What Is Body Fat Percentage? The Navy Method Explained

Body weight is a single number made of many things: fat, muscle, bone, organs and water. Body fat percentage splits out one of them. It is the weight of fat divided by total weight, so a person who weighs 80 kg and carries 12.9 kg of fat has a body fat percentage of 16.1. What remains, 67.1 kg in that case, is everything that is not fat, often called lean mass.

The same weight can hide very different mixes, which is the reason the measure exists. Two people of equal height and weight can have different body fat percentages, and the scale alone cannot tell them apart.

How CalorTracker estimates it

Measuring body fat directly takes specialised equipment. The calculator uses the U.S. Navy circumference method instead, from Hodgdon and Beckett's 1984 reports, which predicts body fat from a tape measure and a height. For men it uses height, neck and waist. For women it adds the hip. All measurements are in centimetres, the result is rounded to one decimal place, and if the inputs do not give a valid number the calculator shows none instead of a wrong one. The formulas are written out on how we calculate, and the report is listed on sources.

The method is a prediction from a few circumferences. It does not see inside the body, so it gives an estimate with a margin of error.

Worked example

Use the man from the BMR page: 80 kg and 180 cm tall, with a neck of 38 cm and a waist of 85 cm. The body fat calculator gives:

Fat mass is simply the percentage applied to total weight, and lean mass is what is left. Re-measuring after some weeks with the same tape and the same technique tells you more than the first number, because a consistent error in the method tends to repeat and so shows up less in the change than in the level.

Why it appears elsewhere

Some equations for BMR use lean mass rather than total weight, on the idea that lean tissue accounts for more of the energy used at rest. CalorTracker's default is the Mifflin-St Jeor equation, which uses weight, height, age and sex, so body fat is an optional extra for those who want to track it. A body fat estimate is also a different question from ideal body weight, which looks only at height.

What the number does not tell you

Tape-measure methods are estimates. They can be off by several percentage points compared with a DEXA scan, and the result is sensitive to where exactly you place the tape and how relaxed you are. Treat a single reading as a rough guide and look at the trend across measurements taken the same way.

A percentage is also not a verdict about anyone's health or fitness. Healthy ranges differ by age and sex, and a calculator cannot judge them for you. If a body-composition number worries you, a doctor or a trained professional with proper equipment is the right place to take it.