Body mass index, or BMI, is a measure of body size. It combines a person’s weight with their height. The results of a BMI measurement can give an idea about whether a person has the correct weight for their height.
BMI is a screening tool that can indicate whether a person is underweight or if they have a healthy weight, excess weight, or obesity. If a person’s BMI is outside of the healthy range, their health risks may increase significantly.
BMI does not measure body fat directly, and it does not account for age, sex, ethnicity, or muscle mass in adults.
However, it uses standard weight status categories that can help doctors to track weight status across populations and identify potential issues in individuals.
Calculating BMI involves measuring a person’s height and body weight.
- To calculate BMI in metric units, use the following method: BMI = kg/m2
- So, to calculate an adult’s BMI: Divide their weight in kilograms (kg) by the square of their height in meters (m2)
Since most people measure height in centimeters (cm), divide height in cm by 100 to get height in meters.
- When using imperial units, the formula is: BMI = lbs x 703/in2
- In other words: Multiply a person’s weight in pounds (lbs) by 703. Then divide by their height in inches, squared (in2)
To avoid using the math, a person can use a calculator or a chart to find their BMI.
Enter height or weight in either imperial or metric measurements to find your BMI.
People can also work out their BMI using a chart. Click here to see a chart provided by the National Heart, Lung, and Blood Institute (NHLBI).
Locate your height in inches on the side of the chart, then look across to find your body weight in pounds. Scan to the top to see if the result corresponds to a normal weight, overweight, or obesity.
The following table shows standard weight status categories associated with BMI ranges for adults.
|30.0 and above||Obese|
A BMI of less than 18.5 indicates that you are underweight, so you may need to put on some weight. You are recommended to ask your doctor or a dietitian for advice.
A BMI of 18.5-24.9 indicates that you are at a healthy weight for your height. By maintaining a healthy weight, you lower your risk of developing serious health problems.
A BMI of 25-29.9 indicates that you are slightly overweight. You may be advised to lose some weight for health reasons. You are recommended to talk to your doctor or a dietitian for advice.
A BMI of over 30 indicates that you are heavily overweight. Your health may be at risk if you do not lose weight. You are recommended to talk to your doctor or a dietitian for advice.
In adults, BMI values are not linked to age and are the same for both sexes.
However, measuring BMI in children and teens is slightly different. Girls and boys develop at different rates and have different amounts of body fat at different ages. For this reason, BMI measurements during childhood and adolescence take age and sex into consideration.
Doctors and other health professionals do not categorize children by healthy weight ranges because:
- they change with each month of age
- male and female body types change at different rates
- they change as the child grows taller
Doctors calculate BMI for children and teens in the same way as they do for adults, by measuring height and weight. Then they locate the BMI number and person’s age on a sex-specific BMI-for-age chart. This will indicate whether the child is within a healthy range.
Calculator and charts for child and teen BMI
The Centers for Disease Control and Prevention (CDC) have produced a calculator that provides BMI and the corresponding BMI-for-age percentile on a CDC growth chart for children and teens.
First, click here for the calculator.
Next, use the charts to see if a child’s weight is suitable for their age.
Click here for the charts:
What do the results mean?
The following categories explain the meaning of the results:
|Weight status category||Percentile range|
|Underweight||Below the 5th percentile|
|Healthy weight||5th percentile to less than the 85th percentile|
|Overweight||85th to less than the 95th percentile|
|Obesity||Equal to or greater than the 95th percentile|
BMI is not accurate enough to use as a diagnostic tool, but it can screen for potential weight problems in adults and children.
If someone has a high or low BMI, a doctor or other healthcare professional might then consider other factors, such as:
- skinfold thickness measurements, which indicate how much fat is in the body in adults and children
- evaluations of diet and physical activity
- discuss any family history of cardiovascular disease and other health problems
- recommend other appropriate health screenings
The doctor or healthcare professional can then make diet and exercise recommendations based on these results.
Excess weight has the following effects on the body:
- It increases how hard the heart has to work.
- It raises blood pressure, blood cholesterol and triglyceride levels.
- It lowers high-density lipoprotein (HDL), or good cholesterol levels.
- It can make diabetes and other health problems more likely.
According to the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), carrying extra weight can increase the risk of the following conditions:
- hypertension, or high blood pressure
- dyslipidemia, which involves high LDL cholesterol, low HDL cholesterol, or high levels of triglycerides
- type 2 diabetes
- coronary heart disease
- gallbladder disease
- sleep apnea and respiratory problems
- some cancers, including endometrial, breast and colon cancer
Carrying extra weight as a child or teenager can also pose significant health risks, both during childhood and into adulthood.
As with adult obesity, childhood obesity increases the risk of various health problems, including cardiovascular disease, diabetes, and sleep apnea.
The American Heart Association (AHA), point out that children with a high BMI also have a higher risk of:
Benefits of a healthy weight
Apart from reducing the risk of the health conditions, maintaining a healthy weight offers additional benefits:
- fewer joint and muscle pains
- increased energy and ability to join in more activities
- improved regulation of bodily fluids and blood pressure
- reduced burden on the heart and circulatory system
- improved sleep patterns
BMI is a useful tool, but it cannot identify whether a person’s weight is made up of muscle or fat.
For example, an athlete with a lot of muscle tissue may have a higher BMI than a person who is not very active. But, this does not mean that the athlete is overweight or unhealthy.
In addition, cardiovascular disease and high blood pressure are more likely to occur in people who have additional fat — known as visceral fat — around their middle rather than their hips.
Other measures of body size include waist-hip ratio, waist-to-height ratio, and body composition, which measures body fat and lean body mass. These measurement systems focus more on the amount of fat a person has and its distribution around the body.
Together with BMI, these additional measures can help to assess more accurately the health risks associated with an individual’s weight.
BMI can be a useful screening tool for predicting certain health risks. However, people should use it with caution, as it does not take other factors — such as activity levels and body composition — into account.
For children and teens, it is important to include their age and sex when taking a BMI measurement, because their bodies continuously change as they develop.