The US Navy method estimates body fat from a few tape measurements. It is less accurate than a DEXA scan, but it needs nothing but a tape measure and it tracks change over time well - which for most purposes matters more than the absolute number being exactly right.
Neck just below the larynx, sloping slightly downward at the front. Waist at the navel for men, at the narrowest point for women. Hips at the widest point. Measure relaxed, not holding your breath in, and keep the tape snug but not compressing the skin.
Typically within 3 to 4 percentage points of a DEXA scan, and it can be further out for very lean or very heavy people. Its real strength is consistency: measured the same way each time, the trend is reliable even when the absolute figure is not.
Because it distinguishes muscle from fat, which BMI cannot. Two people of identical height and weight have identical BMI whether the difference is muscle or fat. Body fat percentage separates them, which is why a muscular athlete flagged as overweight by BMI usually looks entirely normal here.