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what is bmi

Body mass index (BMI) is a person’s weight in kilograms divided by the square of height in meters. BMI is an inexpensive and easy screening method for weight category—underweight, healthy weight, overweight, and obesity.

BMI does not measure body fat directly, but BMI is moderately correlated with more direct measures of body fat 1,2,3. Furthermore, BMI appears to be as strongly correlated with various metabolic and disease outcome as are these more direct measures of body fatness 4,5,6,7,8,9.On This Page

How is BMI used?

BMI can be a screening tool, but it does not diagnose the body fatness or health of an individual. To determine if BMI is a health risk, a healthcare provider performs further assessments. Such assessments include skinfold thickness measurements, evaluations of diet, physical activity, and family history10.
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What are the BMI trends for adults in the United States?

The prevalence of adult BMI greater than or equal to 30 kg/m2 (obese status) has greatly increased since the 1970s. Recently, however, this trend has leveled off, except for older women. Obesity has continued to increase in adult women who are age 60 years and older.

To learn more about the trends of adult obesity, visit Adult Obesity Facts.
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Why is BMI used to measure overweight and obesity?

Because calculation requires only height and weight, BMI is an inexpensive and easy tool. To see the formula based on either kilograms and meters or pounds and inches, visit How is BMI calculated?
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What are other ways to assess excess body fatness besides BMI?

Other methods to measure body fatness include skinfold thickness measurements (with calipers), underwater weighing, bioelectrical impedance, dual-energy x-ray absorptiometry (DXA), and isotope dilution 1,2,3. However, these methods are not always readily available, and they are either expensive or need to be conducted by highly trained personnel. Furthermore, many of these methods can be difficult to standardize across observers or machines, complicating comparisons across studies and time periods.
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How is BMI calculated?

BMI is calculated the same way for both adults and children. The calculation is based on the following formulas:

Measurement UnitsFormula and Calculation
Kilograms and meters (or centimeters)Formula: weight (kg) / [height (m)]2With the metric system, the formula for BMI is weight in kilograms divided by height in meters squared. Because height is commonly measured in centimeters, divide height in centimeters by 100 to obtain height in meters.Example: Weight = 68 kg, Height = 165 cm (1.65 m)
Calculation: 68 ÷ (1.65)2 = 24.98
Pounds and inchesFormula: weight (lb) / [height (in)]2 x 703Calculate BMI by dividing weight in pounds (lbs) by height in inches (in) squared and multiplying by a conversion factor of 703.Example: Weight = 150 lbs, Height = 5’5″ (65″)
Calculation: [150 ÷ (65)2] x 703 = 24.96

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How is BMI interpreted for adults?

For adults 20 years old and older, BMI is interpreted using standard weight status categories. These categories are the same for men and women of all body types and ages.

BMIWeight Status
Below 18.5Underweight
18.5 – 24.9Healthy Weight
25.0 – 29.9Overweight
30.0 and AboveObesity

For example, here are the weight ranges, the corresponding BMI ranges, and the weight status categories for a person who is 5′ 9″.

HeightWeight RangeBMIWeight Status
5′ 9″124 lbs or lessBelow 18.5Underweight
125 lbs to 168 lbs18.5 to 24.9Healthy Weight
169 lbs to 202 lbs25.0 to 29.9Overweight
203 lbs or more30 or higherObesity

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For children and teens, the interpretation of BMI depends upon age and sex. For more information about interpretation for children and teens, read – What is a BMI percentile and how is it interpreted?

Is BMI interpreted the same way for children and teens as it is for adults?

BMI is interpreted differently for children and teens, even though it is calculated using the same formula as adult BMI. Children and teen’s BMI need to be age and sex-specific because the amount of body fat changes with age and the amount of body fat differs between girls and boys. The CDC BMI-for-age growth charts take into account these differences and visually show BMI as a percentile ranking. These percentiles were determined using representative data of the US population of 2- to 19-year-olds that was collected in various surveys from 1963-65 to 1988-9411.

Obesity among 2- to 19-year-olds is defined as a BMI at or above the 95th percentile of children of the same age and sex in this 1963 to 1994 reference population. For example, a 10-year-old boy of average height (56 inches) who weighs 102 pounds would have a BMI of 22.9 kg/m2. This would place the boy in the 95th percentile for BMI – meaning that his BMI is greater than that of 95% of similarly aged boys in this reference population – and he would be considered to have obesity.

For more information and to access the CDC Growth Charts

For adults, the interpretation of BMI does not depend on sex or age. Read more about interpreting adult BMI.
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How good is BMI as an indicator of body fatness?

The correlation between the BMI and body fatness is fairly strong1,2,3,7, but even if two people have the same BMI, their level of body fatness may differ12.

In general,

  • At the same BMI, women tend to have more body fat than men.
  • At the same BMI, the amount of body fat may be higher or lower depending on the racial/ethnic group13-15.
  • At the same BMI, older people, on average, tend to have more body fat than younger adults.
  • At the same BMI, athletes have less body fat than do non-athletes.

The accuracy of BMI as an indicator of body fatness also appears to be higher in persons with higher levels of BMI and body fatness16. While, a person with a very high BMI (e.g., 35 kg/m2) is very likely to have high body fat, a relatively high BMI can be the results of either high body fat or high lean body mass (muscle and bone). A trained healthcare provider should perform appropriate health assessments to evaluate an individual’s health status and risks.
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If an athlete or other person with a lot of muscle has a BMI over 25, is that person still considered to be overweight?

According to the BMI weight status categories, anyone with a BMI between 25 and 29.9 would be classified as overweight and anyone with a BMI over 30 would be classified as having obesity.

However, athletes may have a high BMI because of increased muscularity rather than increased body fatness. In general, a person who has a high BMI is likely to have body fatness and would be considered to be overweight or obese, but this may not apply to athletes. A trained healthcare provider should perform appropriate health assessments to evaluate an individual’s health status and risks.
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What are the health consequences of obesity for adults?

People who have obesity are at increased risk for many diseases and health conditions, including the following: 10, 17, 18

  • All-causes of death (mortality)
  • High blood pressure (hypertension)
  • High LDL cholesterol, low HDL cholesterol, or high levels of triglycerides (dyslipidemia)
  • Type 2 diabetes
  • Coronary heart disease
  • Stroke
  • Gallbladder disease
  • Osteoarthritis (a breakdown of cartilage and bone within a joint)
  • Sleep apnea and breathing problems
  • Chronic inflammation and increased oxidative stress19,20
  • Some cancers (endometrial, breast, colon, kidney, gallbladder, and liver)
  • Low quality of life
  • Mental illness such as clinical depression, anxiety, and other mental disorders21,22
  • Body pain and difficulty with physical functioning23

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