Body Fat Calculator
Estimate body fat using a legacy circumference equation or a BMI-based screening formula.
Formula used
Density = 1.0324 - 0.19077 x log10(waist - neck) + 0.15456 x log10(height)
Body fat % = 495 / density - 450
Height and circumferences are measured in centimeters.
This implements the legacy Hodgdon-Beckett circumference equation as a screening estimate. It is not a medical measurement and is not the Navy's current official Body Composition Assessment. For an official assessment, follow the current Navy guide and its required tables and PRIMS process.
Current Navy Body Composition Assessment GuideEstimated Body Fat
17.4%
Screening estimate only; individual error can be meaningful.
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How to interpret this estimate
Tape and BMI equations estimate body composition from a few measurements; they do not directly measure body fat. Small differences in tape placement or tension can change the result. Use the same method and measurement technique when comparing changes over time, and do not use this number alone for medical, nutrition, or eligibility decisions.
Current Navy assessments
The current official Navy process is defined by the current Body Composition Assessment Guide and can change independently of this educational estimator. Consult that guide and authorized personnel for an official result.
For a simpler screening, use our BMI calculator.
- Verified: results checked against independent formulas on every release
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How this is calculated
The calculator uses the Hodgdon and Beckett circumference method: body density is estimated from height and tape measurements, then converted to a body-fat percentage.
Formulas
Men: body density from the waist-minus-neck difference and height, all in centimeters.
density = 1.0324 − 0.19077 × log10(waist − neck) + 0.15456 × log10(height)
Women: the waist, hip and neck measurements are combined.
density = 1.29579 − 0.35004 × log10(waist + hip − neck) + 0.22100 × log10(height)
Body density becomes a body-fat percentage with the Siri equation.
body fat % = 495 ÷ density − 450
Sources
- Hodgdon and Beckett, Prediction of percent body fat for U.S. Navy men from body circumferences and height (Naval Health Research Center, Report 84-11, 1984). The Navy research report that developed the circumference equations for estimating body density from height and body measurements.
How we test this
This tool is checked against independent formulas every time the site is released, and a release does not ship if a check fails. Example from the test suite: Man, 178 cm tall, neck 38 cm, waist 86 cm gives 17.2% body fat. The test applies the density and Siri equations above itself and checks the calculator agrees within 0.05 percentage points.
The test is “body-fat: male 178 cm, neck 38, waist 86” in tests/golden-calculators.test.mjs. More on how all tools are tested: our testing approach.
Assumptions and limits
- This is a screening estimate and is not the Navy's current official body-composition assessment. Like any tape-measure method it can over- or under-estimate body fat, especially at the low and high ends.
- Measure the neck, waist (and hip for women) the same way each time, in centimeters.
- Measurements that make the formula impossible (for example a waist no larger than the neck) are rejected rather than guessed.
- An estimate for general information, not medical advice. Talk to a health professional about your own situation.
Last reviewed , the date of the latest change to this tool's calculation or content.
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When to use this
BMI told you a number, but you suspect it is not the whole story. Maybe you lift weights and your BMI says "overweight" even though you can see your abs. Or maybe your BMI is normal but you feel softer than you would like. Body fat percentage is the metric that fills in the gap — it tells you what fraction of your total weight is fat versus lean mass (muscle, bone, water, organs). Two people at the same height and weight can have dramatically different body fat percentages and, by extension, very different health profiles.
This calculator offers two screening estimates. The legacy Hodgdon-Beckett circumference equation uses neck, waist, and hip circumference along with height. The BMI-based equation derives an estimate from height, weight, age, and sex. The interface identifies the selected formula and required units.
Neither screening equation replaces an assessment performed and interpreted by a qualified healthcare professional. Laboratory and imaging methods use different protocols and can produce materially different estimates. Repeated tape measurements are also sensitive to landmark choice, tape tension, and hydration, so do not treat a single result as a diagnosis.
Good to know
The circumference equation is a legacy screening estimate. It is not the Navy's current official Body Composition Assessment process and it is not a substitute for a clinical body-composition measurement.
Measure consistently for useful data. Take circumference measurements first thing in the morning, before eating, at the same anatomical landmarks each time. Waist at the navel, neck at the narrowest point below the Adam's apple, hips at the widest point. Small measurement inconsistencies can swing your result by several percentage points.
Healthy ranges differ by sex and age. Women naturally carry more essential fat than men — about 10–13% versus 2–5%. A body fat percentage of 20% means something very different for a 25-year-old male athlete than for a 50-year-old woman. The categories below reflect these differences.
Essential fat is not optional. Dropping below essential fat levels (roughly 5% for men, 12% for women) can disrupt hormone production, immune function, and organ protection. Competitive bodybuilders reach these levels temporarily for competition but do not sustain them year-round.
Body fat is one piece of the puzzle. Where you carry fat matters as much as how much you carry. Visceral fat around the organs (reflected in waist circumference) is more strongly linked to metabolic disease than subcutaneous fat under the skin. A healthcare provider can help you interpret your results in context.
Quick Reference
| Category | Men | Women |
|---|---|---|
| Essential Fat | 2 – 5% | 10 – 13% |
| Athletes | 6 – 13% | 14 – 20% |
| Fitness | 14 – 17% | 21 – 24% |
| Average | 18 – 24% | 25 – 31% |
| Above Average | 25%+ | 32%+ |