FreeTools Labs Glossary
Understanding BMI
Learn how BMI relates weight to height, how values are classified, and why it does not directly measure body fat.
Relevant tool
Body mass index (BMI) relates body weight to height. The FreeTools BMI Calculator calculates this classic value only and shows its adult screening category.
What BMI measures
BMI is a mathematical ratio, not a direct measurement of body fat, muscle mass or health.
BMI formula
BMI = body weight in kg ÷ height in m²How the calculator works
Enter height in centimetres and body weight in kilograms. The calculator converts height to metres and calculates the unrounded BMI.
The category uses that unrounded value. The one-decimal number is display-only, so display rounding cannot by itself move a result across a threshold.
Adult categories
Below 18.5: Underweight; 18.5 to under 25: Healthy weight; 25 to under 30: Overweight; 30 and above: Obesity.
These thresholds are intended here only for adults aged 20 and over. This calculator does not provide child or adolescent percentiles.
Example
At 180 cm and 80 kg, the calculation is 80 ÷ 1.80² = 24.69…. The displayed result is 24.7 and the category remains Healthy weight because the unrounded value is below 25.
Limits
BMI is a screening measure, not a diagnosis. It does not directly distinguish fat mass from muscle mass and cannot provide a complete individual health assessment.
For additional body-composition values explicitly labelled as estimates, use the separate Body Metrics & Body Fat Calculator.
What the calculator does not model
It does not use sex, activity profile, reference weight or weight difference, and it does not create an additional health category.
Sources and foundations
- CDC – Adult BMI Categories — Centers for Disease Control and Prevention (External link)
Adult scope, BMI categories and screening context.