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Assessing body weight

Someone’s ideal body weight can be defined as the weight that carries the least risk to general health. In practice there’s a specific range of body weights for a given height that is associated with best health.

Being overweight or obese means having an excess of fat in the body and it’s the excess body fat that impairs health. Therefore to assess a person’s health risk we need to determine how much body fat they’re carrying. However, it can be difficult to measure body fat directly, but it can be estimated in various ways.

The simplest way of quickly assessing whether or not a person is at a healthy weight is to calculate their Body Mass Index (BMI). BMI assesses weight in relation to height.

BMI gives a good reflection of body fat and is a method commonly used to identify those at risk of ill health.

BMI = Weight (kg) / Height (m²)

The result indicates whether a person is underweight, within the healthiest weight range, overweight or obese.

BMI below 18.5kg/m² – Under weight

  • Intentionally trying to lose weight to reach and stay within this range can be unhealthy.
  • Being underweight can lead to problems such as osteoporosis, anaemia and also affect fertility.
  • Someone in this range may not be eating enough food to supply their body with the nutrition they need.

BMI 18.5-25kg/m² – Healthy weight

  • Being in this range puts someone at least risk of developing weight-related complications.
  • Someone’s health may not necessarily benefit from further weight loss and it’s important not to aim to go below this range.
  • It’s important to encourage healthy eating and regular physical activity to help prevent excess weight gain and the related risks.

BMI 25-30kg/m² – at risk of developing weight-related health problems

  • Being in this range means that someone may be at greater risk of developing weight-related health problems such as heart disease, stroke and type 2 diabetes.
  • Losing weight will reduce this risk and could also improve existing health conditions.

BMI greater than 30kg/m² – at greatest risk of developing weight-related health problems

  • Being in this range means that someone is at greater risk of developing weight-related health problems such as heart disease, stroke and type 2 diabetes.
  • Losing weight and keeping it off will reduce the risk and improve health and fitness

Important notes:

  • The point at which someone’s health is at risk due to their weight varies between ethnic groups. It’s recommended that Asian (South Asian and Chinese), black African and African Caribbean populations use slightly lower BMI cut offs (BMI 23 for ‘at risk’ and BMI 28 for ‘at greatest risk’) as an indicator for when their health may be at increased risk.
  • A BMI guide may not be accurate for people with a high muscle mass, pregnant women or teens and children. BMI charts specific to children are available.

For the majority of adults, BMI gives a good indication of the amount of body fat they have and their health risk. It is a simple and useful way of making an initial assessment of health risk and for monitoring weight loss progress. However, in some circumstances BMI may not always give a true picture of someone’s health risk. This is because BMI does not distinguish between weight associated with body fat and weight associated with muscle. For example, someone who does a lot of exercise and is very muscular may be heavy but not have a lot of body fat – they could be misclassified as overweight or obese by the BMI method because their muscle makes them heavier.