Lean Body Mass Calculator

A body fat calculator estimates the percentage of your total body mass that is fat tissue. Body fat percentage is a more meaningful health metric than BMI for many people because it distinguishes between fat mass and lean mass (muscle, bone, organ tissue, water). A muscular person can have a high BMI but a healthy body fat %; a "skinny fat" person can have a normal BMI but an elevated body fat % and associated metabolic risk. and are useful companion tools for body composition assessment.

Body fat is estimated through various methods of varying accuracy: DEXA scan (gold standard, ±1–2%); hydrostatic weighing (±2%); air displacement plethysmography (Bod Pod, ±2–3%); bioelectrical impedance (BIA scales, ±3–5% but sensitive to hydration); skinfold callipers (±3–5% with trained practitioner); circumference-based formulas (US Navy method, ±3–4%). The online calculator uses circumference-based or BMI-based estimates — convenient but with wider error margins. Use for trend tracking and general guidance rather than precise clinical measurement.

1. Enter height, weight, age, and sex.
2. For the US Navy method: also enter waist circumference (measured at navel), neck circumference (measured below larynx), and for women, hip circumference (measured at widest point).
3. Measure all circumferences first thing in the morning, standing upright, tape parallel to the floor, without sucking in.
4. Read the estimated body fat % and compare against the healthy range for your sex and age.
5. Use the result to set a target — most people seeking body recomposition aim to reduce body fat % by 3–8 percentage points rather than tracking absolute weight.
6. Re-measure monthly using the same method, same time of day, and same conditions to track trends.

Body fat estimation formulas

US Navy method (men): BF% = 86.010 × log₁₀(waist − neck) − 70.041 × log₁₀(height) + 36.76

US Navy method (women): BF% = 163.205 × log₁₀(waist + hip − neck) − 97.684 × log₁₀(height) − 78.387

where all measurements are in cm or all in inches.

BMI-based estimation (Deurenberg, 1991): BF% = (1.20 × BMI) + (0.23 × age) − (10.8 × sex) − 5.4 (sex: 1 for male, 0 for female)

Worked example (US Navy, male): height 178 cm, waist 85 cm, neck 38 cm. BF% = 86.010 × log₁₀(47) − 70.041 × log₁₀(178) + 36.76 ≈ 17.0%.

Interpreting your body fat result

Healthy body fat % ranges

General body fat categories (widely cited in fitness and clinical literature): Men — essential fat 2–5%, athletes 6–13%, fitness 14–17%, acceptable 18–24%, high risk 25%+. Women — essential fat 10–13%, athletes 14–20%, fitness 21–24%, acceptable 25–31%, high risk 32%+. Body fat naturally increases with age — healthy ranges shift upward by approximately 1–2% per decade after age 30. Visceral fat (fat around internal organs) is metabolically more dangerous than subcutaneous fat and is not accurately measured by circumference methods. Waist circumference above 94 cm (men) or 80 cm (women) is a WHO risk threshold for metabolic disease, regardless of total body fat %.

Health tips and best practices

• Track trend over time rather than absolute values — circumference-based estimates can vary by ±3–5%; what matters is consistent reduction over weeks and months.
• Adequate protein intake (1.6–2.2g per kg of body weight) is critical when cutting calories to preserve muscle mass during a deficit and increase satiety.
• Resistance training during a caloric deficit preserves or builds lean muscle, improving body composition beyond what scale weight alone reflects.
• Very low body fat (below 8% for men, below 15% for women) impairs hormonal function, immunity, and recovery — avoid targets in the essential fat range.
• Sleep quality and quantity significantly affect body composition — sleep deprivation increases cortisol, promotes fat storage, and reduces muscle protein synthesis.

• 1 kg of body fat contains approximately 7,700 kcal; a 3,500 kcal/week deficit produces approximately 0.45 kg of fat loss per week.
• Reducing body fat from 25% to 18% while maintaining muscle mass typically requires 6–12 months of consistent training and nutrition for a natural athlete.
• Visceral fat area above 100 cm² (measurable via CT scan) is associated with significantly elevated cardiovascular risk independent of overall body fat % (multiple meta-analyses).
• Consumer bioelectrical impedance (BIA) scales can vary by ±5% body fat depending on hydration, time of day, and food intake — use them for trends, not absolute values.

Common mistakes to avoid

• Relying on consumer BIA scales for precise measurements — accuracy varies enormously based on hydration, time of day, food intake, and device quality.
• Confusing healthy body fat targets with the extremely low levels seen in fitness photography — these levels are not sustainable or healthy for most people long-term.
• Treating body fat % as the only health metric — muscle mass, bone density, visceral fat, and metabolic health markers are equally or more important.
• Measuring at inconsistent times — measure at the same time of day (ideally morning, post-toilet, pre-food) to minimise hydration and food-volume variability.

Body fat percentage estimates from online calculators are mathematical approximations with error margins of 3–5%. They are not clinical measurements and should not be used to diagnose obesity, eating disorders, or any medical condition. Body composition assessment in clinical settings uses validated methods (DEXA, Bod Pod, hydrostatic weighing). If body fat levels are a clinical concern, assessment should be conducted by a qualified healthcare professional. For guidance on healthy weight, consult your national health authority or a registered dietitian.