Body Fat Calculator

How it works

Researchers Hodgdon and Beckett measured hundreds of Navy personnel by underwater weighing and found that a few circumferences plus height predicted body fat well. The equations below (inches) are the widely used form:

Men: %BF = 86.010 × log₁₀(waist − neck) − 70.041 × log₁₀(height) + 36.76
Women: %BF = 163.205 × log₁₀(waist + hip − neck) − 97.684 × log₁₀(height) − 78.387

Metric measurements are converted to inches before calculating. In the original studies, the standard error was about 4 percentage points, so a result of 20% is best read as roughly 16–24%.

How to measure

• Neck: just below the larynx (Adam's apple), tape sloping slightly down toward the front.
• Waist: men at the navel; women at the narrowest point of the waist. Measure after a normal breath out, without sucking in.
• Hips (women): at the widest part of the buttocks.
• Use a flexible, non-stretch tape, snug but not compressing skin. Take each measurement twice and average.

Body fat categories (ACE)

Essential fat is needed for normal body function; going below it is dangerous.

How the Navy method compares with other methods

Tracking change over time

Because errors in tape measurements tend to repeat in the same direction for the same person, the Navy method is better at showing trends than at giving an exact number. Measure every 2–4 weeks under the same conditions, and pay more attention to the direction of change than to any single reading.

Body fat and health

Both too little and too much body fat carry risks. Very low body fat can disrupt hormones, menstrual cycles and bone health. Higher body fat, especially around the abdomen, is linked with type 2 diabetes and heart disease. Pair this estimate with the Waist-to-Hip Ratio Calculator to see where fat is stored.

From body fat to lean mass

Multiply your weight by your body fat percentage to get fat mass; the rest is lean mass (muscle, bone, organs and water). For example, 180 lb at 20% is 36 lb of fat and 144 lb lean. Lean mass can feed the Katch-McArdle equation in our BMR Calculator, or compare it with the Lean Body Mass Calculator.

Common measuring mistakes

Pulling the tape too tight, measuring the waist at the wrong level, or holding the breath in are the most common errors. Ask someone to help, keep the tape level all around, and record measurements to the nearest quarter inch or half centimeter.

Worked examples

Example 1: Man, 70 in tall, neck 15 in, waist 34 in

86.010 × log₁₀(19) − 70.041 × log₁₀(70) + 36.76 = 109.98 − 129.23 + 36.76 ≈ 17.5% → Fitness.

Example 2: Woman, 64 in tall, neck 13 in, waist 30 in, hips 40 in

163.205 × log₁₀(57) − 97.684 × log₁₀(64) − 78.387 ≈ 286.6 − 176.4 − 78.4 ≈ 31.7% → Average.

Limitations

• Typical error is about ±3–4 percentage points, more for very lean or very heavy people.
• Small measuring errors matter: 1 inch on the waist can shift the result by 1–3 points.
• Developed in military personnel; may be less accurate in other groups.

Who should not rely on this calculator

Not for children, pregnancy, or people with fluid retention, abdominal swelling or conditions that change body shape. For clinical decisions, methods such as DXA are more accurate.