What is BMI?

Body Mass Index helps you see if you're a healthy weight for your height.
What is BMI?

BMI (or Body Mass Index) is a way of working out if you’re a healthy weight for your height.

How do you work it out?

BMI = [ Weight in Kilograms / ( Height in Metres x Height in Metres ) ]

or to put it another way: weight in kilograms divided by height in metres squared.

So, for example, if you weigh 80kg and are 1.75m tall, your BMI = 80/(1.75x1.75) = 26.1

There is a calculator on the NHS website.

What does the result mean?

People come in all shapes and sizes so any BMI from 18.5 to 24.9 is considered to be healthy.

  • If your BMI is below 18.5, you are probably underweight.
  • If your BMI is 25 or over, you are probably overweight.
  • If your BMI is over 30, you are classified as obese. 

Being overweight and particularly being obese increases your risk of risk of heart disease, stroke and type 2 diabetes. 

How accurate is BMI?

BMI is guidance not a magic formula and not the only factor a doctor will be interested in.

Muscle weighs more than fat so professional athletes with high muscle bulk may have high BMIs without being unhealthily overweight.

Ethnicity matters too. Because of the increased risk of diabetes in Asian men, these men are advised to keep their BMI below 23. Although the evidence is less clear-cut, black people and other minority groups are also advised to maintain a BMI below 25 to reduce their risk of type 2 diabetes.

Anything easier?

Yes. Waist size.

Note that this is not necessarily the same as your trouser size. Measure your waist at the bottom of your ribs and the top of your hips and breathe out naturally (no holding it in.)

  • As a man, you have a higher risk of health problems if your waist size is more than 94cm (37 inches)
  • You are at an increased risk if your waist size is more than 102cm (40 inches).
How do I lose weight?

Check out our Food and Exercise FAQs.


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MAIN IMAGE: Tape Measure by Pink Sherbet licensed under CC BY 2.0

Date published 03/08/15
Date of last review 03/08/15
Date of next review 08/08/17


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