Body fat calculator

The calculator estimates what percentage of your weight is fat, using a few circumferences and your height. The method is based on the Hodgdon-Beckett formula. In men the neck and the waist are used, in women the hips are added, where fat is deposited differently.

The result is an estimate, not a measurement. Compared with reference methods such as hydrostatic weighing, the formula can be off by a few percent, and the error grows at high values [1]. The figure is still useful as a starting point and as a trend over time (with the same tape measure). The badge groups those five ranges into three. Below 6% in men and below 14% in women it reads Low range, which covers only essential fat. From there up to 25% in men and 32% in women it reads Reference range, and above those it reads Obesity range.

You need a flexible tailor's tape and a mirror. Keep the tape horizontal, flat against the skin, but without pulling it tight. Measure in the morning, before a meal, then write the figures down so you can compare them in a few months.

• the neck — just below the Adam's apple, with the tape sloping slightly forward;
• the waist — in men at the level of the navel, in women at the narrowest point;
• the hips — in women, at the widest point, over the buttocks;
• height — barefoot, with your back against the wall.

Measure twice and use the average. Breathe normally and do not pull your stomach in while you measure, otherwise the figure comes out falsely better.

Reference values differ between men and women, because a woman's body needs more essential fat. The ranges in common use are:

• essential fat — 2–5% in men, 10–13% in women;
• athletic range — 6–13% in men, 14–20% in women;
• good range — 14–17% in men, 21–24% in women;
• acceptable range — 18–24% in men, 25–31% in women;
• obesity range — 25% or more in men, 32% or more in women.

The colored badge under the result groups these steps into three, so they are easy to read. Below essential fat the body no longer has the reserve it needs for hormones and for temperature, so an extremely low percentage is not a good result.

It is not the total amount of fat that matters most, but where it sits. Fat around the abdominal organs releases fatty acids and inflammatory substances straight to the liver. This raises insulin resistance far more strongly compared with the subcutaneous fat on the thighs.

That is why the waist sometimes says more compared with the scales. Two people with the same body mass index can have very different risks, depending on how much of their fat sits in the abdomen. Weight loss reduces abdominal fat first, so the benefit for blood glucose appears quickly.