Body Mass Index (BMI) is based on two simple measurements: height and weight. The calculation compares your weight with the square of your height, giving you a number that can be used to place your weight into a general BMI category.
For metric measurements, the formula is:
BMI = Weight in kilograms ÷ (Height in meters × Height in meters)
For example, someone who weighs 70 kg and is 1.75 meters tall would have a BMI of about 22.86.
If you use pounds and inches, the formula changes slightly because the measurements come from a different system:
BMI = (Weight in pounds × 703) ÷ Height in inches²
BMI Categories for Adults
Once your BMI has been calculated, the result can be compared with the standard adult BMI ranges below. These categories are commonly used to describe weight status in adults.
| BMI | Weight Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Healthy Weight Range |
| 25.0 to 29.9 | Overweight |
| 30.0 or higher | Obesity |
These ranges are intended for adults. They should not be used to assess children or teenagers in the same way because body composition changes as young people grow.
What Does Your BMI Result Mean?
Your BMI result gives you a general idea of how your weight compares with your height. A BMI between 18.5 and 24.9 is commonly classified as being within the healthy weight range for adults, while results below or above that range fall into different weight categories.
However, BMI does not tell the whole story. Two people with exactly the same BMI can have very different body compositions. For that reason, BMI is most useful when considered alongside other factors rather than viewed on its own.
Why People Use BMI
BMI is popular because it is quick and requires only height and weight. You do not need specialized equipment to calculate it, which makes it convenient for a basic assessment of weight status.
It can also be useful for tracking changes over time. For example, someone monitoring changes in body weight may use BMI as one of several measurements to see how their weight changes relative to their height.
Limitations of BMI
One of the main limitations of BMI is that it cannot distinguish between muscle and body fat. A person with a large amount of muscle may have a higher BMI because muscle contributes to body weight, even if they have a relatively low amount of body fat.
BMI also does not show where fat is stored. Fat distribution can provide additional information about health that a BMI number alone cannot capture.
Age, sex, muscle mass, and other individual characteristics can also affect how meaningful a BMI result is. This is why BMI should be treated as a general indicator rather than a complete picture of someone's health.
Other Measurements to Consider
BMI can be considered alongside other measurements when someone wants a broader view of body composition and weight-related health. Waist circumference is one example because it provides information about abdominal fat that BMI does not show.
Body fat percentage is another measurement that can provide additional information. Unlike BMI, body fat percentage attempts to estimate how much of a person's total body weight comes from fat.
These measurements are not necessarily replacements for BMI. They simply provide different pieces of information that may help create a more complete picture.
BMI for Children and Teenagers
The standard adult BMI categories are not appropriate for children and teenagers. Young people are still developing, and their expected body composition changes with age.
For children and adolescents, BMI is generally interpreted using age- and sex-specific growth charts. A result that may be considered unusual for one age group can be different for another, so adult BMI cutoffs should not be applied directly.
How to Use the BMI Calculator
Enter your height and weight into the appropriate fields and select the measurement units you are using. After the values are entered, the calculator works out your BMI and displays the corresponding result.
Make sure your height and weight are entered correctly. Even a small error in height can affect the calculation because height is squared in the BMI formula.
BMI Calculation Example
Consider an adult who weighs 80 kilograms and is 1.80 meters tall. The calculation would be:
80 ÷ (1.80 × 1.80) = 24.69
The resulting BMI is approximately 24.69, which falls within the commonly used healthy weight range for adults.
Another Example Using Pounds and Inches
Suppose an adult weighs 154 pounds and has a height of 68 inches. Using the imperial BMI formula:
(154 × 703) ÷ 68² = 23.41
The BMI is approximately 23.41, which is within the standard healthy weight range for adults.
Why Height Matters in BMI
Height has an important effect on BMI because the calculation uses height squared. This means BMI is not simply a comparison between weight and height as two equal numbers.
For this reason, entering an accurate height is important when using a BMI calculator. If your height is entered incorrectly, the final result can also be noticeably different.
When a BMI Result Needs More Context
A BMI result that falls outside the standard adult range does not automatically explain why the number is higher or lower. Muscle mass, body composition, age, and fat distribution can all influence the meaning of the result.
If you are using BMI to understand your personal health, consider the result together with other relevant measurements and lifestyle factors. If you have concerns about your weight or health, a qualified healthcare professional can provide advice based on your individual situation.
Important: BMI is a general screening measure and does not provide a diagnosis or a complete assessment of individual health. If you have concerns about your weight, growth, or health, speak with a qualified healthcare professional.
Health Disclaimer
This calculator is provided for general informational and educational purposes only. Its result is an estimate based on the information you enter and should not be considered medical advice, a diagnosis, or a substitute for advice from a qualified healthcare professional. If you have a medical condition, take medication, are pregnant, or have concerns about your health or nutrition, consult a qualified healthcare professional before making decisions based on the result.