Body Fat Calculator

How the US Navy method works

Fat storage shows up in the circumference of the waist and hips relative to the neck and overall height, so a tape measure can stand in for expensive equipment. The circumference method was developed for the US Navy in the 1980s to screen sailors for body composition standards, and it is still one of the few body fat estimates you can make at home with no devices.

With measurements in centimetres the equations are:

Men: body fat % = 100 × (4.95 ÷ (1.0324 − 0.19077 × log₁₀(waist − neck) + 0.15456 × log₁₀(height)) − 4.5)

Women: body fat % = 100 × (4.95 ÷ (1.29579 − 0.35004 × log₁₀(waist + hip − neck) + 0.22100 × log₁₀(height)) − 4.5)

The part inside the brackets estimates whole-body density from the measurements; 4.95 ÷ density − 4.5 is the Siri relation that converts density into a fat fraction. If you enter inches the calculator converts them to centimetres at 2.54 cm per inch before applying the formula.

The published standard error of the method is about 3.5 percentage points. Roughly two out of three people will be within 3.5 points of their true value and nearly everyone within 7, which is why the result is shown as a figure plus or minus 3.5.

How to use the calculator

1. Pick US units or Metric units and your gender. The Hip field only appears for women, because the women's equation uses it and the men's does not.
2. Enter your age and your weight. The weight is read in pounds with US units and in kilograms with metric units.
3. Enter your height. US units take feet and inches, metric units take centimetres or metres.
4. Enter your neck, waist and (for women) hip circumference in centimetres or inches.
5. Press Calculate.

You will see five lines: the body fat percentage with its ±3.5 margin, the category, body fat mass, lean body mass, and a reference value of ideal body fat for your age. Fat mass is your weight times the percentage, and lean mass is whatever is left.

If the formula produces a negative percentage the calculator asks you to check your measurements, because a waist barely larger than the neck is almost always a measuring slip. A waist smaller than the neck cannot be put through the logarithm at all and the result comes back as "nan".

How to take the measurements

Use a flexible, non-stretch tape, record each reading to the closest half inch or centimetre and measure on bare skin or over thin clothing. Ideally have someone else read the tape, because bending and twisting to see it changes the numbers. Keep the tape level and snug but not squeezing.

• Height: stand barefoot against a wall with heels together and eyes level, and mark the highest point of the head.
• Neck: just below the larynx (Adam's apple), where the neck is narrowest, with the tape sloping slightly down toward the front.
• Waist: around the belly at the level of the navel, relaxed, after a normal breath out. Do not pull the stomach in.
• Hip (women): around the widest part of the buttocks, standing with feet together.

Take each measurement twice and use the average. Repeat under the same conditions, for instance in the morning before eating, when you check progress; day-to-day swings from food and water can move the waist by a centimetre or more.

Worked examples

A man, 70 in (177.8 cm) tall, with a 34 in (86.36 cm) waist, a 15 in (38.1 cm) neck, weighing 170 lb. Waist minus neck is 48.26 cm, whose log₁₀ is 1.6836. The height term is log₁₀ 177.8 = 2.2499. The bracket becomes 1.0324 − 0.19077 × 1.6836 + 0.15456 × 2.2499 = 1.05897, so 4.95 ÷ 1.05897 = 4.6743 and the result is 100 × (4.6743 − 4.5) = 17.43 percent. Body fat mass is 170 × 0.1743 ≈ 29.6 lb and lean mass about 140.4 lb.

A woman, 165 cm tall, with a 75 cm waist, 98 cm hips, 32 cm neck, weighing 60 kg. Waist plus hip minus neck is 141 cm, with log₁₀ 2.1492. The bracket is 1.29579 − 0.35004 × 2.1492 + 0.221 × 2.2175 = 1.03354, giving 4.95 ÷ 1.03354 = 4.7894 and 28.94 percent. Fat mass is 60 × 0.2894 ≈ 17.4 kg and lean mass about 42.6 kg.

Body fat categories

The calculator labels your result with the five bands used by the American Council on Exercise. A boundary value belongs to the lower band, so a man at exactly 13.0 percent is still in "Athletes" and at 13.1 percent moves up to "Fitness".

Essential fat is the minimum needed for normal function: hormones, nerves and organ protection. Sitting at the bottom of the scale for a long time is not a goal for most people, and women in particular can lose their menstrual cycle at the lowest levels. The bands are population guides, not pass/fail marks, and what is right for you depends on health, sport and age.

Ideal body fat for age

Body fat tends to rise with age even in people who stay active, so the last line of the result gives an age-adjusted reference figure. The calculator looks up the first age band whose upper limit is at least your age and reports its value; ages above 55 use the final row.

A 33-year-old man falls in the "up to 35" band and gets 13.7 percent. Compare your result with this figure for direction only; being a few points above or below it carries no special meaning.

Other ways to measure body fat

• Skinfold calipers pinch the fat layer at several sites and feed the readings into an equation. Accuracy depends heavily on the person pinching.
• Bioelectrical impedance (smart scales, handheld meters) sends a weak current through the body and infers fat from how much resistance it meets. Hydration changes the answer a lot.
• Hydrostatic (underwater) weighing works out body density from displacement and was long the reference standard.
• Air displacement in a sealed chamber gives density without submersion.
• DEXA scans use two low-dose X-ray beams to separate fat, lean tissue and bone, and are the usual choice in research clinics.
• CT and MRI image fat directly, including the fat around internal organs, but are used for medical reasons rather than routine tracking.

Different methods can disagree by several points for the same person, so pick one and stick to it when tracking change.

Lowering body fat

Fat loss comes from a sustained energy deficit. The calorie calculator and weight loss calorie calculator estimate how much to eat, and your BMR shows the floor you should not drop far below. Resistance training and enough protein help you lose fat while keeping muscle, which is why the scale can overstate the progress of someone who is also getting stronger.

For a single-number cross-check use the BMI calculator, and for the military standard see the army body fat calculator.

This tool provides an estimate with a margin of error of several points. It is general information, not medical advice, and no replacement for a clinical assessment. See the disclaimer.