by Laura Dunkley

Created on: March 05, 2010   Last Updated: March 06, 2010

What is the body mass index? It is an individuals weight divided by their height squared. To calculate your BMI, you need a few common items, a bathroom scale, a tape measure, pencil, paper and a calculator. Take your height and weight and after writing them down, using your calculator, use this simple formula to determine your BMI. The formula is as follows:

BMI = Body weight in pounds ÷ (Height in inches)(height in inches) x 703

The BMI can be calculated in metric also but because I am writing from the US, I will use Imperial measures.

Once you have calculated your BMI, what does it mean? In adults it is as follows:

<16 severely underweight

16.5 to 18.4 underweight

18.5 to 24.9 normal weight

25 to 30 overweight

>30 obese, of which there are three classes.

In children over two years through 18 years, the BMI is looked at on a percentile based on age.

<5th percentile is underweight for age

5th to 25th percentile is thin for age

25th to 75th percentile is normal for age

75th to 85th percentile is overweight

85th to 95th percentile is at risk for obesity

The BMI is based on stature (standing height) and as a result is never used on children less than 24 months whose length is recumbent height.

BMI is highly subjective but is a common and easily used diagnostic tool to determine weight and health status. A more accurate determination of weight status is the mid arm circumference and the waist hip ratio. An basketball player who is seven feet tall and weighs 300 lbs using the BMI would be considered obese because his BMI is 30 but we are not considering his muscle mass. Remember that muscle is heavier than fat. Visually, he would also not appear to be obese. I personally would not consider him to be obese, but simply a big guy. Similarly if you have a child who is tall for their age, with a height >95th percentile for age and a weight also > 95th percentile for age, this child may have a high BMI percentile, but would not necessarily be obese despite what the BMI chart says for his age.

Now that we know how to calculate our BMI, get out your scale, tape measure, pencil paper and calculator and jump on the scale, have someone measure you and work that calculator. If you are at either of the extremes, it is time to get to work, neither extreme represents good health.

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