BMI - Body Mass Index

BMI, formerly called the Quetelet index, is a measure for indicating nutritional status in adults. 

It is defined as a person’s weight in kilograms divided by the square of the person’s height in metres (kg/m2). 

For example, an adult who weighs 70 kg and whose height is 1.75 m will have a BMI of 22.9.

70 (kg)/1.752 (m2) = 22.9 BMI

For adults over 20 years old, BMI falls into one of the following categories.

Overweight and obesity lead to adverse metabolic effects on joints, muscles, blood pressure, cholesterol, triglycerides and insulin resistance. 

Risks of coronary heart disease, ischemic stroke and type 2 diabetes mellitus increase steadily with increasing body mass index (BMI), a measure of weight relative to height. 

Raised body mass index also increases the risk of cancer of the breast, colon, prostate, endometrium, kidney and gall bladder. 

Mortality rates increase with increasing degrees of overweight, as measured by body mass index. 

To achieve optimum health, the range of 21 to 23 kg/m2...... 

while the goal for individuals should be to maintain body mass index in the range 18.5 to 24.9 kg/m2. 

There is increased risk of co-morbidities for body mass index 25.0 to 29.9, and moderate to severe risk of co-morbidities for body mass index greater than 30.

 Table - Nutritional status

BMINutritional status

Below 18.5



Normal weight




Obesity class I


Obesity class II

Above 40

Obesity class III

The BMI ranges are based on the effect excessive body fat has on disease and death and are reasonably well related to adiposity. 

BMI was developed as a risk indicator of disease; as BMI increases, so does the risk for some diseases. 

Some common conditions related to overweight and obesity include: 
premature death, 
cardiovascular diseases, 
high blood pressure, 
some cancers and diabetes.

BMI is also recommended for use in children and adolescents. 

In children, BMI is calculated as for adults and then compared with z-scores or percentiles. 

During childhood and adolescence the ratio between weight and height varies with sex and age, so the cut-off values that determine the nutritional status of those aged 0–19 years are gender- and age-specific. 

The cut-off points of the 2006 BMI-for-age reference for children aged 0–5 years for the diagnosis of overweight and obesity were set as the 97th and the 99th percentile, respectively. 

For those aged 5–19 years, overweight is defined as a BMI-for-age value over +1 SD and obesity as a BMI-for-age value over +2 SD.

Your BMI result


Being underweight could be a sign that you're not eating enough or that you may be ill. If you're underweight, your GP can help. Find out more in underweight adults.

Healthy weight

Keep up the good work. For tips on maintaining a healthy weight, check out our food and diet and fitness sections.


The best way to lose weight is through a combination of diet and exercise. The BMI calculator will give you a personal calorie allowance to help you achieve a healthy weight safely.


The best way to lose weight is through a combination of dietand exercise and in some cases medication. Contact your GP for help and advice.

Ethnicity and diabetes risk

Black, Asian and other minority ethnic groups (BMEs) have a higher risk of developing some chronic conditions, such as type 2 diabetes.

BME adults with a:

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

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 diseases.

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 (men)
  • 80cm (31.5ins) or more (women)

You are at very high risk and you should contact your GP if your waist is:

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

Children's BMI

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

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

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

A child's BMI is expressed as a "centile" to show how their BMI compares to 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 advised for children because it doesn't take their height into account.

If you're concerned about your child's weight, contact your GP who may be able to refer you to your local healthy lifestyle programme for children, young people and families.

Limitations of the BMI

Your BMI can tell you if you're carrying too much weight but it can't tell if you're carrying too much fat. The BMI can't tell the difference between excess fat, muscle, or bone.

The adult BMI does not take into account age, gender or muscle mass. This means that: 

  • 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 in the "healthy weight" range even though they may be carrying excess fat

However, the BMI is a relatively straightforward and convenient method of assessing someone's weight. 

Your can use your BMI result as a starting point for further discussion with your doctor about your weight and your general health.

If you're suffering from an eating disorder, the BMI calculator results do not apply. 
Please seek further advice from your doctor.

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