BMI Calculator
Body mass index (BMI) relates weight to height. Calculate it to see where a measurement falls within standard adult screening categories.
For adults ages 20 and older. Children and teenagers need BMI-for-age interpretation.
Adult BMI categories
The CDC adult BMI categories apply to ages 20 and older. Thresholds use the unrounded BMI.
| Category | BMI (kg/m²) |
|---|---|
| Underweight | Less than 18.5 |
| Healthy weight | 18.5 to less than 25 |
| Overweight | 25 to less than 30 |
| Obesity | 30 or greater |
The obesity range is further divided into class 1 (30 to less than 35), class 2 (35 to less than 40), and class 3 (40 or greater).
What BMI can and cannot tell you
BMI is a screening measure, not a diagnosis or a direct measurement of body fat. It does not distinguish muscle from fat or describe fat distribution. High muscle mass can contribute to a higher BMI even with low body fat.
A BMI category alone does not determine someone’s overall health. Medical history, physical examination, and other measurements provide context. See the CDC explanation of BMI and its limitations.
Pregnancy needs a different clinical context: current pregnancy weight should not be interpreted using this general adult calculator. The NHS adult BMI guidance also excludes pregnancy from use of its general calculator.
How BMI is calculated
BMI = weight in kilograms ÷ (height in meters)²
Divide centimeters by 100 to obtain meters. In US customary mode, height in inches is feet × 12 + inches. Convert using exactly 0.0254 meters per inch and 0.45359237 kilograms per pound, then use the same formula. BMI has units of kg/m².
Worked example: 70 kg and 175 cm
175 cm = 1.75 m. BMI = 70 ÷ (1.75 × 1.75) = 22.857142… kg/m², displayed as 22.9. The unrounded value is in the healthy weight category.
Frequently asked questions
How do I calculate BMI?
Divide weight in kilograms by height in meters squared. Enter both measurements above to calculate it automatically. A zero height or weight cannot produce a valid adult BMI result.
Do metric and US customary inputs give the same BMI?
Yes, equivalent measurements give the same BMI. This calculator converts pounds and inches to kilograms and meters using exact conversion factors. Switching units preserves full precision; manually entering rounded equivalents can slightly change a result.
What are the limitations of BMI?
BMI describes weight relative to height, not body composition or overall health. Muscle mass, fat distribution, and pregnancy all require context beyond a category label.
Why do children and teenagers need a different interpretation?
Body composition changes during growth. For ages 2 through 19, BMI is interpreted using percentiles specific to age and sex, rather than fixed adult thresholds. Use the CDC Child and Teen BMI Calculator. The CDC BMI FAQs explain the different interpretations.