Calculating and plotting BMI

Having obtained an accurate weight and height for a child, it is then possible to calculate their BMI and plot this onto the correct BMI chart. BMI charts are valid for children aged 2 years upwards and they are gender specific.

How to plot BMI

Once the BMI is calculated, the next step is to plot it onto the gender specific BMI chart, to find out the BMI percentile.

Gender specific BMI charts

Boys and girls have separate BMI charts because their body compositions differ. It is very important that the correct BMI chart is used, otherwise a child may be incorrectly categorised.

Steps in plotting BMI

Plotting BMI is very similar to plotting height and weight on a centile chart. The instructions below provide the main steps in plotting BMI:

  1. Select the correct BMI chart (i.e. Girls’ chart or Boys’ chart).
  2. Have the child’s calculated BMI ready.
  3. Know the child’s exact age.
  4. On the BMI chart, track along the X axis (bottom line) to find the age of the child.
  5. Track up the Y axis (side line) to find the BMI for the child.
  6. Join the lines for the child’s age and BMI.
  7. Make a cross or dot at the point at which the age and BMI lines meet.
  8. Make a note of the BMI percentile as you would for height or weight, such as:
    • “on the 75th percentile”
    • “just below the 50th percentile”
    • “between the 85th and 95th percentile”.

Things to watch out for when plotting BMI

The main points to look out for when plotting BMI are:

  • the BMI is correct
  • the correct chart is being used (BMI is gender specific)
  • the child is two or more years of age
  • the BMI is plotted against the correct age.

Once the BMI percentile has been plotted it is worth double-checking that the point you have made on the percentile chart is for the correct age and the correct BMI. This will only take a relatively quick glance over the chart.

Interpreting the BMI chart

Healthy weight range

Most of the children you see will be a healthy weight for their height. Children with a healthy weight have a BMI between the 5th and 85th percentile.

Encouragement and healthy lifestyle advice should be given to all parents, even if their children are a healthy weight. Based on current statistics, at least half of all children will be overweight by the time they themselves become parents.

Interpretation of BMI percentiles

Below the 5th percentile

Child may be failing to thrive. However, a proportion of children will be naturally thin and healthy.

You will need to use your clinical judgment to determine whether a low BMI is indicative of concurrent illness.

Between the 5th and 85th percentile

This is the healthy BMI range.

On or above the 85th percentile but below the 95th percentile

This is the overweight BMI range.

On or above the 95th percentile

This is the obese BMI range (although care should be taken to limit the use of the word obese with parents as it is generally unacceptable. Generally obese children should be referred to as being overweight when talking to parents).

For information on the ways to discuss a child's BMI with their parents, see Discussing BMI with parents.

What inaccuracies can occur with BMI?

The following extract is taken from the Community Paediatric Review “Body Mass Index (BMI) for children”, Volume 15 Number 1, May 2006.

The extract demonstrates how inaccuracies in weighing and/or measuring a child, and/or calculating BMI can impact on their placement on the BMI percentile chart.

CPR Extract
Jenny is 4 years old. She weighs 18.5 kg and her height is 102.4 cm. Using these figures her BMI is 17.6kg/m2 which is in the 90th percentile and classifies Jenny as overweight.

Variation detailsCalculationBMIPercentileClassification
Actual BMI18.5 ÷ 1.024 ÷ 1.02417.690thOverweight
Weight overstated by 0.5kg19 ÷ 1.024 ÷ 1.02418.195thObese
Height understated by 1.2cm18.5 ÷ 1.012 ÷ 1.01218.195thObese
Weight understated by 1.5kg17 ÷ 1.024 ÷ 1.02416.275thNormal
Height overstated by 2.5cm18.5 ÷ 1.049 ÷ 1.04916.875thNormal
Weight divided by height once18.5 ÷ 1.02418.195thObese

There are four stages in acquiring the BMI percentile for each child, these are:

  • obtaining an accurate weight
  • obtaining an accurate height
  • calculating the BMI
  • plotting the BMI.

Inaccuracies or errors at any one of the stages can cause the child to be wrongly classified and lead to inappropriate management. It is vital to ensure that each stage is correct. Practicing each stage in real life situations will assist with proficiency and minimise the likelihood of errors.

More information

For an overview of this section, you can download: Calculating BMI  (pdf - 36.37kb)