Body Fat Calculator

What this tool does

The Body Fat Calculator uses the US Navy circumference equations (Hodgdon and Beckett, 1984) to estimate body fat from height, neck, waist and, for women, hip measurements. It splits your weight into fat and lean mass, places the result in an American Council on Exercise category and shows a BMI-based Deurenberg estimate for comparison.

Two people with the same weight and BMI can look and perform very differently because their bodies hold different amounts of fat and muscle. Body fat percentage captures that difference. Laboratory methods such as DEXA or hydrostatic weighing are accurate but not something you can do at home, and bathroom bioimpedance scales swing with hydration. A tape measure, used carefully, offers a repeatable middle ground.

The US Navy method was developed by Hodgdon and Beckett to screen personnel quickly. It uses logarithms of circumference differences and height: for men, waist minus neck; for women, waist plus hip minus neck. The equations are entered here in their metric form, with imperial measurements converted first. Studies report a typical error of about 3 to 4 percentage points against DEXA, so it is best for tracking change over time with the same tape and technique.

How to use Body Fat Calculator

1. Choose units and sex, and enter your age, weight and height.
2. Measure your neck just below the larynx, keeping the tape slightly sloped downwards to the front.
3. Measure your waist: men at the navel, women at the narrowest point, after a normal breath out.
4. Women: measure the hips at the widest part of the buttocks.
5. Enter the measurements and read your body fat percentage and ACE category.
6. Check fat mass versus lean mass in the chart, and note the Deurenberg BMI estimate for comparison.

Tips for better results

• Take each measurement three times and use the average.
• Measure at the same time of day, ideally in the morning before eating.
• Keep the tape snug but not compressing the skin, and parallel to the floor for waist and hips.
• Re-measure every two to four weeks; weekly changes are usually smaller than the measurement error.

Common use cases

• Tracking body recomposition when the scale barely moves but your clothes fit differently.
• Estimating lean mass to use in the Katch-McArdle BMR formula.
• Checking military or police fitness standards that use the Navy tape method.
• Giving clients a low-cost baseline before a training programme.

Common mistakes to avoid

• Sucking in the stomach or measuring after a large meal, which shifts the waist reading by centimetres.
• Mixing inches and centimetres between measurements.
• Comparing a tape-based number directly with a smart-scale reading; each method has its own bias.