The Body Mass Index (BMI) is a number calculated from a person's weight and height. The result of the calculation is what’s referred to as the status of body fat. The status is then used to identify two characteristics of the BMI factors. The first, is the BMI level classification you fall into and the second are the potential health problems because of the BMI level identified.

Calculating the BMI is easy to compute, which is why it's used by the general public and it works fairly well as a self-evaluation means. The downside to this simple method of calculation is that there are inconsistencies that affect the intrepretation of the BMI classifications that need to be considered.

How reliable is the BMI calculation as a gage of body fat?

Several major factors that play an important role when determining an accurate overall view of a person’s health.  In deciphering the BMI calculation these discriminators; sex, race and age need to be considered.  A person’s lifestyle can affect the outcome of the BMI status as well.

When we look at the differences between men and women, the same BMI for a man and woman may not equate to the their weight or height.  Women’s BMI’s tend to be higher in comparison to men. The reason is simply that women have more body fat by nature than men.

Age is another a factor, older adults have higher body fat content due to a natural aging process in comparison to younger adults.  Athletic activities or active lifestyles tend to increase the amount of body muscle making the body heavier while reducing overall body fat.  An inactive lifestyle may increase the amount of body fat even though for some it may not affect the body weight.

Children's body fat is constantly changing during their growing years.  Infants or toddlers, may not have a wide gap of differences between boys and girls body fat, but as they enter puberty the differences become more prevalent and lifestyles continue to play a part in their BMI’s.

How to Calculate Your BMI

1. First you take your weight in pounds and convert your height to inches
2. Take your weight and divide it by the height 
3. Take that result and divide it again by the height
4. Multiply that result by 703
5. You have your BMI status level

Examples of Calculations:

Adult Male - Weight: 225 lbs. Height: 6’3”

1. Convert into inches:  6 feet x 12 inches =72 + 3 = 75 inches
2. Calculate BMI Formula:  225 ÷ 75 = 3 ÷ 75= .04 x 703 = 28.1
3. BMI Status: Overweight: 25.0 – 29.9  Normal 18.5 – 24.9
4. Normal Weigh should be 144 – 199

Adult Female - Weight: 150 lbs. Height: 5’3”

1. Convert into inches: 5 feet x 12 inches =60 +3 = 63 inches
2. Calculate BMI formula:  150 ÷ 63 = 2.3 ÷ 63 = .03 x 703 = 26.56
3. BMI Status: Overweight 25.0 – 29.9  Normal 18.5 – 24.9
4. Normal Weight should be 104 lbs. -141 lbs.

Adolescent - Weight: 105 lbs. Height: 4’3”

1. Convert into inches 4 feet x 12 inches = 48 +3 = 51 inches
2. Calculate BMI formula: 105 ÷ 51= 2.0 ÷ 51 = .04 x 703 = 28.3
3. BMI Status: Overweight 25.0 – 29.9 Normal 18.5 – 24.9
4. Normal Normal Weigh should be 68 lbs. - 92 lbs.

The standard BMI status table:

• Below 18.5 Underweight
• 18.5 – 24.9 Normal
• 25.0 – 29.9 Overweight
• 30.0 and above Obese

Interpretation of BMI

Remember that other factors need to be taken into account when determining health risks in relation to the BMI calculation formula status.  BMI is not a direct measure of body fat since the formula includes both muscle and fat.  Most people falling into the overweight levels with higher BMI's may have higher muscle weight versus body fat content.  Those falling into the obese levels equal or greater to 30 is attributed to higher body fat.   As with all health concerns, you should seek professional advice.

Sources: Right Health, WebMD, CNN Health, Kids Health

Photo: Flickr