What is BMI?
Body Mass Index (BMI) is a simple screening measurement that estimates whether your weight is healthy for your height. It's calculated by dividing your weight by the square of your height, and is used worldwide by doctors, insurers, and fitness professionals as a quick first check — not a diagnosis. Because it only uses two numbers, BMI can't tell the difference between muscle and fat, which is why it works best as a starting point rather than the final word on your health.
How is BMI Calculated?
The formula is straightforward. In metric units: BMI = weight (kg) ÷ height (m)². In imperial units: BMI = 703 × weight (lbs) ÷ height (in)². For example, someone weighing 70 kg at 1.75 m would have a BMI of 70 ÷ (1.75 × 1.75) = 22.9 — right in the middle of the "Normal weight" range. Our calculator does this instantly, and also converts freely between metric and imperial so you don't have to do the maths yourself.
BMI Categories & Chart
The World Health Organization defines four standard adult BMI categories:
| Category | BMI Range |
|---|---|
| Underweight | Below 18.5 |
| Normal weight | 18.5 – 24.9 |
| Overweight | 25.0 – 29.9 |
| Obese | 30.0 and above |
These bands are what our color-coded bar above is based on, so you can see at a glance where your result sits.
Population-specific interpretation
This calculator uses the standard adult thresholds 18.5, 25, and 30. It does not implement an Indian or Asian diagnostic classification. Risk can differ by ancestry and individual factors; a clinician may use different action thresholds and additional measurements. A category label alone does not establish a person's health or the right weight target.
BMI for Men vs. Women
The BMI formula itself doesn't change between men and women, but its interpretation can. Women naturally carry a higher percentage of body fat than men at the same BMI, while men tend to carry more muscle mass — which is denser than fat and can push BMI higher without extra body fat. Neither effect is large enough to need a separate formula, but it's a useful reason not to over-interpret a single number.
Limitations of BMI
BMI cannot distinguish muscle from fat or show fat distribution. This adult calculator is not for children, adolescents, or pregnancy. Interpretation in older adults and athletes needs additional context. The displayed weight interval is a mathematical reference band, not a prescription.
Reference and scope of check
The WHO obesity and overweight fact sheet, checked on 2026-10-15, supports the BMI formula and adult overweight/obesity thresholds of 25 and 30. The implementation also has arithmetic regression tests; neither is clinical validation or a personalized health assessment.