What is BMI – How to calculate BMI for Adults and children

In modern society, the most common way for doctors to determine whether a person is overweight is by calculating their BMI. What is BMI I hear you ask?

BMI is the abbreviation of ‘Body Mass Index’. BMI is a measure of an individuals weight compared to their height. It is used to provide an estimate of how much body fat an individual has. It can also indicate what risk factors a person has of suffering from obesity related health issues such as heart disease, stroke, diabetes and high blood pressure.

There are four main levels of interpretation of BMI results in adults, although these can also be split into six:

1) BMI below 16 – Severely underweight
2) BMI of 16-18 – underweight
3) BMI of 18-25 – Healthy weight range
4) BMI of 25-30 – overweight
5) BMI of 30-40 – Obese
6) BMI over 40 – Morbidly Obese

While the calculation of BMI in children is the same as it is for adults, the interpretation is different because children are still growing. BMI results for children are broken down into BMI for Age and BMI for sex. A childs BMI is calculated and compared to other children of the same age and sex and displayed as a percentile. In children, it is possible to have a BMI of 18 and still be classed as obese depending on their age and sex. The breakdown for children is

1) Lowest 5% of BMI – underweight
2) Within 5-85% of BMI – Healthy weight range
3) Within 85-95% of BMI – overweight
4) In top 5% of BMI – Obese

How to calculate BMI.

There are two ways to calculate BMI depending on whether your system of measurement is imperial (pounds and inches) or metric (kilograms and metres):

In the Imperial system of measurement the formula is weight (lb) divided by height (in) divided by height (in) x 703

So if you weigh 198.42lb and are 66.5″ tall then the formula comes out as 198.42 / 66.5 / 66.5 x 703 = BMI of 31.54

By Metric the formula is weight (kg) divided by height (cm) divided by height (cm) x 10,000

So using the same figures converted is 90kg / 168.91 / 168.91 x 10,000 = BMI of 31.54

It should be noted that it is possible to have a high BMI without being overweight because BMI is calculated on overall weight and does not distinguish between the weight of fat or muscle. Therefore, a person who is extremely fit or does weight training can have a high BMI without being overweight. In such cases doctors also use tests such as the skinfold thickness measurement to determine whether a person is carrying unaceptable amounts of fat.

Children with high BMI for their age must also undergo other testing (such as the skinfold thickness measurement) for doctors to determine whether they are in fact overweight. It should be noted that it is unadvisable to put children on low fat diets since they need higher fat intake for growth and development.

So while BMI is not an exact science, it provides a good indication of whether a person is overweight in the majority of cases.

If you believe that yourself or your child may be overweight it is recommended that you consult your family doctor for advice.



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