Patient education: Understanding body mass index (BMI) (The Basics)

What is body mass index? — Body mass index, or "BMI," is a calculation doctors use to help understand a person's health. For adults, your weight and height are used to calculate your BMI (table 1).

Based on this number, you fall into 1 of these categories:

●Underweight – BMI under 18.5

●Healthy weight – BMI between 18.5 and 24.9

●Overweight – BMI between 25 and 29.9

●Having obesity – BMI 30 or greater

Your doctor or nurse will often calculate your BMI as part of a routine check-up. You can also do this yourself at home using a chart or online calculator.

How is a child's BMI calculated? — BMI can be calculated for children using their height and weight. A child's BMI is then compared with a "growth chart." Growth charts have information about the typical heights and weights of children who are the same age and sex. Doctors use BMI and the growth chart together to find out if a child is underweight, a healthy weight, overweight, or has obesity. What counts as being underweight or overweight for children is different than for adults.

If you have a question about a child's weight or BMI, talk to their doctor.

Should I see a doctor or nurse? — If you have concerns about your weight, see your doctor or nurse. If you calculated your BMI at home, the doctor or nurse can help you understand what it means. They can also give you advice on how to maintain a healthy weight and lifestyle.

What does my BMI mean? — Doctors use BMI to help understand the effect that your weight has on your health. In general, having obesity can increase the risk of having other health problems. These include:

●Diabetes

●High blood pressure

●High cholesterol

●Heart disease (including heart attacks)

●Stroke

●Sleep apnea (when you stop breathing for short periods of time while asleep)

●Asthma

●Cancer

You might also have trouble moving, breathing, or doing other things if you have obesity.

Being underweight can be bad for your health, too.

Remember that your weight and BMI are just pieces of your overall health. Someone with a lower BMI might not be healthy overall, and someone with a higher BMI can still be healthy. Other factors, like whether or not you smoke, also play a role.

No matter what your BMI is, try to live a healthy lifestyle. This includes not smoking, eating plenty of healthy foods like fruits and vegetables, and moving your body. Talk to your doctor about your overall health and what you can do to improve it.

What if I don't want to talk to my doctor about my BMI or weight? — If you are not comfortable discussing your weight or BMI with your doctor or nurse, you can tell them so. You can ask them to focus on other parts of your health instead.

Remember that your doctor understands that managing weight can be difficult. If you are interested, they can work with you to develop a weight management plan.

More on this topic

Patient education: Diet and health (The Basics)
Patient education: Exercise and movement (The Basics)
Patient education: Exercise and movement as you get older (The Basics)
Patient education: Health risks of obesity (The Basics)
Patient education: Weight loss treatments (The Basics)
Patient education: Weight and health in children (The Basics)

Patient education: Diet and health (Beyond the Basics)
Patient education: Exercise (Beyond the Basics)
Patient education: Losing weight (Beyond the Basics)