Body Mass Index in seconds. Switch between metric and imperial — nothing leaves your browser.
Colours follow the WHO BMI categories shown in the guide below.
How BMI is calculated
Body Mass Index (BMI) is a simple ratio of weight to height. In metric units the formula is weight in kilograms divided by height in meters squared: BMI = kg ÷ m². In imperial units it is weight in pounds times 703 divided by height in inches squared. The calculator handles both systems and converts behind the scenes, so you can enter your numbers exactly as you know them.
The result is then mapped to the standard World Health Organization categories: underweight (below 18.5), normal (18.5–24.9), overweight (25–29.9) and obese (30 and above).
Common uses
- Health check-ins — track where you sit relative to the population average.
- Fitness goals — set a target weight that lands you in the healthy range.
- Screening — a quick, free first step before a more detailed assessment with a clinician.
- Conversations with doctors — bring a number to discuss weight-related risks.
Frequently asked questions
What is a healthy BMI?
For most adults a BMI between 18.5 and 24.9 is considered normal weight.
Is BMI accurate for everyone?
Not perfectly. BMI ignores muscle mass, so athletes and very muscular people can read as “overweight” despite low body fat. It also doesn’t distinguish fat distribution, which matters for health risk.
Does it work for children?
No. Children’s BMI is interpreted against age- and sex-specific growth charts, not the adult cut-offs shown here.
Why does the result show a range too?
The healthy-weight range tells you the weight that would put a given height into the normal BMI band, which is often more actionable than a single number.
Should I worry about small BMI changes?
Day-to-day weight fluctuates with water and food, so a BMI that moves a fraction of a point isn’t meaningful. Look at the trend over months, not single readings.
WHO category bands for adults
| BMI | Category | What it signals |
|---|---|---|
| Below 16.0 | Severe thinness | Significant underweight |
| 16.0 – 18.4 | Underweight | Below the healthy range |
| 18.5 – 24.9 | Healthy range | Lowest associated risk |
| 25.0 – 29.9 | Overweight | Elevated risk; some countries use 23 |
| 30.0 – 34.9 | Obesity I | Substantially elevated risk |
| 35.0 – 39.9 | Obesity II | High risk |
| 40.0 + | Obesity III | Very high risk |