Why it beats the scale
A scale cannot tell whether a kilogram lost was fat or muscle. Body fat percentage can, because it measures the composition of your weight rather than just its total. A person whose weight is unchanged but whose body fat has fallen has made real progress the scale hides.
Healthy ranges
For general health, roughly 14–24% is a reasonable band for men and 21–31% for women. Athletic ranges are lower: about 6–13% for men and 14–20% for women. Essential fat — the minimum needed for basic physiology — is about 2–5% for men and 10–13% for women, and going below it is unsafe.
How it is measured
Common methods, from cheapest to most accurate: tape-based circumference formulas (Navy, Army), bioelectrical impedance scales and handheld devices, skinfold callipers, and laboratory methods such as DEXA and hydrostatic weighing. Each trades cost and convenience against precision.
Accuracy in practice
Tape methods and consumer scales are typically within 3–5 percentage points of a laboratory measurement, and impedance scales can swing several points with hydration. The absolute number matters less than consistency: measure the same way at the same time and watch the trend.
Using it as a goal
When losing weight, aim to lose fat while holding lean mass. Track both: if lean mass falls steeply, the deficit is too aggressive, protein too low, or resistance training missing. Combining a moderate deficit with high protein and lifting is the reliable way to change body composition rather than just weight.