What is BMI?
Body Mass Index (BMI) is a simple screening number that relates your weight to your height. It's calculated as your weight in kilograms divided by the square of your height in metres:
BMI = weight (kg) ÷ height (m)²
BMI categories (adults)
| Category | BMI range |
|---|---|
| Underweight | Below 18.5 |
| Healthy weight | 18.5 – 24.9 |
| Overweight | 25 – 29.9 |
| Obesity | 30 and above |
What BMI does — and doesn't — tell you
BMI is a quick, useful screen for populations, but it has real limits for individuals. It doesn't distinguish muscle from fat, so very muscular people can read as "overweight" while being lean. It also doesn't account for age, sex, or where body fat is stored.
Treat BMI as a starting point for a conversation — not a diagnosis. Waist circumference, body-fat percentage, and a doctor's assessment give a fuller picture.
Your healthy weight range
The result panel shows the weight range that would put your BMI between 18.5 and 24.9 for your height. It is more useful than the single number: instead of a pass/fail, it gives you a band to aim for. A kilogram or two either way is normal and shifts from day to day with water and food.
When BMI reads differently
The adult thresholds above don't fit everyone equally:
- Athletes and muscular builds — muscle is denser than fat, so a lean, strong person can land in "overweight" without carrying excess fat.
- Older adults — a little extra reserve can be protective later in life, and height loss with age changes the maths.
- Children and teens — under-18 BMI is read against age-and-sex percentile charts, not these fixed adult bands, so the categories here don't apply to them.
- People of South and East Asian descent — health risks can rise at a lower BMI, so some guidelines use about 23 rather than 25 as the "overweight" mark.
Moving toward a healthy range
If your goal is to shift your BMI, the real lever is daily energy balance — calories in versus calories out — not the scale on its own. Small, steady changes hold far better than crash diets. It helps to know your daily calorie needs first, then adjust from there gradually.
Tools that pair with this one
- Calorie Calculator — find the daily calories to maintain, lose, or gain weight toward the healthy range above.
This tool is for general information only and is not medical advice. Consult a healthcare professional about your individual health. Formula and categories reviewed July 2026.