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US Navy tape method: an estimate typically within 3-4% of lab methods. Measure the waist at the navel (relaxed, not sucked in) and the neck below the larynx. A general fitness guide, not medical advice or a diagnosis.
What The Body Fat Calculator Works Out
- Estimated body fat: the US Navy tape method, which combines waist and neck (plus hip for women) with height in a logarithmic formula. Developed across thousands of service members, it typically lands within 3% to 4% of laboratory methods. Measure the waist at the navel, relaxed; the neck just below the larynx; and always at the same time of day.
- Category: American Council on Exercise reference ranges. For men, 6% to 13% reads as athletic, 14% to 17% fitness and 18% to 24% average; for women, whose essential fat is naturally higher, 14% to 20% athletic, 21% to 24% fitness and 25% to 31% average.
- Fat mass (80 kg example): what the percentage means in kilograms for a reference 80 kg body, splitting it into fat mass and lean mass. Body fat answers a question BMI cannot, distinguishing a muscular 90 kg from a sedentary one.
- Measurements entered in (advanced): switch to inches if your tape measure is imperial; everything is converted at 2.54 cm per inch before the formula runs.
- Your weight (advanced): in kg, stones or pounds, so the fat and lean mass figures are yours rather than the 80 kg example. Tracked over time, a falling percentage alongside stable weight means fat lost and muscle kept.
- BMI-based estimate (advanced): the Deurenberg formula (1.2 x BMI + 0.23 x age - 10.8 for men - 5.4) gives a second opinion from weight, height and age alone. It is less accurate than the tape method, especially for muscular people, but a big gap between the two is worth noting.
- Weight at target body fat (advanced): the body weight you would be at your chosen percentage if lean mass stayed the same, with the difference shown as fat to lose or gain and a rough timescale at 0.5 kg a week. The category table shows the fat mass each band represents at your weight.
- Caveats: tape estimates drift most at the extremes (very lean or very heavy physiques) and single readings matter less than the trend, so measure monthly under the same conditions rather than daily. Smart scales using bioelectrical impedance swing with hydration; the tape method is at least consistent. This is a general fitness guide, not medical advice, and anyone making significant changes to diet or training with a health condition should speak to a GP first.
Source and further reading: NHS: managing a healthy weight.