Body Fat Calculator

You need a soft tape measure and about five minutes. Measure each site twice and enter the average, because a centimetre of error at the waist moves the answer noticeably.

1. Choose your Sex, Male or Female. The formula and the category table differ between the two.
2. Set Units to Metric (cm, kg) or Imperial (in, lb). Every length field then uses centimetres or inches together.
3. Enter your Age, Height and Weight. Age is used only for the BMI-based comparison figure, not for the main estimate.
4. Enter your Neck circumference, taken just below the larynx, and your Waist at the navel, taken relaxed at the end of a normal breath out.
5. If you chose Female, also fill in Hip at the widest point, around the fullest part of the buttocks. Men can leave this field alone because the male formula does not use it.
6. Read the estimated body fat percentage and its category, then the fat mass, lean mass, BMI and waist to height ratio listed beside it.

The method was published by Hodgdon and Beckett in 1984 for the US Navy, which needed a way to screen personnel without callipers or scanners. Lengths are converted to centimetres and put into one of two equations built on base-10 logarithms:

• Men: %BF = 495 / (1.0324 - 0.19077 x log10(waist - neck) + 0.15456 x log10(height)) - 450
• Women: %BF = 495 / (1.29579 - 0.35004 x log10(waist + hip - neck) + 0.22100 x log10(height)) - 450

Take a man aged 30, 175 cm tall, weighing 75 kg, with a 38 cm neck and an 85 cm waist. The waist minus neck figure is 47 cm, and the equation returns 16.9 percent, which falls in the Fitness category. Fat mass is 75 kg x 16.9% = 12.7 kg and lean mass is 62.3 kg. His BMI is 24.5, so the Deurenberg comparison (1.20 x BMI + 0.23 x age - 10.8 for men - 5.4) gives 20.1 percent. The three-point gap is normal: the BMI formula guesses from height and weight, while the tape-measure method sees where the weight is carried.

Now a woman aged 30, 165 cm, 62 kg, with a 32 cm neck, 72 cm waist and 98 cm hips. The result is 27.4 percent (Average), with 17.0 kg of fat and 45.0 kg of lean mass. The calculator labels the result with the American Council on Exercise categories:

Most of the error in a circumference estimate comes from the tape, not the maths. Keep the tape level and snug without pressing into the skin, measure at the same time of day, and do not pull in your stomach. The neck goes just below the larynx and the waist sits level with the navel. If you track change over weeks, repeat the exact same routine each time, and compare your own results with each other rather than with a friend.

The tool also shows a waist to height ratio, which is the waist divided by height. In the first example it is 85 / 175 = 0.49. Health screening guidance often treats a ratio at or above 0.5 as a prompt to look more closely, so it is a useful second signal that needs no formula at all. Because it uses only the waist and height, it also stays meaningful when the other measurements are slightly off.

Expect a margin of error. Even at its most favourable, a tape-measure estimate lands within a few percentage points of a DEXA scan, and it is less reliable for very muscular or very lean people, whose neck and waist measurements do not follow the average pattern. If the measurements are impossible, for example a neck larger than the waist, or a result outside roughly 2 to 65 percent, the tool shows a warning rather than a number. Check each figure and the unit you chose.

The result is an estimate for general information and is not medical advice or a diagnosis. For related figures, try the BMI calculator, the ideal weight calculator or the calorie calculator, and use the length converter if your tape is marked in a unit you did not expect.