About BMI
What is BMI?
Body Mass Index (BMI) is a number calculated from a person’s weight and height. It is a reliable measure to understand whether a person is normal weight or overweight or underweight.
What are some of the other ways to measure obesity?
Other methods to measure body fatness include skinfold thickness measurements (with calipers), underwater weighing, bioelectrical impedance, dual-energy x-ray absorptiometry (DXA), and computerized tomography. However, these methods are not always readily available, and they are either expensive or need highly trained personnel. Calculating BMI is one of the best as it requires only height and weight, it is inexpensive and easy to use.
How is BMI calculated?
Calculation of BMI
BMI is calculated the same way for both adults and children. Go to BMI calculator. The calculation is based on the following formulas:
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Measurement Units |
Formula and Calculation |
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Kilograms and meters (or centimeters) |
Formula: weight (kg) / [height (m)]2 With the metric system, the formula for BMI is weight in kilograms divided by height in meters squared. Since 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) |
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Pounds and inches |
Formula: weight (lb) / [height (in)]2 x 703 Calculate 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″) |
How is BMI interpreted?
Interpretation of BMI for adults
For adults 20 years old and older, BMI is interpreted using standard weight status categories that are the same for all ages and for both men and women. For children and teens, on the other hand, the interpretation of BMI is both age- and sex-specific. For more information about interpretation for children and teens, visit BMI for children and teens.
The standard weight status categories associated with BMI ranges for adults are shown in the following table.
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BMI |
Weight Status |
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Below 18.5 |
Underweight |
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18.5 – 24.9 |
Normal |
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25.0 – 29.9 |
Overweight |
|
30.0 and Above |
Obese |
For example, here are the weight ranges, the corresponding BMI ranges, and the weight status categories for a sample height.
|
Height |
Weight Range |
BMI |
Weight Status |
|
5’ 9” |
124 lbs or less |
Below 18.5 |
Underweight |
|
125 lbs to 168 lbs |
18.5 to 24.9 |
Normal |
|
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169 lbs to 202 lbs |
25.0 to 29.9 |
Overweight |
|
|
203 lbs or more |
30 or higher |
Obese |
Some variations:
- At the same BMI, women tend to have more body fat than men.
- At the same BMI, older people, on average, tend to have more body fat than younger adults.
- Highly trained athletes may have a high BMI because of increased muscularity rather than increased body fatness.
Importance of waist circumference:
In addition to BMI, an individual’s waist circumference is also an important risk factor for heart attack or stroke. Abdominal fat as evidenced by increased waist circumference is a risk factor for heart attack or stroke.
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Classification of Overweight by BMI, Waist Circumference, and Disease Risks for heart attack, diabetes and high blood pressure |
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Disease Risk (for heart attack, diabetes and blood pressure) Relative to Normal Weight and Waist Circumference |
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BMI |
Obesity |
Men 102 cm (40 in) or less |
Men > 102 cm (40 in) |
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Underweight |
< 18.5 |
|
- |
- |
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Normal |
18.5 - 24.9 |
|
- |
- |
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Overweight |
25.0 - 29.9 |
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Increased |
High |
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Obesity |
30.0 - 34.9 |
I |
High |
Very High |
|
|
35.0 - 39.9 |
II |
Very High |
Very High |
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Extreme Obesity |
40.0 + |
III |
Extremely High |
Extremely High |