Data Source and Methodology
Authoritative Data Sources:
- Hodgdon JA, Beckett MB. Prediction of Percent Body Fat for U.S. Navy Men and Women from Body Circumferences and Height. NHRC Report 84-11, Naval Health Research Center, San Diego, 1984. Direct PDF.
- Deurenberg P, Weststrate JA, Seidell JC. Body mass index as a measure of body fatness in adults. Br J Nutr. 1991;65(2):105–114. DOI:10.1079/BJN19910073.
All calculations are strictly based on the formulas and data provided by these sources.
The Formulas Explained
U.S. Navy (measurements in inches, log base 10):
Male: \( \%BF = 86.010 \cdot \log_{10}(\text{waist} - \text{neck}) - 70.041 \cdot \log_{10}(\text{height}) + 36.76 \)
Female: \( \%BF = 163.205 \cdot \log_{10}(\text{waist} + \text{hip} - \text{neck}) - 97.684 \cdot \log_{10}(\text{height}) - 78.387 \)
BMI-based (Deurenberg):
\( \%BF = 1.20 \cdot BMI + 0.23 \cdot \text{Age} - 10.8 \cdot \text{Sex} - 5.4 \), where \( BMI = \frac{weight\,(kg)}{height\,(m)^2} \) and Sex = 1 for male, 0 for female.
Glossary of Variables
| Field / Result | Definition |
|---|---|
| Units | Choose Metric (cm, kg) or US (in, lb). |
| Method | U.S. Navy (measurements) or BMI estimate (weight, height, age). |
| Sex | Required to select proper equation and category thresholds. |
| Age | Used only in the BMI estimate. |
| Height | Stature without shoes. |
| Neck | Neck circumference just below the larynx. |
| Waist | Abdominal circumference at the level of the navel. |
| Hip | Widest hip/buttocks circumference (women only). |
| Weight | Body mass; enables fat and lean mass outputs and the BMI estimate. |
| Body fat % | Proportion of body mass composed of fat tissue. |
| Fat mass | Body fat % multiplied by body mass. |
| Lean mass | Body mass minus fat mass (fat-free mass). |
| Category | Classification by sex per ACE: Essential, Athlete, Fitness, Acceptable, Obese. |
All inputs are validated and must be physiologically plausible.
How It Works: A Step-by-Step Example
Scenario: Male, Metric units, Height 180 cm, Neck 38 cm, Waist 85 cm, Weight 80 kg.
- Convert to inches for the Navy formula: height 180/2.54 ≈ 70.87 in; neck 38/2.54 ≈ 14.96 in; waist 85/2.54 ≈ 33.46 in.
- Compute: \( \log_{10}(33.46 - 14.96) = \log_{10}(18.50) \approx 1.267 \); \( \log_{10}(70.87) \approx 1.851 \).
- Apply: \( \%BF = 86.010 \cdot 1.267 - 70.041 \cdot 1.851 + 36.76 \approx 108.2 - 129.7 + 36.76 \approx 15.3\% \).
- Fat mass: 0.153 × 80 kg ≈ 12.2 kg; Lean mass: 80 − 12.2 ≈ 67.8 kg.
Frequently Asked Questions (FAQ)
Which method should I use?
The U.S. Navy method is recommended when you can measure neck, waist, and (for women) hips. Use the BMI estimate when you only know height, weight, and age.
How many times should I measure?
Take at least two measurements for each site and use the average. Measure at the end of a normal exhale and keep the tape level.
Do hydration and time of day matter?
Yes. Circumferences and scale weight can change throughout the day. Measure under consistent conditions, ideally in the morning.
Is this medical advice?
No. This tool is informational and not a substitute for professional medical advice or diagnosis.
What are healthy body fat ranges?
Ranges vary by sex, age, and goals. As a general guide, ACE categories: Men 6–24% (athletic to acceptable), Women 14–31%. Consult a professional for individualized targets.