Your height-to-weight ratio, or body mass index (BMI), is sometimes used to determine if you're at a healthy weight, but body fat percentage is generally considered a more reliable indicator.
Body fat percentage is a comparison between the amount of your body composed of fat cells and the other parts of your body, such as bones and muscles. "Normal" body fat percentage can refer either to the "ideal" percentage recommended by health experts or the "average" percentage of a general population.
The National Institutes of Health suggest a healthy body fat percentage for men is 13 to 17 percent. The average American man's body fat is 17 to 19 percent.
The NIH guidelines for body fat in women is higher than in men, setting the ideal at 20 to 21 percent. However, the average American woman's body fat percentage is a little higher at 22 to 25 percent.
Athletes often have extremely low body fat percentages. This is usually not dangerous until it reaches between 3 and 6 percent for men and below 12 percent for female athletes.
Lower body fat isn't always better. Being underweight can lead to symptoms such as no menstrual cycle in women and memory problems in both sexes.