personnel Research and Development Center. Healthy body fat ranges reference American Council on Exercise (ACE) body fat percentage categories. BMI-to-fat conversion uses the Deurenberg formula (1991). Results are estimates — for clinical body composition measurement, consult a healthcare professional or use DEXA scanning.
Researched and maintained by Iulian, founder of Flux Media Systems. General information, not professional advice — about this site & our sources →