Body fat from a tape measure,
and what it means for lean mass.
The US Navy method estimates body fat from neck, waist (and for women, hip) circumferences and height. It is not a DEXA scan, but it is repeatable, free, and within a few percentage points for most people. From that, lean mass and FFMI follow.
ESTIMATED BODY FAT
—How it’s calculated
Men: %BF = 495 ÷ (1.0324 − 0.19077 × log₁₀(waist − neck) + 0.15456 × log₁₀(height)) − 450 Women: %BF = 495 ÷ (1.29579 − 0.35004 × log₁₀(waist + hip − neck) + 0.22100 × log₁₀(height)) − 450 (all measurements in cm) Hodgdon & Beckett, 1984 Lean mass = weight × (1 − %BF) FFMI = lean mass (kg) ÷ height (m)² normalised FFMI = FFMI + 6.1 × (1.8 − height m)
Assumptions and limits
- Measure waist at the navel (men) or narrowest point (women), neck below the larynx, hips at the widest point. Tape snug, not compressing.
- Circumference methods are least accurate at very low or very high body fat and in very muscular people.
- FFMI above roughly 25 is rare without pharmacological assistance in men (Kouri et al., 1995), which is why it is used as a plausibility check, not a goal.
References
- Fat-Free Mass Index in Sport: Normative Profiles and Applications for Collegiate Athletes.J Strength Cond Res 2024.PubMed 39074219 · doi:10.1519/JSC.0000000000004864
- Fat-Free Mass Index for body composition analysis in pediatric sport: a cross-sectional study.J Sports Med Phys Fitness 2024.PubMed 37955930 · doi:10.23736/S0022-4707.23.15377-1
- Body mass index and fat free mass index in obstructive lung disease in Korea.Int J Tuberc Lung Dis 2014.PubMed 24365561 · doi:10.5588/ijtld.13.0212
- Predicting fat-free mass index and sarcopenia in assisted-living older adults.Age (Dordr) 2014.PubMed 24994536 · doi:10.1007/s11357-014-9674-8
- Differential impacts between fat mass index and fat-free mass index on patients with COPD.Respir Med 2023.PubMed 37390978 · doi:10.1016/j.rmed.2023.107346
- Influence of fat-free mass index on the survival of patients with head and neck cancer.Eur Arch Otorhinolaryngol 2023.PubMed 36437380 · doi:10.1007/s00405-022-07732-w
Literature retrieved from PubMed on October 18, 2026, favouring human studies and reviews. Numbered markers in the text show which paper supports which statement; uncited entries are background reading.