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BMI Calculator

The best way to understand your BMI is by checking it regularly.

Enter your measurements into our free BMI calculator provided by the NHS to understand what your reading means.

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BMI is your body mass index. Use this tool to find out if you’re a healthy weight. If you are under 18 please use the child’s BMI tab.

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. 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 labelled 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.

What Your BMI Result Means

Underweight – Being underweight could be a sign you’re not eating enough or you may be ill. It can affect your health by increasing your risk of heart disease, reducing the strength of your immune system, reducing fertility and causing osteoporosis (brittle bones). If you’re underweight you can contact your local GP, who 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, or start reading more about obesity risks and treatment here.

Black, Asian and other minority ethnic groups

If you’re from a Black, Asian or minority ethnic group, you’re at increased risk of type 2 diabetes and other long term conditions with a BMI of 23 or more.

Tips for losing weight at home

Read more

Achieve your health goals with our easy guide

Read more

Limitations of the BMI

Your BMI can tell you if you’re carrying too much weight, but it cannot tell if that weight is fat or muscle.

The BMI cannot tell the difference between excess fat, muscle or bone.

The adult BMI does not take into account age, gender or muscle mass.

This means:

  • very muscular adults and athletes may be classed “overweight” or “obese” even though their body fat is low
  • adults who lose muscle as they get older may fall into the “healthy weight” range even though they may be carrying excess fat

Pregnancy will also affect a woman’s BMI result. Your BMI will go up as your weight increases. You should use your pre-pregnancy weight when calculating your BMI.

Apart from these limitations, the BMI is a relatively straightforward and convenient way of assessing someone’s weight.

Eating disorders

If you have an eating disorder, the BMI calculator results do not apply. Please get further advice from a GP.

Children’s BMI

For children and young people aged 2 to 18, the BMI calculator takes into account age and gender as well as height and weight.

Overweight children are thought to be at increased risk of a variety of health conditions, and they’re also more likely to be overweight as adults.

The BMI calculator works out if a child or young person is:

  • underweight – on the 2nd centile or below
  • healthy weight – between the 2nd and 91st centiles
  • overweight – 91st centile or above
  • very overweight – 98th centile or above

A child’s BMI is expressed as a “centile” to show how their BMI compares with children who took part in national surveys.

For example, a girl on the 75th centile is heavier than 75 out of 100 other girls her age.

Measuring waist size is not routinely recommended for children because it does not take their height into account.

See a GP if you’re concerned about your child’s weight. They may be able to refer you to your local healthy lifestyle programme for children, young people and families.

Please use these links to find out the NHS guidence concerning underweight children aged 6 to 12 and very overweight children.

Why waist size matters

Measuring your waist is a good way to check you’re not carrying too much fat around your stomach, which can raise your risk of heart disease, type 2 diabetes and stroke.

You can have a healthy BMI and still have excess tummy fat, meaning you’re still at risk of developing these conditions.

To measure your waist:

  1. Find the bottom of your ribs and the top of your hips.
  2. Wrap a tape measure around your waist midway between these points.
  3. Breathe out naturally before taking the measurement.

Regardless of your height or BMI, you should try to lose weight if your waist is:

  • 94cm (37ins) or more for men
  • 80cm (31.5ins) or more for women

You’re at very high risk and should contact a GP if your waist is:

  • 102cm (40ins) or more for men
  • 88cm (34ins) or more for women


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