How to calculate BMI

November 12, 2012

How to calculate BMI

BMI is a commonly used measurement to calculate the healthiness of a person’s weight in relation to their height.

BMI – or Body Mass Index – is a calculation involving weight and height in order to determine the amount of body fat in an individual and whether their height/weight ratio is appropriate. The BMI measurement is one tool used by medical professionals in assessing the overall health of both men and women. Whilst the BMI figure is useful it does, however, have limitations. BMI does not consider body type, muscle density or lean mass – all of which may cause a higher BMI despite being good for health. This means, for example, that athletes or those who are very muscular may have a high BMI when, in fact, they have a healthy level of body fat. A medical practitioner will consider other factors such as blood pressure, cardiac health, exercise levels and waist measurements when evaluating the health of any individual.

What do the numbers mean?

The desired range of BMI is between 18.5 and 24.9, at this weight an individual is considered to be a normal weight and should continue with their current lifestyle.

A BMI of below 18.5 indicates an individual who is underweight and who may need advice from their medical practitioner in order to increase their overall calorie intake.

If, when you calculate BMI, the result is between 25.0 and 29.9 you are considered to be overweight and may need to make changes to your diet and increase levels of exercise.

A BMI score of over 30 means an individual is considered to be obese and needs to make immediate changes to diet and lifestyle in order to reduce the many risks of obesity.

Why calculate BMI

For anyone who is concerned about his or her weight – whether under or overweight – it may well be helpful to calculate BMI; it is not necessary to calculate BMI yourself – your medical practitioner will have a pre-printed chart available, but, understanding the calculation is often useful. Anyone who is exceptionally frail, pregnant or breastfeeding, as well as children, should not be concerned with their BMI.

How to calculate BMI

  • Weigh yourself and record the result – in pounds OR kilograms

  • Calculate your height in inches OR metres and square the result (that is, if your height is 61 inches then the square will be 61 x 61 = 3721)

  • Divide your weight by the final number found in the previous step – remembering to use imperial OR metric measurements and not a mixture.

  • Multiply this number by 703

Check your frame size and calculate BMI

  • Measure your wrist size in inches and record

Use this measurement to determine the size of your frame -

  • Women Wrist size over 5.75″ height under 62″ = Large frame

  • Wrist size over 6.25″ height 62 – 65″ = large frame

  • Wrist size over 6.5″ height over 65″ = large frame

  • Men wrist size over 7.5″ height over 65″ = large frame

When working to calculate BMI a large framed adult should increase the calculated BMI by 10% – the ideal range for a large framed adult is 23 -25.

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