What is BMI?

Body Mass Index, or BMI is the customary method used to measure obesity. It’s a calculation of your weight-to-height ratio and can provide insight into risk for diseases. While BMI can’t be used as a sole indicator for disease risk, it does help. It’s one element of your wellness that helps you and your medical professionals manage your health.

BMI is used for risk assessment for the general population. In general, as a person’s BMI increases, so does his or her risk of certain diseases. However, BMI doesn’t distinguish between body fat and lean body mass and doesn’t take into account location of body fat. For this reason, it’s not an accurate measure of health for certain populations, such as people with higher than average muscle mass or people whose body composition may be skewed for other reasons, this includes athletes, children, pregnant women and the elderly.

BMI and Your Health

BMI is not a direct measure of excess body fat, but it does provide a more accurate measure than body weight alone.

Numerous studies show a link between BMI and the risk of serious diseases as well as premature death. As BMI increases, so does the risk for:

  • diabetes
  • cardiovascular disease
  • stroke
  • hypertension
  • gallbladder disease
  • osteoarthritis
  • sleep apnea
  • some cancers

The good news is, many of these diseases are lifestyle diseases and the threat of contracting them can be lessened and even eliminated by changing your lifestyle. Changes in diet, exercise and mind set go a long way toward elimination of these diseases at best and at worst, lessening their symptoms.

How is BMI Calculated?

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

For pounds and inches:

Formula: weight (lbs.) / [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″)
Calculation: [150 ÷ (65)2] x 703 = 24.96

For kilograms and meters:

Formula: weight (kg) / [height (m)]2

With 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

BMI Categories

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.

You can use the formulas above or the IIFYM BMI Calculator to find your BMI. From there, use the information below to determine your weight classification. Ideally, you should be in the “Healthy Weight” range.

If you’re “Underweight,” consider adding some healthy body weight in the form of lean muscle mass. If you’re overweight or obese, focus on losing body weight in the form of fat with a diet and exercise plan overseen by a medical professional.

The standard weight status categories associated with BMI ranges for adults are:

Underweight                                Under 18.5

Healthy Weight                           18.5 – 24.9

Overweight                                 25.9 – 29.9

Obese                                        30 and above

BMI Applied

Here are the weight ranges, the corresponding BMI ranges, and the weight status categories for a person who’s 5 feet 9 inches tall.

At 124 pounds or less their BMI is less than 18.5 and they are underweight.

At 125 to 168 pounds, their BMI is 18.5 to 24.9 and they’re in the healthy weight range.

At 169 to 202 pounds, their BMI is 25.0 to 29.9 and they’re considered overweight.

At 203 pounds or more, their BMI is 30 or higher and they’re in the obese range.

In general,

  • At the same BMI, women tend to have more body fat than men
  • At the same BMI, elderly people, on average, tend to have more body fat than younger adults
  • At the same BMI, athletes have less body fat than non-athletes

So, a person with a very high BMI is very likely to have high body fat, but a relatively high BMI can be the result of either high body fat or high lean body mass (muscle and bone). As always, discuss your diet and exercise plan with a trained medical professional that you trust.