How to measure BMI

A study has shown that in the UK you are more likely to be obese than underweight. The study, conducted by The Imperial College London, compared the BMI (body mass index) of around 20 million men and women over the last 30 years around the globe and found that obesity has more than doubled in women and tripled in men.

The study, which was published in The Lancet, focused on 186 countries worldwide and found that obesity has grown from 105 million in 1975 to 641 million in 2014. Over the same time, the proportion of underweight people fell from 14 percent to 9 percent of men and from 15 percent to 10 percent of women, according to the study. Despite this, starvation is still a serious threat in some regions.

Based on these findings, the researchers estimated that in nine years, a fifth of all adults globally and 40% of American adults will be obese. To qualify as obese, you need a BMI score of over 30. A score of under 18.5 is classed as underweight.

How do I calculate my BMI?

BMI is a recognised method of measuring whether a person's weight is right for their height. BMI is measured by calculating a person's weight in kilograms divided by the square of the person's height in metres (kg/m2).

For example, if a person weighs 60kg and measures 1.7m, you would calculate 1.72 (1.7 x 1.7) which equals 2.89. You'd then divide their weight by that result, so 60/2.89 = 20.7. Round this up to 21 to get the person's BMI. This person would fall into the healthy weight category.


Underweight:

< 18.5

Healthy:

18.5- 24.9

Overweight:

25 - 29.9

Obese:

30 or above


What are the risks of being overweight?

People with an overweight BMI are more likely to develop health problems such as:

  • Heart disease
  • Stroke
  • Type 2 diabetes
  • Some types of cancer
  • High blood pressure
  • Kidney disease

Regular exercise and a healthy diet can help to reduce BMI and eliminate these risks.

What are the risks of being underweight?

People who are underweight are also at risk of health issues such as:

  • Brittle bones (Osteoporosis)
  • Absent periods in women
  • Iron deficiency (Anaemia)

Weight lifting and gradual weight gain can help to target these issues.

BMI Accuracy

Healthcare professionals take multiple factors into consideration when assessing weight, as BMI alone does not take into account muscle. Muscle weighs more than fat and therefore a person who has a lot of muscle mass may have an overweight BMI, despite having a healthy level of body fat. 

Is BMI useful?

Whilst BMI is useful, it can only tell if you’re carrying too much weight. BMI can't tell the difference between excess fat, muscle, or bone.

The NHS states that adult BMI does not take into account age, gender or muscle mass. This means that:

  • very muscular adults and athletes may be classed "overweight" or "obese" even though their body fat is low   
  • adults who lose muscle as they get older may fall in the "healthy weight" range even though they may still be carrying excess fat

However, the BMI is a relatively straightforward and convenient method of assessing someone's weight.

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