By far the easiest and least expensive way to determine body fat percentage is by using a tape measure. There are 2 formulas commonly used with these measurements – the YMCA formula and the Navy formula. The YMCA formula figures out body fat percentage using only your weight and waist measurement. The Navy formula relies on measurements of your neck and abdomen as well and is thought to provide a more accurate calculation of body fat, as it takes into account fat around the hip area.