How to Calculate BMI

What is BMI?

Body Mass Index, commonly known as BMI, is a numerical measure calculated from a person's height and weight. It was developed in the 19th century by Belgian mathematician Adolphe Quetelet and has been used by healthcare professionals worldwide as a simple screening tool to assess whether a person's weight is healthy for their height.

BMI is not a direct measure of body fat. It is a proxy — an estimate that correlates reasonably well with body fat percentage in large populations, which is why it has become so widely used in public health contexts. It requires only two measurements (height and weight) and produces an instantly interpretable number, making it practical for clinical and population-level assessments.

How to Calculate BMI

BMI is calculated by dividing a person's weight in kilograms by the square of their height in metres. The formula is: BMI = weight (kg) ÷ height (m)².

For example, a person who weighs 70kg and is 1.75m tall has a BMI of: 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.86. This falls in the healthy weight range.

In imperial units the formula is: BMI = (weight in pounds × 703) ÷ (height in inches)². For a person who weighs 154 pounds and is 68 inches tall: (154 × 703) ÷ (68 × 68) = 108,262 ÷ 4,624 = 23.4.

Rather than calculating manually, use the ToolBullet BMI Calculator to get an instant result in metric or imperial units.

BMI Categories

The World Health Organisation (WHO) defines the following BMI categories for adults:

Underweight: Below 18.5. May indicate nutritional deficiency or underlying health conditions. People in this range are at increased risk of osteoporosis, immune system problems, and fertility issues.

Healthy weight: 18.5 to 24.9. Associated with lowest risk of weight-related health problems for most adults. This is the target range for most people.

Overweight: 25 to 29.9. Increased risk of developing conditions including type 2 diabetes, heart disease, and certain cancers. Weight management strategies are generally recommended.

Obese: 30 and above. Significantly increased health risks. Often sub-divided into Class 1 (30–34.9), Class 2 (35–39.9), and Class 3 (40+) obesity.

What is a Healthy BMI?

For most adults, a BMI between 18.5 and 24.9 is considered healthy. However, the ideal BMI within this range varies by individual. Some people naturally sit at the lower end of the healthy range while others sit at the higher end — both can be equally healthy depending on body composition, muscle mass, age, and other factors.

For people of Asian descent, including those in India, China, Japan, Singapore, South Korea, and other parts of Asia, health risks associated with excess weight occur at lower BMI values. Many Asian healthcare guidelines recommend taking action at a BMI of 23 or above, rather than the standard 25. This is because research shows that people of Asian ethnicity tend to have higher body fat percentages at the same BMI compared to people of European descent.

Limitations of BMI

BMI is a useful screening tool but it has well-documented limitations. The most significant is that it does not distinguish between muscle mass and fat mass. A professional athlete or bodybuilder may have a BMI in the overweight or obese range despite having very low body fat and excellent cardiovascular health. Conversely, an inactive person with a normal BMI might have a high body fat percentage and elevated health risks — a condition sometimes called "normal weight obesity" or "skinny fat".

BMI also does not account for where fat is distributed in the body. Abdominal fat (fat stored around the waist) is associated with significantly higher health risks than fat stored in other areas. Waist circumference is therefore often measured alongside BMI to give a more complete picture. A waist measurement of more than 88cm (35 inches) in women or 102cm (40 inches) in men indicates increased health risk regardless of BMI.

Age affects the relationship between BMI and health. Older adults tend to have more body fat at the same BMI compared to younger adults, and slightly higher BMI values (23–27) in people over 65 may actually be associated with better health outcomes. Children and adolescents use age and sex-specific BMI charts rather than the adult categories above.

Despite these limitations, BMI remains widely used because it is quick, free, and requires no specialist equipment. For most people in the general population, it provides a reasonable indication of whether weight may be affecting health — but it should always be interpreted alongside other information and in consultation with a healthcare professional for clinical decisions.

What to Do With Your BMI Result

If your BMI falls in the healthy range, it is reassuring — but maintaining that through a balanced diet and regular physical activity remains important. If your BMI is in the overweight or obese range, it suggests that weight management could reduce your risk of several health conditions. A GP or healthcare provider can help you develop a realistic and safe approach.

If your BMI is in the underweight range, it is worth speaking to a healthcare professional to understand the cause and consider whether nutritional support or other interventions are appropriate. Underweight is less commonly discussed but carries its own health risks.

Remember that BMI is one data point, not a diagnosis. Use it as a starting point for a conversation with your doctor, not as a definitive statement about your health.