Mayo Clinic Minute: The Problem With BMI
What Is BMI?
BMI stands for body mass index, and is one way to estimate the amount of excess fat your body carries.
Using both height and your weight, BMI has proven to be a generally reliable and easy way to estimate a person's risk of obesity — though it's not foolproof.
How Is BMI Measured?
BMI is calculated as your weight in pounds, divided by your height in inches squared, multiplied by 703:
Formula:Weight (lb) / height (in)2x 703
Example:For a person who is 5 feet 4 inches tall and weighs 140 pounds: 140 / (64x64) x 703 = 24 BMI.
Or, for those who use the metric system, it's your weight in kilograms divided by your height in meters squared:
Formula:Your weight (kg) / height (m)2
Example:For a person who is 1.7 meters tall and weighs 68 kg: 68 / (1.7 x 1.7) = 23.5 BMI.
BMI numbers are the same for men and women, though children use a slightly different formula than adults (see below).
Online BMI calculators do the math for you and are a handy way to determine a BMI measurement.
There are separate BMI calculators for adults and BMI calculators for children.
Use the BMI calculator for children from 2 years old to 19 years old.
If you are 20 years or older, use the adult BMI calculator.
What Does Your BMI Number Mean?
BMI numbers are used as a screening tool to identify those who may be underweight, overweight, or obese:
- BMI below 18.5 = Underweight
- BMI 18.5 to 24.9 = Normal weight
- BMI 25.0 to 29.9 = Overweight
- BMI 30.0 and higher = Obese
- BMI 40 and higher = Morbidly obese
However, your BMI number does not mean you are definitively at a healthy weight, overweight, or obese, because other factors should be taken into account.
Limitations of BMI
While BMI has been shown to have a strong correlation to the amount of body fat a person has, it does have some drawbacks when it comes to healthy weight measurements.
The most common misleading factors are:
- Women generally have more body fat than men at the same BMI.
- Older adults generally have more body fat and less muscle than younger adults at the same BMI.
- Professional and amateur athletes may have a higher BMI because of greater muscle mass, not increased body fat. (Muscle weighs more than fat.)
What Other Numbers Should Be Considered in Addition to BMI?
BMI is only one indicator of an increased risk for obesity-related diseases.
The National Heart, Lung, and Blood Institute guidelines recommend also considering:
Your waist circumference:Fat around your waist relative to your hips increases your risk for heart disease and type 2 diabetes.
Your waist should be 35 inches or smaller for women, or 40 inches or smaller for a man.
Other risk factors for disease:Healthcare professionals also consider the presence of high blood pressure (hypertension), high cholesterol, high triglycerides, high blood glucose (sugar), family history of premature heart disease, lack of physical activity, and cigarette smoking.
Other Ways to Measure Fat
There are other ways to measure body fat besides a BMI calculation.
However, they require special equipment, require training to perform, and can produce varied results depending on the equipment used and the person performing the procedures.
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