Healthy BMI: Understanding body mass index for all

It’s common to wonder whether someone’s weight is in a healthy range for their height. The good news is that there’s a tool that can help them check. Body mass index (BMI) calculates the relationship between height and weight. To learn more about how BMI works, more information is below.

In this article:

What is body mass index (BMI)?

Body mass index (BMI) is a calculation that measures a person's weight in relation to their height. BMI data has been used as a tool worldwide for decades to find trends across locations and large groups of people. Overtime, its helped public health entities make decisions to improve health.

Health care providers find BMI for adults using this formula:

• If a person is using the U.S. standard measurement system, their BMI is calculated by dividing their weight (in pounds) by the square of height (in inches) and then multiplying that by 703.
• If a person is using the metric system, their BMI is calculated by dividing their weight (in kilograms) by the square of their height (in meters).

There are BMI calculators online, so no one has to do the math on their own. Normally BMI ranges are grouped into categories. For example, adult BMI ranges are grouped into categories: underweight, healthy weight, overweight and obesity. Obesity is then subdivided into three groups. People in certain categories (like obesity) may have a higher risk of conditions like high blood pressure, high cholesterol and type 2 diabetes.

As a result, BMI and other factors can help health care providers understand someone’s heart disease risk. Maintaining a healthy weight can help lower future health risks from happening. Being in the overweight or obesity categories is associated with an increased risk to develop conditions like type 2 diabetes and heart disease.

Key facts about healthy BMI

For adults, a BMI in the healthy weight range is usually between 18.5 and less than 25. A BMI that is lower than 18.5 is considered underweight. A BMI between 25 and less than 30 is considered overweight. If a BMI number is 30 or higher, the category is obesity.

But BMI does not distinguish between how much fat or muscle someone has. It also does not show what types of fat people have or where they carry it. If an athlete has a lot of muscle, their BMI might be higher, even if they don’t have much fat. That’s because both fat and muscle can influence weight on the scale, and weight is just part of the BMI equation.

Additionally, BMI tools don’t consider a person’s age, gender or family medical history.

BMI tools and body composition

Health care providers use more tools than BMI to measure someone’s overall health. They also look at their family history, health behaviors, data from physical exams (like blood pressure and muscle mass) and sugar or cholesterol levels from findings in a lab.

Health care providers can also take body composition into account to understand someone’s health. Body composition helps measure how much fat someone has and where it’s located throughout their body. Tests like dual-energy X-ray absorptiometry (DEXA) find these numbers. However, DEXA scans are expensive and not widely used.

Health care providers might also measure waist circumference. If most of the fat on someone’s body is found around the waist rather than the hips, there is a higher risk of heart disease and type 2 diabetes. People can wrap a measuring tape around their waist (just above their hipbones), breathe out, then record the number in inches. A waist size for men that is more than 40 inches and for women (who aren't pregnant) that is more than 35 inches increases their risk.

BMI by biological sex

Even if two people have the same BMI, their amount of body fat may be different. Women generally have greater amounts of body fat than men. This difference can mean two people may have the same BMI, but their amount of body fat may differ, which can affect their risk for certain diseases. (For the purposes of this article, "men" and "women" refer to biological sex.)

Men and women have different waist size guidelines when checking if they are within a healthy range. Women who have a waist measurement of 35 inches or more who aren’t pregnant may be more likely to have heart disease, high blood pressure or type 2 diabetes. Men who have a waist circumference of 40 inches or more are also more at risk for those conditions.

Calculating BMI in children

BMI for children is measured differently. Since kids are still growing, health care providers don’t focus on one number. They look at BMI-for-age percentiles, which help compare a child’s health to other kids who are the same age and gender.

There are BMI categories for kids between age 2 and age 19. They are:

• Underweight
• Healthy weight
• Overweight
• Obesity
• Severe obesity

An underweight category has a BMI range less than the 5th percentile. A healthy weight BMI range is between 5th and less than the 85th percentile. The overweight category has a BMI range between 85th and less than the 95th percentile. The obese category has a BMI range of 95th percentile or greater. The severe obesity category has a BMI range of 120 percent of the 95th percentile or greater.

For example, a girl who is age 12 and 5 feet tall and weighs 136 pounds has a BMI of 26.6, which means she falls in the 96th percentile (in the obese BMI category). That means her BMI is the same or higher than 96 percent of all age 12 girls in the reference population used to create BMI-for-age growth charts. (For the purposes of this article, “girl” refers to biological sex.)

Of course, there are BMI percentile calculators available online, so people don’t have to do the math by hand.

Health conditions associated with obesity

Having excess body fat can increase the risk of many health conditions, including high blood pressure, heart disease and type 2 diabetes.

Some factors that can raise someone’s risk of being overweight or obese may include:

• Family history of obesity
• Having other health conditions, such as metabolic syndrome or PCOS (polycystic ovary syndrome)
• Not getting enough good quality sleep
• Stress
• Taking certain medications

Ways to maintain a healthy weight

Here are a few ways people can help keep a healthy weight:

• Eating healthy food. People might think about keeping a balanced diet with lots of fruits, vegetables, lean proteins and whole grains.
• Moving their body. 150 minutes of moderately intense exercise each week like dancing, walking or playing a fun sport can help. (If a person has a chronic condition or disability, they can speak with a health care provider about what types and amounts of physical activity are right for them before making too many changes.)
• Paying attention to portions. This might look like considering how much food is on their plate and adjusting based on their weight goals.

Key takeaways

A healthy BMI can be a sign of good health. Health care providers suggest checking it along with other factors, like cholesterol levels and body composition. When a person keeps an eye on their BMI and makes healthy lifestyle choices, they can help improve their health outcomes. A reliable BMI calculator can help determine risk of certain health conditions. For more information, health care providers can offer personalized guidance as well as plans to achieve and maintain a healthy weight.