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28 June 2021

What is BMI and how is BMI calculated?


BMI is your body mass index.

BMI uses your height and weight to work out if you’re a healthy weight, underweight or overweight.

Your BMI result is not always the perfect measure of your overall health, it can’t tell if you’re carrying too much fat or you’ve got a lot of muscle.  Your BMI result should be used as a starting point for further discussion with your doctor.

Tape And Fork


How is BMI Calculated?

We calculate BMI using the standard formula of a person’s mass in kg divided by the square of their height in meters (kg/m2) and display it to one decimal place.

The BMI result is assigned to a standard category:

  • Less than 18.5 – underweight
  • 18.5 to 24.9 – healthy weight
  • 25 to 29.9 – overweight
  • 30 to 39.9 – obese
  • 40 and over – very obese (also known as morbidly obese)
BMI calculations

What Your BMI Result Means

Underweight – Being underweight could be a sign you’re not eating enough or you may be ill. If you’re underweight you can contact your local GP can help and give you advice.

Healthy weight – Good job, you are maintaining a healthy balance of diet and exercise. keep up the good work!  For tips on maintaining a healthy weight keep an eye on our blog.

Overweight – The best way to lose weight if you’re overweight is through a combination of diet and exercise. The one rule you need to understand is; to lose weight you need to be in a calorific deficit, consuming fewer calories than you burn. As a rough guide without exercise women expend roughly 1,600 to 2,400 calories per day and men 2,000 to 3,000 calories per day. Keep an eye out on our blog for more information regarding calorie deficits & tips on losing weight at home.

The BMI calculator will give you a personal calorie allowance to help you achieve a healthy weight safely.

Obese – If your BMI result shows that you’re obese then you need to start losing some weight otherwise you could be compromising your health. See a GP for help and advice.

tape measure

Black, Asian and Other Minority Ethnic Groups

Black, Asian and other minority ethnic groups have a higher risk of developing some long-term (chronic) conditions, such as type 2 diabetes. If you are in one of these groups it is important to be aware that the BMI result may not be 100% realistic.

These adults with a BMI of:

  • 23 or more are at increased risk
  • 27.5 or more are at high risk

Is BMI Accurate?

There are some scenarios where the BMI result could be misleading, please see below:

  • Older adults can have a healthy BMI but still have too much fat. This is because older adults tend to have more body fat than younger adults.
  • Women can have a healthy BMI but still have too much fat. This is because women tend to have more body fat than men with the same BMI.
  • If you’re from a black and ethnic minority group, you’re at increased risk of type 2 diabetes with a BMI of 23 or more.
  • An athletic adult with a lot of muscle may have a high BMI but not be overweight. This is because BMI can’t tell the difference between fat and muscle. Muscle weighs more than fat so some athletic body types may be labeled as overweight by the test.
  • If you are pregnant, you should use your pre-pregnancy weight to work out your BMI. Using your pregnancy weight may not be accurate due to the extra human you are carrying.
BMI scales

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