How to measure correctly
Measure in the morning, before eating, with a non-stretch tape. The tape should touch the skin without compressing it and stay parallel to the floor. Do not suck in your stomach; measure at the end of a normal breath out.
Neck: just below the larynx (Adam's apple), with the tape sloping slightly down toward the front. Waist: at the navel for men, at the narrowest point for women. Hips (women): at the widest point of the buttocks. Repeating each measurement two or three times and averaging reduces error.
The US Navy formula
This method was developed in 1984 by Hodgdon and Beckett for the US Navy and validated against underwater weighing. The metric version is:
Men: body fat % = 495 / (1.0324 - 0.19077 x log10(waist - neck) + 0.15456 x log10(height)) - 450 Women: body fat % = 495 / (1.29579 - 0.35004 x log10(waist + hip - neck) + 0.22100 x log10(height)) - 450
If you enter inches, the values are converted to centimeters first. With your weight, fat mass = weight x body fat %, and lean mass = weight - fat mass.
What does your result mean?
Using the American Council on Exercise (ACE) categories, for men 2-5% is essential fat, 6-13% athletes, 14-17% fitness, 18-24% average and 25% or more obese. For women 10-13% is essential fat, 14-20% athletes, 21-24% fitness, 25-31% average and 32% or more obese.
Essential fat is the minimum needed for hormone production and protecting organs; going below it is unhealthy. Women naturally carry more essential fat for reproductive health. Tracking body fat while losing weight shows you the muscle gain and fat loss that the scale hides.
Limitations
The US Navy method can differ from lab methods such as DEXA by around 3-4 percentage points on average. Even small differences in technique can move the result by a few points, so look at the trend across measurements taken under the same conditions rather than a single number.
It can slightly overestimate body fat in people who store fat around the abdomen, and slightly underestimate it in athletes with very muscular necks. It should not be used during pregnancy or under 18. For a more precise result, consider DEXA, underwater weighing or skinfold measurement by an experienced professional.
When to see a doctor
If your body fat is above 25% for men or 32% for women, especially with a large waist, it is worth getting checked for insulin resistance, fatty liver and cardiovascular risk. On the other hand, values close to essential fat are linked to missed periods, hormonal problems and bone loss; in that case also seek professional support.