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Body Mass Index calculation

Body Mass Index (BMI) weighing scales

BMI / BMI Index(Body Mass Index) is a mathematical formula used by health professionals to establish whether a person is a healthy weight for their height.  Here's how to calculate your BMI Index, and how it can help prevent future health problems.


This article on tthe body mass index / BMI is written by Sarah Dawson, a freelance journalist who writes for national and international newspapers, magazines and websites.


Body Mass Index (BMI): woman measuring her waist

Calculate BMI


BMI stands for Body Mass Index and is a system used to determine whether a patient is overweight or obese.  It is calculated by dividing a person's weight by their height.  As an example, a woman who is 5'6" and weighs 10 stone has a BMI of 22.6. 


Adults can use the NHS Direct BMI calculator to get a personal reading, but BMI is less easy to establish in children because their reading will change as they continue to grow.  Adults should aim for a healthy number which lies between 18.5 and 24.9.  Above this means you're at risk of developing serious conditions such as heart disease, diabetes, stroke or cancer.  The World Health Organisation (WHO) says that a BMI calculator reading of between 25 and 29.9 indicates you're over the recommended weight for your height, a reading of between 30 and 39.9 means you're obese, and a reading over 40 means you're very obese. 

Obesity is a serious issue and levels are rising each year.  A 2005 Health Survey for England reported that over 21 percent of men and women were classified as obese and, more recently, research from Foresight, a Department of Health sponsored organisation looked into the causes of obesity, finding that weight-gain isn't just about over-indulgence or laziness


The report suggests that the technological revolution of the 20th century has led to unavoidable weight gain for many of the population because our bodies and biological make-up are out of synch with our surroundings.  If current obesity growth rates continue, it has been predicted that some 60 percent of men, 50 percent of women and 25 percent of children in Britain will be obese by 2050.

Depending on your BMI reading, you can establish whether you need to make subtle - or serious - lifestyle changes for your long term health.  However, remember the BMI test doesn't take into account a person's body fat content, so your BMI calculator reading may not be accurate if you're a weight-trainer, athlete or just very muscular because muscle weighs more than fat. 


Consequently, you could have a higher BMI reading regardless of a healthy body fat level.  BMI testing may also not be accurate if you're pregnant or breastfeeding and if you're over the age of 60 because bones weigh less as you get older.

BMI - apple or pear

Body shape and BMI

People generally store body fat in one of two places; around the hips and thighs or around the middle, and the shape of your body and the way that you store fat can make you more prone to certain disease.  If you store fat around the middle you're known as an 'Apple', and around the hips/thighs, a 'Pear'.  Health professionals believe that people with more weight around the waist (Apple types) face more health risks than people who carry more weight around their hips (Pear types).  The excess weight around the middle of the body can add extra strain on the body, increasing cholesterol levels which in turn could lead to the risk of heart disease, strokes or diabetes.

Finding out what your waist measures is a good way of calculating fat distribution and measuring your waist circumference and waist hip ratio calculates the proportion of fat stored on your body around your waist and hips to indicate potential health risks.  Let a tape measure rest loosely on your skin and measure your waist at its narrowest point (around your tummy button), then your hips at their widest point (around the buttocks).  Next, divide your waist measurement by your hip measurement.  The figure you get from this calculation is your waist-hip ratio.  For example, a female with a waist measurement of 29 inches and hip measurement of 38 inches gives a waist-hip ratio of 0.76.  You can use the calculator below to get your own waist-hip ratio figure.  The World Health Organisation says you are at increased risk if the waist measurement exceeds 94cm (37 inches) for men and 80cm (32 inches) for women.  Women should aim for a waist circumference of no greater than 80cm (32in) and men should aim for no greater than 94cm (37in). 


Sarah Dawson 60px

Profile of the author

Sarah Dawson is a Brighton based journalist who writes for national and international newspapers, magazines and websites. Sarah has worked as a journalist since 1997, mostly as a freelance.  Her articles have appeared in a diversity of publications from The Guardian to Red magazine.  Sarah specialises in health & wellbeing, holistic travel and lifestyle features.


View Sarah Dawson's website.


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