Confirming a high body mass index (BMI)
To diagnose overweight and obesity, doctors measure BMI using calculations that depend on whether you are a child or an adult.
- Children: A healthy weight is usually when your child’s BMI is at the 5th percentile up to less than the 85th percentile based on growth charts for children who are the same age and sex. To figure out your child’s BMI, use the Center for Disease Control and Prevention (CDC) BMI Percentile Calculator for Child and Teen and compare the BMI with the table below.
- Adults: A healthy weight for adults is usually when your BMI is 18.5 to less than 25. To figure out your BMI, use the National Heart, Lung, and Blood Institute’s online BMI calculator and compare it with the table below. You can also download the BMI calculator app for iPhone(link is external) and Android(link is external). Even if your BMI is in the healthy range, it is possible to be diagnosed as obese if you have a large waist circumference that suggests increased amounts of fat in your abdomen that can lead to complications.
Body mass index (BMI) is used to determine if you or your child are underweight, healthy, or overweight or obese. Children are underweight if their BMI is below the 5th percentile, healthy weight if their BMI is between the 5th to less than the 85th percentile, overweight if their BMI is the 85th percentile to less than the 95th percentile, and obese if their BMI is the 95th percentile or above. Adults are underweight if their BMI is below 18.5, healthy weight if their BMI is 18.5 to 24.9, overweight if their BMI is 25 to 29.9, and obese if their BMI is 30 or above. *A child’s BMI percentile is calculated by comparing your child’s BMI to growth charts for children who are the same age and sex as your child.
Your doctor will ask about your eating and physical activity habits, family history, and will see if you have other risk factors Your doctor may ask if you have any other signs or symptoms. This information can help determine if you have other conditions that may be causing you to be overweight or obese or if you have complications from being overweight or obese.
During your physical exam, your doctor will measure your weight and height to calculate your BMI. Your doctor may also measure your waist circumference to estimate the amount of unhealthy fat in your abdomen. In adults, a waist circumference over 35 inches for women who are not pregnant or 40 inches for men can help diagnose obesity and assess risk of future complications. If you are of South Asian or Central and South American descent, your doctor may use smaller waist circumference values to diagnose your obesity. People from these backgrounds often don’t show signs of a large waist circumference even though they may have unhealthy amounts of fat deep in their abdomens and may be diagnosed with obesity. Visit Assessing Your Weight for more information.
Read Living With for more information about why it is important to monitor your waist circumference to assess your risk for complications.
Tests to identify other medical conditions
Your doctor may order some of the following tests to identify medical conditions that may be causing your overweight and obesity.
- Blood tests. Blood tests that check your thyroid hormone levels can help rule out hypothyroidism as a cause of your overweight or obesity. Cortisol and adrenocorticotropic hormone (ACTH) tests can rule out Cushing’s syndrome. Total testosterone and dehydroepiandrosterone sulphate (DHEAS) tests can help rule out polycystic ovary syndrome (PCOS).
- Pelvic ultrasound to examine the ovaries and detect cysts. This can rule out PCOS.
Reference: National Heart, Lung and Blood Institute
Last updated May 2017