Body composition explained
Body fat, lean mass, FFMI and waist-to-height: the measures that show what your weight is actually made of, and why they beat a single number.
Your bodyweight is a single number that hides the most useful information: what that weight is made of. Body composition splits it into fat mass and lean mass, and that split is what actually relates to health, strength and how you look.
The measures worth knowing
- Body fat percentage is how much of you is fat. It is the headline composition number.
- Lean body mass is roughly everything that is not fat, and it tracks your muscle.
- FFMI (fat-free mass index) scales lean mass to height, so it judges muscle fairly regardless of how tall you are.
- Waist-to-height ratio is a quick proxy for the fat that matters most, the fat around your organs.
Why one number misleads
BMI and the scale both move with total weight, so they cannot distinguish fat loss from muscle loss. That is why a muscular person can read "overweight" on BMI, and why someone at a normal weight can still carry a lot of fat. Looking at composition fixes that blind spot.
Use the calculators to get your baseline, then read the guides to understand what the numbers mean.
In this topic
Weight vs body composition: look beyond the scale
The scale only tells you total weight. Body composition shows whether you are losing fat or muscle, which is what actually matters.
What is FFMI, and why it beats BMI for muscle
FFMI scales your lean mass to your height, so unlike BMI it does not penalise muscle. Here is what it means and how to read it.
Body fat percentage explained
What body fat percentage means, what counts as healthy for women and men, and how the common methods of measuring it compare.
Waist-to-height ratio: the simplest health check
One measurement and one rule of thumb. Why waist-to-height ratio is a better quick health screen than BMI for many people.
Common questions
What is body composition?
Body composition is what your weight is made of, mainly fat mass and lean mass (muscle, organs, water and bone). Two people at the same weight can have very different compositions.
Is body fat percentage better than BMI?
For most purposes, yes. BMI only compares weight to height and cannot tell muscle from fat. Body fat percentage measures fat directly, so it gives a clearer picture of health and progress.
What is a healthy body fat percentage?
It depends on age and sex. As a rough guide, many sources put a healthy range around 21 to 33 percent for women and 8 to 20 percent for men, with athletes often lower.
What does FFMI tell me?
FFMI scales your lean mass to your height. Unlike BMI it does not penalise muscle, so it is a useful way to track muscle as you gain or lose weight.