Free BMI Calculator

Body Mass Index, or BMI, is a single number that describes your weight relative to your height. The Centers for Disease Control and Prevention (CDC) defines it as a person's body weight in kilograms divided by the square of their height in meters. If you enter your height and weight in US units, the calculator above converts them to metric and applies the CDC formula.

The CDC lists being quick and simple to calculate, and inexpensive, among the strengths of BMI, and the CDC describes it as a quick, safe, and reliable screening measure to assess a person's weight relative to their height. It is also what the CDC calls a well-established and valuable population health measure, used to describe and compare groups of people. Those two purposes, screening one person and describing a population, are worth keeping separate as you read your own result. A number that works well for describing thousands of people can still leave a great deal unsaid about you.

For adults aged 20 and older, the CDC groups BMI into the following categories:

Source: CDC, Adult BMI Categories. The National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) publishes the same thresholds for healthy weight, overweight, obesity, and severe obesity.

These categories apply to adults only. The CDC is explicit that BMI is interpreted differently for children and teens, whose BMI is compared to growth charts for their age and sex rather than to the adult table. The calculator on this page is built for adults and should not be used to evaluate a child.

One more detail matters: the CDC applies the adult categories the same way regardless of age, sex, or race. That uniformity is what makes BMI simple and comparable. It is also why the number, on its own, is a starting point for a conversation rather than the end of one.

BMI does not measure body fat directly. The CDC states that BMI cannot distinguish fat mass from lean body mass, meaning muscle and bone, and that it does not indicate where in the body a person carries fat. A person with a lot of muscle can have a BMI in the overweight or obesity category based on the number alone, because muscle, fat, and bone all add to weight.

Age changes the picture too. MedlinePlus, the National Library of Medicine's consumer health resource, notes that healthy weight ranges for older adults differ from those for other age groups, and that an older adult with a normal BMI may have less muscle than someone whose BMI is slightly higher. The CDC's adult table does not adjust for any of this, which is by design: it is a screening table, not a diagnosis.

Fat distribution is another gap that matters for health risk. The CDC notes that BMI does not indicate what types of fat a person has or where that fat sits. The National Heart, Lung, and Blood Institute (NHLBI) explains that fat stored inside the abdomen, called visceral fat, interferes with the body's endocrine and immune systems and contributes to obesity-related complications such as heart disease and diabetes. Two people with the same BMI can carry fat very differently, and BMI cannot tell them apart.

None of this makes BMI useless. The CDC describes it as moderately to strongly associated with more precise measures of body fat, which is why it is used as a first screen. It means the number needs company.

In practice, a clinician reads BMI as one input among several. The CDC recommends considering BMI together with medical history, including existing conditions and family history; health behaviors such as diet, physical activity, and sleep; physical exam findings such as blood pressure and muscle mass; and laboratory findings such as glucose and cholesterol levels.

Waist circumference is the companion measure the NHLBI names. The NHLBI states that a person can have a normal BMI and still carry more fat in the abdomen than elsewhere, which a large waist circumference can reveal. MedlinePlus likewise notes that a provider may take waist circumference and waist-to-hip ratio into account when deciding whether someone has overweight. The NHLBI describes the method: stand, place a tape measure around your middle just above the hipbones, and measure just after breathing out.

A clinician may also look for causes rather than only consequences. The NHLBI notes that some medical conditions and some medicines can lead to weight gain, and that a provider may order blood tests to rule them out. That is one reason a full history matters: the same BMI can have different explanations and call for different responses.

This is also how eligibility for any weight-related treatment works at Majesta Health. A physician considers BMI together with your history; BMI alone does not determine treatment. Medication is prescribed only if a licensed physician determines it is appropriate for you, after reviewing your health history, current medications, and goals. Your BMI result is useful information to bring to that conversation, not a verdict in either direction.

The NHLBI's advice is the right note to end on: talk with your healthcare provider about your BMI result and your body fat distribution, and about what steps, if any, make sense for you.