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Healthy weight: what BMI shows, what it misses and what the NHS offers

Guide Published

BMI is an NHS check on whether weight suits height, but it cannot tell muscle from fat, so waist size and a GP discussion add to it.

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Key points

  • The NHS BMI calculator checks whether a person is a healthy weight for their height.
  • BMI cannot tell muscle from fat, so very muscular people may be rated overweight and people who lose muscle with age may still fall in the healthy range.
  • The NHS says Black, Asian and some other minority ethnic groups have a higher risk of some long-term conditions at a lower BMI.
  • NHS treatment for obesity ranges from lifestyle changes and weight management programmes to medicines and surgery.

What does BMI measure?

Body mass index, or BMI, is calculated from height and weight, and the NHS calculator uses it to check whether an adult is a healthy weight for their height. The calculator shows which category a result falls in, and the NHS gives a different healthy range for Black, Asian and some other minority ethnic groups. The reason given is that these groups have a higher risk of developing some long-term conditions, such as type 2 diabetes, at a lower BMI.

What are the limits of BMI?

BMI cannot tell the difference between muscle and fat. Very muscular people and athletes may be rated overweight or obese despite low body fat. Adults who lose muscle as they get older may fall in the healthy weight range even though they may be carrying excess fat.

Why does waist size matter?

The NHS gives a method for measuring the waist: find the bottom of the ribs and the top of the hips, place a tape measure midway between them, and measure after breathing out naturally. The NHS and NICE describe a waist measurement of more than half a person's height as linked with increased health risks. The NHS advises seeing a GP for help with losing weight, or if the waist measurement is more than half of height.

What treatment does the NHS offer?

The NHS says a GP can advise on weight loss and on options such as weight management programmes, medicines or surgery. Options include lifestyle changes, weight management programmes, medicines and surgery. Local NHS specialist weight management services, often called tier 3, usually have a team that can include a doctor, dietitians, specialist nurses and psychologists. NHS access to weight-loss injections such as semaglutide and tirzepatide depends on eligibility criteria and varies by area; some are offered through specialist weight management services, and tirzepatide is being introduced through GP practices in phases for people with the highest clinical need.

What health risks are linked to excess weight?

The NHS lists type 2 diabetes, heart disease, stroke, high blood pressure, several cancers, osteoarthritis and fatty liver disease among conditions linked to excess weight.

Why does weight change?

The NHS describes overweight and obesity as having more body fat than the body needs. It can happen when a person eats or drinks more than the body needs or does not do enough physical activity. Other factors listed include genes, some medicines, and some health conditions such as an underactive thyroid.

What does a GP check, and what support is free?

The NHS says a GP looks for possible underlying causes of obesity, for example a medicine the person is taking. The NHS offers a free 12-week weight loss plan as an app. The NHS says that for someone living with obesity, losing about 3 per cent of starting body weight can reduce the risk of obesity-related complications.

More guides and news

Educational content. This article summarises public guidance and announcements in plain English for education. It is not medical advice and it cannot say what is right for you: speak to a clinician about your own health, call NHS 111 if you are unsure what to do and 999 in an emergency.