Body Fat Calculator (U.S. Navy Method)
This body fat calculator estimates your body fat percentage with the U.S. Navy tape-measure method: a man 70 in tall with a 15 in neck and a 33.5 in waist comes out at about 16.5%. Measure your neck and waist (women also the hips), enter your height, and you also see how far the formula can be off.
Example: Sex: Male · Unit: inches (in) · Height 70 in · Neck 15 in · Waist / abdomen 33.5 in · Hips (women only) 38.5 in → Estimated body fat: 16.5 %. Source: DTIC ADA143890, Hodgdon J. A., Beckett M. B.: Prediction of percent body fat for U.S. Navy men from body circumferences and height. Naval Health Research Center, Report 84-11, 1984. Updated: .
How it is calculated
How the U.S. Navy method works
The formulas come from two reports by the Naval Health Research Center: Hodgdon and Beckett, Report 84-11 (men) and Report 84-29 (women), both 1984, read in full text on November 2, 2026. Body density is first estimated from height and circumferences, all in centimetres:
- Men: density = 1.0324 − 0.19077 × log₁₀(abdomen − neck) + 0.15456 × log₁₀(height)
- Women: density = 1.29579 − 0.35004 × log₁₀(waist + hips − neck) + 0.22100 × log₁₀(height)
Density is then converted to fat percentage with Siri’s equation (1961): body fat % = 495 ÷ density − 450. Weight and age are not inputs: the reports tested both as variables, and neither entered the final equations.
How to calculate body fat percentage
- Convert to centimetres: 70 in = 177.8 cm, neck 15 in = 38.1 cm, abdomen 33.5 in = 85.09 cm (1 in = 2.54 cm exactly).
- Subtract the neck from the abdomen: 85.09 − 38.1 = 46.99 cm, with log₁₀ = 1.6720. Also log₁₀(177.8) = 2.2499.
- Calculate density: 1.0324 − 0.19077 × 1.6720 + 0.15456 × 2.2499 = 1.0612.
- Convert with Siri: 495 ÷ 1.0612 − 450 ≈ 16.5%.
- Read the range: the calculator adds the standard error, here about 12.9% to 20.0%.
| Sex | Height | Neck | Waist | Hips | Body fat | Range shown |
|---|---|---|---|---|---|---|
| Male | 68 in | 14.5 in | 32 in | 15.3% | 11.8 to 18.8% | |
| Male | 70 in | 15 in | 33.5 in | 16.5% | 12.9 to 20.0% | |
| Male | 72 in | 16 in | 38 in | 22.0% | 18.5 to 25.5% | |
| Female | 63 in | 12.5 in | 27 in | 36 in | 23.7% | 20.0 to 27.4% |
| Female | 65 in | 13 in | 28 in | 38 in | 25.7% | 21.9 to 29.4% |
| Female | 65 in | 13 in | 34 in | 42 in | 38.0% | 34.3 to 41.7% |
How to measure
- Neck: just below the larynx, tape sloping slightly downward at the front.
- Abdomen (men): horizontally at the level of the navel, at the end of a normal breath out.
- Waist (women): at the narrowest point, roughly midway between the lower tip of the breastbone and the navel.
- Hips (women): at the greatest protrusion of the buttocks.
Keep the tape snug without compressing the skin. Measuring twice and averaging reduces errors.
How accurate is it?
The equations were checked against underwater weighing in military personnel. The standard error of estimate was 3.52 percentage points for men (2.70 in a cross-validation with 100 other men) and 3.72 for women (4.36 and 4.04 in two cross-validation samples). In the Canadian Forces sample the women’s equation averaged 27.94% against 29.20% measured, so it can also miss in one direction. All of this was tested on service members, not on the general population.
The calculator deliberately gives no rating such as “good” or “too high”, although many others add fitness categories. The reports mention only the Navy’s 1984 service limits of 22% for men and 30% for women, which are not medical cut-offs. The result is an estimate, not a diagnosis, and does not replace medical advice.
Common measuring mistakes
Small tape errors move the result noticeably, because the formula works on the difference between waist and neck. The same man, 70 in tall with a 15 in neck:
| Abdomen | Body fat |
|---|---|
| 30 in | 8.9% |
| 33.5 in | 16.5% |
| 34.5 in | 18.4% |
| 36 in | 21.1% |
| 40 in | 27.7% |
- Pulling the stomach in: one inch less at the abdomen looks like nearly 2 points less fat in this example.
- Measuring at the wrong height: men use the navel, women the narrowest point. Swapping them changes the input the equation expects.
- Mixing units: choose inches or centimetres first; the calculator rejects values outside its plausible limits, such as a height outside 47 to 90 in.
- Comparing across methods: a result from this tape method is not directly comparable with one from another method. Track change with the same method and the same routine.
- Reading one decimal as precise: a change of 1 point sits well inside the standard error.
More calculators: the BMI calculator for weight relative to height, the ideal weight calculator for common weight formulas, the BMR calculator for resting calories and the TDEE calculator for daily needs.
Frequently asked questions
How accurate is the U.S. Navy method?
It is accurate to about 3.5 percentage points for men and 3.7 for women, the standard error of estimate against underwater weighing in the original reports. That suits a rough estimate and tracking over time, not a precise measurement.
Why do women need a hip measurement?
For women, the combination of waist, hips and neck predicted body density best in the 1984 report; for men, abdomen and neck were enough. The two equations were developed separately.
Where exactly do I measure my waist?
Men measure horizontally at the navel, women at the narrowest point of the waist. Measure at the end of a normal breath out, without pulling the stomach in, and keep the same routine each time.
Do I need my weight or age?
No. Only height and circumferences go into the formula. The Navy researchers tested weight and age as variables, and neither made it into the final equations.
How much does one inch on the waist change the result?
For a 70 in man with a 15 in neck, about 2 percentage points: 33.5 in gives 16.5% and 34.5 in gives 18.4%. That is why careful measuring matters more than the decimal.
Can I compare the result with a body fat scale?
Only to a limited extent, because a scale measures differently from a tape-based formula. Compare values within the same method, ideally under the same conditions.
What body fat percentage is normal?
This calculator gives no normal range, because the underlying reports do not set medical cut-offs. The 22% and 30% in those reports were Navy service limits in 1984. Discuss how to interpret your value with your doctor.
Sources and legal basis
- DTIC ADA143890: Hodgdon J. A., Beckett M. B.: Prediction of percent body fat for U.S. Navy men from body circumferences and height. Naval Health Research Center, Report 84-11, 1984
- DTIC ADA146456: Hodgdon J. A., Beckett M. B.: Prediction of percent body fat for U.S. Navy women from body circumferences and height. Naval Health Research Center, Report 84-29, 1984
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