How BMI is calculated and what it can and cannot tell you
Body mass index is a quick number built from height and weight. It is useful as a rough screening tool for groups of people, and much less useful as a verdict on one person’s health.
Open the BMI CalculatorThe formula
BMI = weight (kg) ÷ height (m)²BMI = 703 × weight (lb) ÷ height (in)²
The idea dates back to the nineteenth-century Belgian mathematician Adolphe Quetelet, who looked for a way to compare body size across a population. Squaring height scales weight to body size so taller people are not automatically given a higher number.
Example: 70 kg at 175 cm. Height in metres is 1.75 and 1.75² = 3.0625. BMI = 70 ÷ 3.0625 = 22.9.
The adult categories
The commonly published World Health Organization categories for adults are:
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Healthy weight |
| 25.0 to 29.9 | Overweight |
| 30.0 and above | Obesity |
These are statistical bands drawn from population studies. They are not sharp lines: a BMI of 24.9 and one of 25.0 describe nearly identical bodies.
What BMI cannot tell you
- Body composition. BMI cannot separate muscle, fat and bone. A strength athlete can land in the overweight range with little body fat.
- Where fat is stored. Fat around the abdomen is associated with different risks than fat elsewhere, and BMI does not see the difference.
- Age and sex. Body composition changes with age, and men and women differ on average.
- Ethnic background. Some health bodies have noted that risks can begin at lower BMI values in some Asian populations, so thresholds may differ in some guidelines.
- Children and teenagers. Their BMI is compared with growth charts for their age and sex, not with adult cut-offs.
How to use BMI sensibly
Think of BMI as a starting point for a conversation, not a result. If your number is outside the “healthy” band, that is a reason to look at other measures, such as waist circumference, blood pressure and blood tests, with a healthcare professional. If your number is inside the band, that does not guarantee good health either.
If you track BMI over time, measure under similar conditions. See the guide to measuring height and weight accurately.
Using the calculator
The SizeKit BMI calculator accepts metric or imperial units, shows the category and also shows the weight range that corresponds to an 18.5–24.9 BMI at your height. That range is a reference, not a target. Nothing you enter is sent anywhere.
SizeKit is not a medical service. For questions about your weight or health, speak with a doctor, nurse or registered dietitian.