Body Fat % Calculator
Estimate your body fat percentage using the US Navy method — a validated tape-measure technique requiring only height, neck, waist, and hip measurements.
Measuring tips: Neck below the larynx, perpendicular to long axis. Waist at navel. Women: hips at widest point. Stand straight, breathe normally.
Body Fat Estimate
Accuracy: ±3–4% compared to DEXA scan. For highest accuracy, use DEXA or hydrostatic weighing.
How to Measure Body Fat Percentage
Body fat percentage is a far more accurate indicator of health and fitness than scale weight alone. While the scale tells you total mass, body fat percentage reveals how much of that mass is metabolically active lean tissue versus stored fat. This free body fat calculator uses the US Navy circumference method — a validated tape-measure technique requiring no special equipment and achieving accuracy within ±3–4% of DEXA scanning in most individuals.
All calculations run in your browser with no data ever sent to a server. Enter your measurements in centimetres (metric) or inches (imperial).
How to Use the Body Fat Calculator Online
- Select Metric or Imperial — note that all measurement inputs use the same unit (cm or in) once selected.
- Choose your biological sex — the male formula uses neck and waist only; the female formula requires neck, waist, and hips.
- Measure and enter height — stand against a wall without shoes for accuracy.
- Measure neck circumference — tape below the larynx (Adam's apple), perpendicular to the long axis of the neck. Do not compress the skin.
- Measure waist circumference — at the level of the navel (belly button), not the narrowest point. Stand relaxed and breathe normally.
- Women: measure hip circumference — at the widest point of the hips and buttocks, keeping the tape level.
- Read your result — body fat %, category, lean mass %, and scale position update instantly.
How the Body Fat Calculator Works Technically
The calculator implements the US Navy circumference formula using JavaScript's Math.log10() function in the browser via React hooks. Imperial inputs are passed directly (the formula was originally developed in inches); metric inputs are kept as-is since the published formula also has a metric version. The formula uses logarithmic relationships between circumference measurements to estimate body density.
Female: BF% = 495 ÷ (1.29579 − 0.35004 × log(waist+hip−neck) + 0.22100 × log(height)) − 450
Healthy Body Fat Ranges — 6 Key Categories
- Essential Fat (men 2–5%, women 10–13%): The minimum fat required for organ protection, nerve insulation, and hormonal function. Below this level causes serious health consequences including hormonal disruption and organ dysfunction.
- Athletic (men 6–13%, women 14–20%): Typical range for competitive athletes and serious fitness enthusiasts. Visible muscle definition is common at these levels, especially below 10% for men.
- Fitness (men 14–17%, women 21–24%): The healthy range for active adults who exercise regularly. Associated with good metabolic health, normal hormonal function, and low cardiovascular risk.
- Average (men 18–24%, women 25–31%): Normal range for moderately active to sedentary adults. Metabolic health markers are generally normal in this range, though risk begins to increase toward the upper end.
- Obese (men 25%+, women 32%+): Elevated risk of metabolic syndrome, insulin resistance, cardiovascular disease, and joint problems. Weight loss through diet and exercise is strongly recommended at these levels.
- Body recomposition zone: Moving from Average to Fitness category typically requires a 6–12 month programme combining a modest calorie deficit, high protein intake (1.6–2.2 g/kg), and resistance training at least 3 days per week. Use our Macro Calculator to set your nutritional targets.
Why Body Fat % Matters More Than Scale Weight
Two people can weigh the same on a scale but have dramatically different body compositions and health profiles. Someone at 75 kg with 15% body fat has approximately 11.25 kg of fat and 63.75 kg of lean mass — very different from a 75 kg person with 30% body fat (22.5 kg fat, 52.5 kg lean). Tracking body fat percentage alongside scale weight tells you whether you are gaining muscle, losing fat, or both — the metrics that actually reflect health improvements. Combine this tool with our BMI Calculator to get both perspectives on your weight status.
Methods for Measuring Body Fat
Beyond tape measurements, body fat can be measured through DEXA (gold standard, ±1–2% accuracy), hydrostatic weighing, air displacement plethysmography (Bod Pod), bioelectrical impedance analysis (BIA scales), and skinfold callipers. The Navy method achieves ±3–4% accuracy with no equipment cost — suitable for regular self-monitoring. For clinical decisions or contest preparation, DEXA is recommended.
Privacy and Security
The Body Fat Calculator runs entirely in your browser using JavaScript arithmetic — no data is transmitted to ToollyX servers at any point. Your body measurements, sex, and calculated body fat percentage never leave your device. This is safe for personal health monitoring, fitness tracking, clinical reference use, and sensitive body composition data you prefer not to store on external services.
✓Verified by ToollyX Team · Last updated June 2026
Frequently Asked Questions
Disclaimer: The US Navy formula provides an estimate only. For clinical accuracy, use DEXA scanning. Consult a healthcare provider before making significant changes to your diet or exercise programme.