What is body fat percentage?

In one line

Body fat percentage is the share of your body weight that is made up of the fat stored in your body.

What is body fat percentage?
Why do people say to watch it, not just weight?
When you diet or take a body composition test, you see a number called body fat percentage. Two people with the same weight get different numbers. What exactly does this number tell you?
In one line
The share of your weight that is fat
Body fat percentage tells you what percent of your body weight is fat. If a person weighs 70kg and has 14kg of fat, their body fat percentage is 20%.
What is body fat?
A group of cells that hold fat
Body fat is the fat stored in your body. It sits as oil droplets inside cells called fat cells. Tissue made of these cells is found mostly right under the skin and around the organs in the belly.
What fat does
An energy store and an insulator
Fat tissue is a store for extra energy. It works as insulation against the cold and cushions organs such as the kidneys against impact. Having some body fat is normal.
When there is too much
Too much fat, higher disease risk
People with a high body fat percentage have a higher risk of heart disease, diabetes, some cancers and early death. So health checks look at how much fat you have, not just your weight.
Men and women differ
At the same height and weight, women are higher
At the same height and weight, women's body fat percentage was about 10 percentage points higher than men's. If a man is at 20%, a woman is at about 30%. That is why cutoffs for high or low body fat are set separately for men and women.
How is it measured?
Fat can't be taken out, so it is estimated
Fat can't be taken out and weighed, so body fat percentage is estimated. Common ways are weighing the body underwater, a low-dose X-ray scan, pinching the skin to measure its thickness, and sending a weak current through the body. Today research treats the X-ray scan as the reference method.
Weight alone can't tell
BMI can't tell fat from muscle
BMI (body mass index), worked out from height and weight, can't tell whether the weight is fat or muscle. So a very muscular person gets a high BMI even with little fat. The amount of fat in the body is found by measuring body fat percentage separately.
A common myth
If your weight is normal, is your fat fine too?
No. Some people have a normal weight but a high body fat percentage, and they had a higher risk of the body problems that lead to heart disease and diabetes. Using BMI to pick out obesity missed more than half of the people who had too much fat.
Summary
Body fat percentage is the fat part of your weight
Body fat percentage is the share of your body weight that is fat. Some people have a normal weight but high body fat, so weight alone does not show how much fat you carry. Related terms: BMI (body mass index), visceral and subcutaneous fat, body composition analysis.