Understanding your health and weight: Body mass index (BMI)

There are many things that affect our weight and health. These include the things that have happened to us and the things that we do.

The things that have happened to us:

  • age
  • income
  • where we live
  • access to services
  • medication
  • family history
  • weight history
  • medical history
  • illness
  • trauma

The things that we do:

  • what and how much we eat and drink
  • our physical activity levels
  • time spent being inactive
  • sleep
  • previous attempts to lose weight

It is difficult to know what an individual’s ideal weight should be.

What is body mass index (BMI)?

BMI is used to categorise people’s weight. BMI charts are mainly used for working out the health of populations rather than individuals.

Within a population there will always be people who are at the extremes (have a high BMI or low BMI).

A high or low BMI may be an indicator of poor diet, varying activity levels, or high stress. Just because someone has a ‘normal BMI’ does not mean that they are healthy.

BMI doesn’t take account of body composition, for example, muscle, fat, bone density. Sex and other factors which can impact your weight can also lead to an inaccurate reading. As such a BMI calculation is not a suitable measure for some people including children and young people under 18, pregnant women and athletes.

BMI ranges

Your BMI will fit into one of 5 bands:

  • under 18.5 – This is described as underweight. 
  • between 18.5 and 24.9 – This is described as the ‘healthy range’.
  • between 25 and 29.9 – This is described as overweight
  • between 30 and 39.9 – This is described as obesity
  • 40 or over – This is described as severe obesity
BMI chart
Use our chart to find out if you are over or under weight using your BMI result NHS inform

If you have lost weight because you have been poorly or not eating very much you could ask your GP or health professional for help. If you don’t know why you have lost weight, see your GP or health professional.

Results of 25 and over are described as a ‘higher BMI’ and may be just one of the many reasons some of us are at higher risk of some health conditions, including type 2 diabetes.

The BMI ranges are slightly different for people from South Asian, Chinese, African-Caribbean and black African backgrounds. People from these backgrounds may be at higher risk of health problems at a lower BMI than people from other ethnicities.

For a lot of people, achieving a BMI within the ‘healthy range’ is not realistic. Whatever your weight there may be changes you want to make to improve your health, making a few small sustainable changes to your lifestyle can improve your health. Losing a small amount of weight, 5-10%, and maintaining this weight loss can have a significant positive impact on your overall health. We have a self-managed 12 week weight management programme available if you feel you are able to manage your weight without the support of a health professional

If you are concerned about your weight you should contact your health professional.

Why your weight range is important

Health problems associated with a BMI in the obesity range include:

  • type 2 diabetes
  • stroke
  • heart disease
  • some cancers
  • issues with fertility in women
  • high blood pressure
  • osteoarthritis
  • fatty liver disease
  • kidney disease

Health problems associated with a BMI in the underweight range include:

  • weakened immune system
  • osteoporosis
  • anaemia
  • issues with fertility in women
  • palpitations

Last updated:
30 April 2020

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