Two estimates with different jobs
BMI relates body weight to height. It is quick, widely used, and helpful for broad population screening, but it does not separate fat mass from muscle, bone, or water.
Body-fat estimates aim to describe how much body weight is fat tissue. Circumference formulas are convenient, yet they remain estimates and depend heavily on measurement technique.
When BMI is useful
BMI can provide a simple reference range and can help identify when weight-related health discussion may be worthwhile. It is best interpreted alongside waist measurement, health history, fitness, and clinical context.
When body-fat trends help
A consistently measured body-fat estimate may help track body-composition direction during training or nutrition changes. Use the same method, measuring sites, time of day, and conditions whenever possible.
A better decision rule
Do not search for one perfect number. Combine BMI, waist-to-height ratio, performance, habits, and professional assessment when the decision affects health.