BMI Calculator

What is Body Mass Index?

Body Mass Index, usually shortened to BMI, is a simple calculation that compares a person's weight with their height. It is widely used as a general screening measure for adult weight status because it requires only two basic measurements.

BMI does not directly measure body fat, muscle, bone density, fitness or overall health. Two people can have the same BMI while having very different body composition and health circumstances.

How to use the BMI Calculator

The calculator supports both metric and imperial units. Select the measurement system you normally use and enter your current height and weight.

In metric mode, enter weight in kilograms and height in centimetres. In imperial mode, enter weight in pounds and height in feet and inches. The result updates using the entered values.

BMI formula

The formula depends on the measurement system being used. Both formulas produce the same BMI when equivalent measurements are entered.

BMI calculation example

Suppose an adult weighs 70 kg and is 170 cm tall. First convert the height to metres:

170 cm = 1.70 metres.

The BMI calculation is:

Under commonly used adult BMI categories, a BMI of approximately 24.2 falls within the 18.5–24.9 healthy-weight range. This category is only a screening reference and does not by itself describe the person's overall health.

Common adult BMI categories

Standard adult BMI categories are commonly used as a broad screening reference. They do not replace an individual health assessment.

What do the BMI numbers 18.5, 25 and 30 mean?

These values are commonly used adult category boundaries. A BMI below 18.5 is generally classified as underweight, while 18.5 to 24.9 is commonly labelled healthy weight.

A BMI from 25.0 to 29.9 is commonly classified as overweight, while a BMI of 30 or above falls within the obesity range in standard adult BMI classification.

These thresholds are population-level screening categories. They should not be interpreted as a complete health assessment for an individual person.

What does the healthy-weight range mean?

The calculator can estimate the body-weight range that corresponds to an adult BMI between 18.5 and 24.9 for the entered height.

For example, if a person is 170 cm tall, the displayed range represents the approximate weights that would produce BMI values within the standard adult healthy-weight category.

This range is a mathematical reference based only on height and BMI thresholds. It should not be treated as a personalised target weight, because appropriate weight and health assessment can depend on many other factors.

How should BMI be interpreted?

BMI is most useful as a quick screening number rather than a final conclusion about health. A healthcare professional may consider BMI together with other information when evaluating health risk.

Other relevant information can include medical history, blood pressure, laboratory results, waist measurement, body composition, physical activity, nutrition and other individual circumstances.

BMI vs body fat percentage

BMI and body fat percentage are not the same thing. BMI is calculated only from height and weight, while body fat percentage attempts to estimate how much of the body is made up of fat tissue.

Because BMI cannot distinguish fat from muscle, a muscular person may have a relatively high BMI without having the same body composition as another person with the same BMI.

Limitations of BMI

BMI is convenient because it is simple, but that simplicity also creates limitations. It does not show where body fat is stored and does not distinguish muscle, fat, bone or other body tissues.

BMI can therefore be less informative when interpreted without additional context.

BMI and people with high muscle mass

Muscle contributes to body weight, so people with significant muscle mass can have a higher BMI even when their body-fat level is not unusually high.

This is one reason BMI should be interpreted alongside body composition, activity level and other relevant information rather than used as a diagnosis.

BMI for children and teenagers

Standard adult BMI categories are not intended for children and teenagers. During growth, body composition changes with age and development.

BMI for younger people is generally interpreted using age- and sex-specific growth charts or percentiles. If a child's growth or weight is a concern, a qualified healthcare professional can provide appropriate assessment.

Pregnancy and older adults

During pregnancy, body weight changes for reasons that standard adult BMI calculation does not account for. BMI calculated during pregnancy should therefore not be interpreted in the same way as a general adult screening result.

In older adults, changes in muscle mass, bone density and body composition can also affect how informative a BMI result is. Clinical context remains important.

BMI is only one part of health assessment

Weight and height alone cannot describe every aspect of health. Someone whose BMI falls within a standard category may still have health considerations that require other measurements or clinical evaluation.

Likewise, a BMI outside the commonly labelled healthy range does not by itself diagnose a disease. It is better understood as one piece of information that may help identify when further assessment could be useful.