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What is the Body Mass Index?

All you need to know about your BMI


Body mass is calculated from a formula using height and weight and gives an indication of health risk and longevity.


It is by no means a perfect formula because it doesnt take into account whether your weight includes a high or low percentage of body fat, nor where fat is distributed over the body. For example, carrying a lot of fat around your waist is more of a health risk than carrying it on your hips and thighs.


Nonetheless, it is a good guide. Generally speaking, a Body Mass Index figure of 20-25 is considered the normal healthy range. Between 25-30 you should aim to prevent any further gain and think about losing some weight. If its over 30, you really should be making some changes to your lifestyle that will help you lose weight. At over 40, your health could be at serious risk. Equally, being very underweight a BMI of 18.5 or less is also a health risk.


The health risks are the same for both men and women, but the formula does not apply to children under 18, nor to women who are pregnant or breastfeeding. It may not be accurate for athletes who have a high proportion of muscle mass, or for over-60s who may have lighter bones through bone loss.
Calculating your BMI
The BMI formula is weight in kilos divided by height in metres squared


If you want to work out your own BMI first find a calculator(!) and follow each step below:

  • 1 To work out your current weight in kilos, multiply the stones by 14 then add on the pounds. Now multiply your answer by 0.4536 to convert your weight into kilos.
  • 2 To work out your height in metres, multiply feet by 12 and add on the inches. Now multiply your answer by 0.0254 to convert your height into metres.
  • 3 Now multiply your height in metres by itself (height in metres times height in metres).
  • 4 To find your BMI, divide your weight in kilos (answer to step 1) by your height in metres squared (answer to step 3).
If you weighed 11st 6lb and were 5ft 4in tall
1 11 x 14 = 154: 154 + 6 = 160: 160 x 0.4536 = 72.58
2 5 x 12 = 60: 60 + 4 = 64: 64 x 0.0254 = 1.63
3 1.63 x 1.63 = 2.66
4 72.58/2.66 = 27.29


Your BMI would be 27.29
BMI Chart
If the calculation is too taxing for you, use this chart as a guide to your BMI.
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