A measurement, with perspective.
Calculate adult body mass index and understand what the number can—and cannot—tell you.
Your measurements
Adult categories apply from age 20. For younger people, age- and sex-specific growth references are needed.
A screening measure, not a diagnosis.
BMI relates weight to height. It does not directly measure body fat or capture your complete health picture.
| Category | BMI |
|---|---|
| Underweight | Below 18.5 |
| Healthy weight · your result | 18.5 to less than 25 |
| Overweight | 25 to less than 30 |
| Obesity — class 1 | 30 to less than 35 |
| Obesity — class 2 | 35 to less than 40 |
| Obesity — class 3 | 40 or higher |
Classification uses the unrounded value. Near a boundary, the rounded number may look like the adjacent category. Discuss individual interpretation with a health professional.
Your health is bigger
than one calculation.
BMI compares weight with height. It is a screening measure, not a diagnosis, and should be interpreted alongside other health information. The adult reference categories used here are for people aged 20 and older.
The calculator does not ask for a name or save your measurements. If you export a result, review which details you include before sharing it.
Children and teenagers
Adult thresholds do not apply to ages 2–19. Use an age- and sex-specific child and teen BMI calculator instead.
How BMI is calculated
BMI = kg / m²
For example, 70 kg at 1.70 m gives 70 ÷ 1.70² = 24.22. For US inputs, we convert inches to centimetres using 2.54 and pounds to kilograms using 0.45359237 before calculating.
The result is displayed to one decimal place; the category uses full precision. PDF and image reports show two decimals to make boundary cases easier to understand.
Reference
CDC adult BMI categories and CDC adult calculator guidance. Reference checked 11 September 2026.
Questions about interpretation
Why is age required?
It checks that adult reference categories apply. Within the adult range, age does not enter the weight divided by height-squared formula.
Why does switching units slightly change a boundary result?
Unit conversion preserves measurements to three decimal places. Very small rounding differences can affect results exactly on a category boundary. Use your original measurement units where possible.