Body Fat Calculator

What this calculator answers

Body mass index tells you how heavy you are for your height, but it cannot tell muscle from fat. Body fat percentage estimates what share of your total weight is fat, so it can show a change that the bathroom scale hides: the same weight with a smaller waist. This page uses the U.S. Navy circumference method, which needs only a soft tape measure, so you can track a trend at home without special equipment. If you also want the BMI view, theBMI Calculator works well alongside it.

This is an estimate for general information, not medical advice or a diagnosis. Use it to watch direction over weeks, not to label yourself with a single number.

How the U.S. Navy method works

The method comes from work by Hodgdon and Beckett in the 1980s, when the Navy wanted a way to assess body composition without lab equipment. It is a regression equation: it converts body measurements into an estimated body density, then converts density into a fat percentage. The neck stands in for lean tissue, because a thicker neck usually goes with more muscle. The waist, and for women the hips, stand in for where fat is stored. All lengths are in centimeters and log10 is the common logarithm.

• Men: body fat % = 495 ÷ (1.0324 − 0.19077 × log10(waist − neck) + 0.15456 × log10(height)) − 450
• Women: body fat % = 495 ÷ (1.29579 − 0.35004 × log10(waist + hip − neck) + 0.22100 × log10(height)) − 450

The key quantity is the gap between waist and neck. A smaller waist or a larger neck lowers the estimate. Inch-based versions of the same equation exist and give nearly identical results.

How to measure properly

• Neck: just below the larynx (Adam's apple), standing relaxed with shoulders down, tape parallel to the floor.
• Waist (men): at navel height after a normal exhale. Do not suck in or push out your stomach.
• Waist (women): the original protocol uses the narrowest point of the torso. If there is no clear narrowest point, use navel height.
• Hips (women): at the widest point of the buttocks, tape level all the way around.
• Keep the tape against the skin without pinching. Measure three times and average. Use the same tape, time of day (for example, first thing in the morning) and conditions each time, otherwise the changes you see are partly noise.

Body fat categories (ACE)

The ranges below follow the general classification published by the American Council on Exercise (ACE). They are reference bands, not diagnostic cut-offs. Typical body fat rises with age, and the health risk attached to a given percentage can differ between ethnic groups and body shapes.

Women's essential fat band is higher because fat supports reproductive and hormonal function. Going below essential fat is harmful, and the athlete range is not a goal most people should try to hold for long periods.

How sensitive is the result to your waist reading?

The table below is calculated when the page is built: a man 175 cm tall with a 38 cm neck, changing only the waist.

In this range each centimeter of waist moves the estimate by roughly 0.8 percentage points. That means a loosely held tape or a relaxed stomach can shift the answer by several points. A single reading matters far less than repeated readings taken the same way.

Worked examples

Example A: man, 175 cm, neck 38 cm, waist 85 cm, 80 kg

Waist minus neck is 47 cm, which gives an estimate of 16.9%. That sits in the ACE "Fitness" band for men (14-17%). Fat mass is about 80 × 16.9% = 13.6 kg and lean mass is about66.4 kg. If the waist shrinks by just 1 cm to 84 cm, the estimate drops to 16.2%.

Example B: woman, 162 cm, neck 32 cm, waist 76 cm, hips 95 cm

Waist plus hips minus neck is 139 cm, giving 28.7%, which falls in the "Average" band for women (25-31%). With hips of 100 cm the estimate becomes 31.2%. This shows a weakness of the method: someone who builds glute muscle and gains hip circumference can be scored as having gained fat.

Limits and error

• Accuracy: compared with DEXA scans or hydrostatic weighing, circumference estimates can be off by several percentage points for an individual. The "±4 points" shown above is a rough guide, not a statistical confidence interval.
• Body shape: people who store fat mostly in the legs, mostly in the abdomen, or who are very muscular can be misjudged.
• Conditions: a large meal, bloating or water retention changes the waist reading, so keep conditions consistent.
• Not suitable for: pregnancy, anyone under 18, marked swelling or edema, or situations where a medically precise measurement is needed.

Bioimpedance scales estimate body fat from electrical resistance and are affected by hydration, so they have their own error. Whichever method you choose, sticking with one method gives the most useful comparisons.

How to use the result

• Measure once or twice a month and log it next to your weight and waist. If your weight is flat but your estimate falls, you may be gaining muscle while losing fat.
• Look at fat mass and lean mass separately. If lean mass drops sharply during weight loss, check protein and strength training with the Protein Intake Calculator.
• To plan the pace of change, use the Calorie Calculator or estimate daily energy needs with the BMR and TDEE Calculator.
• If the result lands in the highest category or your waist keeps growing, discuss it with a doctor, who can look at blood pressure, blood sugar and lipid results.

Important note

This tool offers a rough estimate from tape measurements and does not replace professional assessment. If you have health concerns, or a history of eating disorders, speak with a qualified clinician before setting body composition targets. Your inputs are calculated in your browser and are not sent to a server.

Frequently Asked Questions