PIF guide: BMI – What you need to know

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BMI – What you need to know

Body Mass Index (BMI) has been rubbished in the media. So why do doctors use it? This will help you make sense of BMI.

What is BMI?

BMI stands for body mass index. It divides your weight by your height to see if you are in the healthy weight range. There are four key ranges for BMI.

UnderweightHealthy weightOverweightObese
Below 18.5Between 18.5 – 24.9Between 25 – 29.9Between 30 – 39.9

Why do doctors use it?

BMI is a tried and tested way of checking if you are in the healthy weight range. It may not be accurate if you are pregnant, an older person or extremely athletic.

Being underweight can lead toBeing overweight or obese can lead to
Fragile bones
Feeling tired1
Some cancers2
Type 2 diabetes
Heart disease
Heart attack

Why does ethnicity matter?

Ethnicity can affect how much fat you have compared to muscle and where you carry the fat. This can increase health risks. For example, adults of South Asian origin may have a higher risk of diabetes with a BMI of 23 or over4.

Why can BMI feel like a judgement?

You may feel upset if your BMI shows you are in the underweight, overweight or obese range. However, you should not feel judged or shamed. 

Based on NHS data collected from people’s medical records, 6 in 10 UK adults are classed as overweight or obese3

Talk to your doctor about your BMI and your health and wellbeing. They can help you to make changes you want to make.

Is BMI used to restrict access to NHS treatments?

Surgery is safer for people in the healthy weight range. If your BMI is high or low, you might be offered support to reduce your risks before treatment. Small changes can make a big difference.

The Royal College of Anaesthetists offers advice to help you get fitter before an operation:

How do I check my BMI?

The NHS has a simple calculator you can use to check your BMI:

If you have any concerns contact your doctor for advice and support.

Download the PDF guide

BMI – What you need to know

Publish date: September 2022
Review date: September 2024

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