Your body fat percentage from a tape measure — the US Navy method.
Men can ignore the hip field — the Navy formula for men uses waist and neck only.
It estimates body fat from circumference measurements instead of weight. For men it uses waist and neck; for women it adds the hip. The formula combines the waist-minus-neck difference (plus hip for women) with height, using base-10 logarithms, because body volume grows in a predictable way with those circumferences.
Studies put it within about 3–4% of underwater weighing for most people, which is good for a free tape-measure method. Very muscular people can read a little high, and exactly where you measure makes a difference, so track the trend over time rather than one single reading.
Measure the neck just below the Adam's apple, the waist at navel level for men or the narrowest point for women, and the hip at the widest point. Keep the tape snug but not tight, stand relaxed, and measure in the morning before eating for the most consistent results.
For men, roughly 14–17% is considered fit and 18–24% average; for women, roughly 21–24% is fit and 25–31% average. Athletes often sit lower. Very low levels (under 5% for men, under 13% for women) are not healthy for most people.