Body Mass Index (BMI) is a ratio of a person’s weight and height and weight, and does not directly measure a person’s health status, muscle, bone, body fat, or body water.

How is BMI measured?

BMI is a mathematical calculation of your weight (in pounds) times 703 divided by your height (in inches) squared. You can figure out your BMI on your own using this formula:

BMI = Weight (in pounds) x 703 divided by Height (in inches) squared.

What does my BMI mean?

If you are under the age of 19, your BMI is plotted onto a growth chart. Whether your weight falls into the “underweight,” “normal weight,” “overweight” or “obese” category depends on where your BMI falls on the chart. While categories have been given these names, the term “normal” does not necessarily reflect what a “normal weight” should be for each individual.

• BMI percentile <5^th: underweight
• BMI percentile 5^th-85^th: normal weight
• BMI percentile 85^th-95^th: overweight
• BMI percentile >95^th: obese

If you are over the age of 19, your BMI classification is based simply on the number:

• BMI <18.5 kg/m²: underweight
• BMI 18.6-24.9 kg/m²: normal weight
• BMI 25-29.9 kg/m²: overweight
• BMI >30 kg/m²: obese

If you are under 19 and want to figure out your BMI percentile, you’ll need to ask your health care provider to look at your growth chart. Remember, BMI is not a perfect tool. Even if your BMI places you into a category other than “normal”, ask your health care provider if you need to make any changes to your health behaviors for your overall health. A health care provider will assess your weight history, activity level, diet, sleep, mental health, family history and body composition (such as how muscular you are) before they decide if you need to make any changes.

No matter what your BMI percentile is, it’s important to ask if it has changed over the last few years. Although BMI can change when you’re growing or going through puberty, teens should ideally stay at about the same BMI percentile during their teen years. If your BMI percentile changes a lot either up or down, you should talk to your health care provider about possible reasons for the change.

What if my BMI is high?

Talk to your health care provider about whether you should be concerned about a high BMI percentile. They will likely look at your growth chart to see if you have been following roughly the same percentile as you have grown, or whether the percentile has dropped. Keep in mind that BMI is just an estimate of body size, and alone is not a good indicator of overall health.

Check out our health guide on Weight, Health, and Weight Stigma for more information.

What if my BMI is too low?

Talk to your health care provider about whether you should be concerned about a low BMI percentile. They will likely look at your growth chart to see if you have been following roughly the same percentile as you have grown, or whether the percentile has dropped. Having a BMI classification of underweight can put you at risk for health complications such as a delay in puberty and weakened immune system.

Remember, BMI is not a perfect tool. Even if your BMI places you into a category other than “normal”, ask your health care provider if you need to make any changes to your eating and exercise habits for your overall health. They can also refer you to a  registered dietitian to make sure you are getting all the nutrients you need.