The simplest home method for calculating body fat percentage is the U.S. Navy method: measure height, neck and waist (plus hips for women) in centimeters, calculate body density with the Hodgdon-Beckett equation, then apply body fat % = 495 ÷ density − 450. A 178 cm man with a 38 cm neck and 89 cm waist measures about 19.4%.
What is body fat percentage?
Body fat percentage is the share of total body weight that is fat tissue. Someone who weighs 80 kg with 20% body fat carries 16 kg of fat and 64 kg of lean mass (muscle, bone, organs and water). Unlike BMI, which uses only height and weight, body fat percentage separates fat from everything else, so it describes body composition more directly.
How do you calculate body fat with the Navy method?
The Navy method was developed by James Hodgdon and Marcie Beckett at the Naval Health Research Center in 1984. They measured body density by underwater weighing and found the combination of circumferences and height that predicted it best.
Step 1: Take the measurements
Use a flexible, non-stretch tape, keep it level and snug without compressing the skin, and record each measurement to the nearest half centimeter (or quarter inch).
| Measurement | Where to measure | Who |
|---|---|---|
| Height | Standing straight, without shoes | Everyone |
| Neck | Just below the larynx (Adam's apple), tape sloping slightly down at the front | Everyone |
| Waist (abdomen) | Men: at the level of the navel. Women: at the narrowest point of the waist | Everyone |
| Hip | Around the widest part of the buttocks | Women only |
If the measurements are in inches, multiply each by 2.54 to convert to centimeters before using the formulas below.
Step 2: Calculate body density
All measurements in centimeters:
- Men: density = 1.0324 − 0.19077 × log10(waist − neck) + 0.15456 × log10(height)
- Women: density = 1.29579 − 0.35004 × log10(waist + hip − neck) + 0.22100 × log10(height)
"log10" is the base-10 logarithm (the LOG key on most calculators).
Step 3: Convert density to body fat
Use the Siri equation:
body fat % = 495 ÷ density − 450
Combining the steps gives the single-line versions used by most calculators, including CalcWisp's:
- 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
Worked example: Navy body fat for a man
A man is 178 cm tall (about 5 ft 10 in), weighs 82 kg, and measures 38 cm at the neck and 89 cm at the navel.
- Waist − neck = 89 − 38 = 51 cm; log10(51) = 1.70757
- log10(178) = 2.25042
- Density = 1.0324 − 0.19077 × 1.70757 + 0.15456 × 2.25042 = 1.05447
- Body fat = 495 ÷ 1.05447 − 450 = 19.4%
- Fat mass = 82 × 0.194 = 15.9 kg; lean mass = 82 − 15.9 = 66.1 kg
On the ACE chart below, 19.4% falls in the "average" range for men.
Worked example: Navy body fat for a woman
A woman is 165 cm tall, weighs 65 kg, and measures 33 cm at the neck, 76 cm at the waist and 100 cm at the hips.
- Waist + hip − neck = 76 + 100 − 33 = 143 cm; log10(143) = 2.15534
- log10(165) = 2.21748
- Density = 1.29579 − 0.35004 × 2.15534 + 0.22100 × 2.21748 = 1.03140
- Body fat = 495 ÷ 1.03140 − 450 = 29.9%
- Fat mass = 65 × 0.299 = 19.4 kg; lean mass = 65 − 19.4 = 45.6 kg
That result sits near the top of the "average" range for women.
How accurate is the Navy method?
In the original studies, the equations predicted underwater-weighing results with a standard error of estimate of about 3.5 percentage points for men and 3.7 for women. In practical terms, a reading of 20% could reasonably correspond to a true value somewhere between about 16.5% and 23.5%.
Accuracy depends heavily on technique. A 1–2 cm error in the waist measurement shifts the result noticeably, so measure two or three times and use the average. The method is also less reliable for people whose fat distribution differs from the Navy populations it was built on, such as very muscular individuals with thick necks.
What other ways can you measure body fat?
| Method | How it works | Notes on accuracy and use |
|---|---|---|
| DEXA (DXA) scan | Low-dose X-rays at two energies separate fat, lean tissue and bone | Often used as a reference method in research; also shows where fat is stored; requires a clinic or lab |
| Hydrostatic (underwater) weighing | Body density from weight in air versus underwater | Long-standing laboratory reference, used to build the Navy equations; requires full submersion and a full exhale |
| Air displacement (Bod Pod) | Body volume from air displaced in a sealed chamber | Laboratory-grade alternative to underwater weighing |
| Skinfold calipers | Pinch thickness at several sites, entered into a prediction equation | Inexpensive and reasonably consistent with a trained, experienced tester; poor with an inexperienced one |
| Bioelectrical impedance (BIA) | Small electrical current; fat conducts less than lean tissue | Used in smart scales and handheld devices; sensitive to hydration, food and recent exercise |
| Tape (Navy) method | Circumferences and height | Free and repeatable at home; an estimate with a few points of error |
For tracking change over time, consistency matters more than the method chosen. Using the same method, at the same time of day and under the same conditions, makes month-to-month trends meaningful even if the absolute number is off by a few points.
What is a healthy body fat percentage?
The American Council on Exercise (ACE) publishes widely used general categories:
| Category | Women | Men |
|---|---|---|
| Essential fat | 10–13% | 2–5% |
| Athletes | 14–20% | 6–13% |
| Fitness | 21–24% | 14–17% |
| Average | 25–31% | 18–24% |
| Obese | 32% and above | 25% and above |
Women carry more essential fat than men for reproductive and hormonal functions, which is why every range is higher. ACE notes that body fat below about 14% for women or 6% for men may be dangerously low. The categories are general guides and do not account for age; typical body fat tends to rise with age.
What to do with the result
Body fat percentage turns a single body weight into two numbers that can guide planning:
- Lean mass is useful for setting protein intake, which is often expressed per kilogram of body or lean weight.
- Fat mass shows how much of a change in weight is fat rather than water or muscle.
- Energy needs can be estimated more precisely: the Katch-McArdle BMR equation uses lean body mass instead of total weight.
This guide is educational and is not medical advice; consult a healthcare professional for an assessment of body composition and health risk.