BMI- Body Mass Index

See also: Dieting for Weight Loss

BMI, Body Mass Index is a measure of a person’s body shape – how fat or thin they are.

BMI is calculated from a person’s height and weight which give an indication of the levels of fat in their body. 

BMI is not a perfect system for measuring body fat, however in most cases BMI is reliable and gives a fair indication of body fat levels.

BMI is widely used, internationally by many healthcare professionals to help establish a patient’s risk of illness - especially diseases and health complaints associated with being overweight or obese.  Body Mass Index is also used to help establish effective drug doses – people with a higher BMI may require a higher dose of medicine.

Your Body Mass Index is therefore an important reference, this page includes simple tools that you can use to calculate and monitor your BMI.


Calculate your BMI

In order to calculate your Body Mass Index you need to know your height and weight.  Most people know their height – if you don’t know yours then ask somebody to measure you – approximate values within a few centimetres or an inch are fine. 

Use some reliable scales to measure your weight, remove as much clothing as appropriate before weighing yourself.

BMI Calculator

Use our calculator to work out your Body Mass Index.

You can use metric or imperial measurements or a combination of the two.

Understanding BMI

BMI is not always accurate, your BMI may not be a valid indicator of body fat if:

  • You are aged between 2 and 20 – Although the same method is used to calculate BMI in children and adolescents, an extra step is required to gain a meaningful value. See BMI for children below.

  • You are pregnant.

  • You have a very athletic, muscular build. Muscles weigh more than fat and therefore for athletes BMI is unrepresentative of true fat levels. Athletes' BMI may indicate that they are overweight when they are not in reality.

Furthermore:  Women generally have more body fat at the same BMI as men.  Older people tend to have more body fat than younger people with the same BMI.


BMI Categories

The following categories are used to determine adult body types based on BMI scores. 

The World Health Organisation (WHO) defines a healthy adult BMI as being between 18.5 and 25.  A BMI less than 18.5 indicates underweight (possibly caused by disease, malnutrition or eating disorders such as anorexia nervosa) and a BMI over 25 as overweight (possibly caused by poor diet choices or lack of exercise). 

These categories do not apply to children and adolescents aged between 2 and 20 years, see BMI for children below.

BMI BMI Category
Less than 15 Very severely underweight
Between 15 and 16 Severely underweight
Between 16 and 18.5 Underweight
Between 18.5 and 25 Healthy weight
Between 25 and 30 Overweight
Between 30 and 35 Moderately obese
Between 35 and 40 Severely obese
Over 40 Very severely or morbidly obese

If you are worried about your weight then you should seek professional help and advice.


BMI Chart

It can be useful to view BMI categories in a tabular form – especially to see your ideal weight range.  The chart below shows BMI scores and categories for a range of the most common heights and weights.

BMI Chart

 

Need a bigger chart with more heights and weights? Download our printable PDF - BMI Reference Sheet.


Body Mass Index for Children and Adolescents

Calculating the BMI for children and adolescents is the same as that for adults, you can use our calculator or chart or work out BMI manually.

However, BMI for people aged between 2 and 20 years has a different meaning and therefore needs different interpretation.

As children grow, so their ratio of body fat changes – age is taken into account in order to gain a useful meaning of a child’s BMI. Furthermore, girls and boys have different body fat ratios during development and therefore the sex of the child is also important when measuring BMI.

For children and adolescents BMI categories are based on age and gender within percentile ranges.  Percentiles are used to determine a child’s BMI category based on data from other children of the same age and sex.  A percentile is a measure representing the value below a given percentage of BMI in the data group.  For example, the 5th percentile refers to the lowest 5%, the 95th percentile refers to the lowest 95% (or top 5%) of BMI readings in the collected data.

BMI Categories for Children and Adolescents

Child/Adolescent
BMI Category
Percentile Range
Underweight Less than the 5th percentile.
In the lowest 5% of the sample.
Healthy weight Between the 5th and 85th percentiles. 
Not in the bottom 5% or top 15% of the sample.
Overweight Between the 85th and 95th percentiles.
Not in the bottom 85% or top 5% of the sample.
Obese Above the 95th percentile.
In the top 5% of the sample.

Percentile Data

Compare your child's BMI to the data provided below. Find their age along the bottom axis and their BMI along the vertical axis. Plot their calculated position on the chart.

  • If your child's point falls below the blue line then they are less than the 5th percentile and may be underweight.
  • If your child's point falls between the blue and black lines then they are a healthy weight.
  • If your child's point falls between the black and red lines then your child is overweight or in danger of becoming overweight.
  • If your child's point falls above the red line then they are above the 95th percentile and may be obese.

If you are worried about your child's weight then you should seek professional help and advice.

BMI Percentile Data for Children and Adolescents

Percentile graphs based on Centers of Disease Control and Prevention original data.

Parents might also find our pages Feeding Babies and Feeding Toddlers helpful.


Manual BMI Calculation

If you require a very accurate BMI reading, or simply want to be able to calculate your BMI without the use of an online calculator or chart, then you can use the following formulas.

Using metric measurements (metres and kilograms)

BMI = weight ÷ (height2)

Example:

For this example we use a height of 1.85 metres and a weight of 92 kilograms.

  • Work out your height squared – that is, multiply your height by itself.
    Using our example height 1.85 × 1.85 = 3.42 (rounded to two decimal places).
  • Divide your weight by your height squared.
    Using our example weight and height squared 92 ÷ 3.42 = 26.9.

In this example the BMI is 26.9.


Using US or imperial measurements (inches and pounds)

BMI = weight ÷ (height2) × 703.

Example

For this example we use a height of 72inches (6ft) and a weight of 202 pounds (14st 6lbs).

  • Work out your height squared – that is, multiply your height by itself.
    Using our example height 72 × 72 = 5184.
  • Divide your weight by your height squared and multiply the answer by 703.
    Using our example weight and height squared 202 ÷ 5184 × 703 = 27.34.

In this example the BMI is 27.34.


Want a quick BMI reference guide to use offline? Download our free and printable BMI Reference Sheet.

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