Body Fat Calculator
Estimate body-fat percentage from neck, waist (and hip) circumferences using the US Navy method — a tape-measure alternative to BMI when you want a composition-style check before a DEXA scan.
Calculate
Estimated body fat
The formula
The US Navy method estimates body fat from circumference measurements. It is more useful than BMI for fit individuals.
Worked example
- US Navy method (male equation).
- Height 178 cm, neck 38 cm, waist 85 cm.
- Estimated body fat ≈ 16.5%.
Result: 16.5%
How the Navy estimate works
Circumference equations were developed to assess body composition without underwater weighing or a scanner. Measurement technique drives accuracy.
Inputs and units
Measure height and circumferences consistently (typically centimeters here; imperial values are converted). In this method, waist is usually measured at the navel and neck below the Adam’s apple. Women also measure hips at the widest point. Keep the tape snug without compressing tissue.
Why logarithms?
The published Navy equations are empirical fits between circumferences and densitometry-based body fat. They do not directly compute the geometric volume of adipose tissue.
Accuracy limits
Expect roughly ±3–4 percentage points versus DEXA for many adults, with larger errors at very low or very high body-fat levels and with inconsistent tape placement. Fluid retention, pregnancy, and some medical conditions make casual use unreliable.
BMI versus body-fat percentage
BMI uses only height and weight. Two people with the same BMI can have very different body-fat percentages — that is why circumference methods exist.
Interesting facts
Navy method
Developed by the US Navy for quick field assessments of personnel fitness without laboratory equipment.
Sex-specific equations
Men use waist and neck; women add hip circumference because typical fat distribution differs. Using the wrong equation skews results.
Essential fat is not zero
The body needs some fat for hormones, organs, and temperature regulation. Very low readings can signal underfueling rather than “better” fitness.
Tape placement matters most
A one-centimeter shift at the waist often moves the estimate by a full percentage point — more than many people expect from the formula itself.
Frequently asked questions
Within about 3–4 percentage points of a DEXA scan for many adults, but less accurate for very lean or very obese individuals and when measurements are inconsistent.
For the Navy method, measure at the navel level while standing relaxed — not at the narrowest point of the torso, which is a common mistake.
Lab methods are more precise for research or clinical tracking. Circumference estimates are useful for trends when you measure the same way every time.
It answers a different question. Body-fat percentage describes composition; BMI is a quick height–weight screen. Athletes often prefer a fat estimate over BMI alone.
Healthy ranges depend on sex, age, and sport. Broad adult guides often place essential fat lower for men than women, with “fitness” and “average” bands above that. Treat Navy estimates as a trend tool — not a medical diagnosis — and confirm concerns with a clinician.
BMI uses only height and weight; the Navy method uses circumferences to estimate fat percentage. Muscular people often look “overweight” on BMI while Navy (or DEXA) better reflects composition. Neither replaces clinical body-composition testing when precision matters.
References
- Body composition assessment overview Circumference methods estimate body fat; they are not laboratory DXA.
- Obesity and overweight — fact sheet Population health context for adiposity screening tools.