Body mass index, or BMI, is a measure of a person’s weight relative to their height that some experts use to estimate health status. As an indicator of overall health, BMI has several flaws.
A note about sex and gender
Sex and gender exist on spectrums. This article will use the terms “male,” “female,” or both to refer to sex assigned at birth. Click here to learn more.
BMI is a screening tool that some doctors use to asses how a person’s weight relates to their age and height. It uses numerical ranges to determine whether a person is underweight, at an ideal weight, or overweight. These standard weight status categories can help doctors track weight status across populations and identify potential issues in individuals.
However, BMI does not measure body fat directly, and it does not account for age, sex, ethnicity, or muscle mass in adults.
Carrying too much weight
A weight that is too low can increase the risk of malnutrition, osteoporosis, and anemia. A person’s doctor will make suitable recommendations based on their individual circumstances and results.
Calculating BMI involves measuring a person’s height and body weight. It does not consider several other factors, such as activity levels, muscle composition, and race.
Learn more about how useful BMI calculations can be.
Metric
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.
Imperial
- When using imperial units, the formula is BMI = lb x 703/in2.
- In other words: Multiply a person’s weight in pounds (lb) by 703. Then divide by their height in inches, squared (in2).
To avoid using math, a person can
The following table shows standard weight status categories associated with BMI ranges for adults.
| BMI | Classification |
|---|---|
| below 18.5 | underweight |
| 18.5 to 24.9 | healthy weight |
| 25.0 to 29.9 | overweight |
| 30.0 and above | obese |
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. People 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 healthcare 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 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) has a
BMI is not accurate enough to use as a diagnostic tool, but it is useful in screening for potential weight problems in adults and children.
If someone has a high or low BMI, a doctor or other healthcare professional might consider other factors, such as:
- skinfold thickness measurements, which indicate how much fat is in the body
- evaluations of diet and physical activity
- family history of cardiovascular disease and other health problems
The doctor or healthcare professional can then recommend other appropriate health screenings and make diet and exercise recommendations based on the 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.
Carrying extra weight
- 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
- stroke
- gallbladder disease
- osteoarthritis
- 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
Benefits of a moderate weight
Apart from reducing the risk of several health conditions, maintaining a moderate 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
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 measure 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.
Learn more about the limitations of BMI calculations.
BMI can be a useful screening tool for predicting certain health risks. However, it should be used 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.