Maternal and Child Health Nurse
BMI education resource
Section 3: Calculating and plotting
BMI
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Contents
Section 3: Calculating and plotting BMI
BMI formula and units calculation
Completing a BMI calculation
How to plot BMI
Interpreting the BMI chart
Practice plotting and interpreting BMI
Section 3 Answers
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Section 3: 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.
The areas addressed in this section are:
• An understanding of what information is needed to calculate BMI.
• How to calculate BMI.
• Plotting BMI and obtaining BMI percentiles.
• Interpreting BMI charts.
Did you know?
Based on a survey completed by 175 Maternal and Child Health Nurses in February 2006:
• Only 22% respondents knew the correct formula to calculate BMI, and
• Only 10% actually calculated it for young children.
To help tackle the childhood obesity epidemic at a population level, both these figures would
ideally be close to 100%.
BMI formula and units calculation
The formula for calculating BMI is:
2
BMI (kg/ m )
=
Weight (kg)
---------------Height (m) 2
In words, the calculation is expressed as “the weight in kilograms divided by the height in
metres squared”.
In practical terms, when using a calculator the weight in kilograms is divided by the
height in metres twice (ie the weight is divided by the height, then the result is divided by
the height again).
NB: There are a number of ways the BMI can be calculated using a calculator. For example
BMI can be calculated by dividing the weight in kilograms by the height in centimetres twice,
then multiplying the result by 10,000. Although other methods may be valid, the formula in
the box is universally recognised, is the simplest and will be used throughout this CD ROM.
Units required for weight and height for the BMI calculation
To calculate the BMI you need:
• the weight in kilograms, to the nearest 0.1kg, and
• the height in metres, to the nearest 0.1cm.
Height is often recorded in centimetres, accurate to 0.1cm (1mm). For BMI, height needs to
be converted to metres.
If you are unsure how to convert centimetres to metres, read the information on page 16.
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Centimetres to metres
The formula for BMI requires the height to be in metres. People often record children’s
heights in centimetres, accurate to 0.1cm. There are 100 centimetres in a metre, therefore:
100cm = 1.00m
132cm = 1.32m
92cm = 0.92m
The conversions on the left demonstrate how centimetres (without
millimetres) are converted to metres. The decimal place is moved
two positions to the left. However heights should always be
measured to the nearest 0.1cm (1mm), this is shown below.
78.6cm = 0.786m
117.4cm = 1.174m
100.3cm = 1.003m
83.0cm = 0.830m
120.1cm = 1.201m
The conversions on the left demonstrate how measurements
accurate to 0.1cm (or 1mm) are converted into metres. For the
BMI calculation the height needs to be in metres. The decimal
place is still moved two places to the left. When the height is
accurate to 1mm, there are three numbers after the decimal place.
Practice converting the following heights which are in centimetres (accurate to 1 millimetre)
into metres, then compare your answers with the answers provided on page 26.
Height in cm
105.3
113.7
96.4
84.0
110.0
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© 2007
Height in m
16
Completing a BMI calculation
Practicing calculating BMI
Thomas’ weight is 17kg and height is 1.061m. Have a calculator ready, and work through
each step.
•
•
•
•
•
•
Type 17 into the calculator
Press the divide key (either / or ÷)
Type 1.061 into the calculator
Press the divide key again (/ or ÷)
Type 1.061 into the calculator for the second time
Press equals (=)
You now have Thomas’ BMI. Write your answer in the box then check your answer on page
26.
17 kg ÷ 1.061 m ÷ 1.061 m =
Rounding
Numbers can be rounded to make them easier to use and understand. BMI is rounded to
one decimal place, which means that there is one number after the decimal point. If the
second number after the decimal point is 4 or under the number is rounded down and if it is 5
or over the number is rounded up. Any more numbers past the decimal point just provide
more minute detail, which you probably don’t need.
The following table shows how the BMI is rounded. The numbers in red indicate the
numbers that are important for rounding for BMI.
Calculator reading
Rounding
BMI result
13.2357
Round down
13.2 kg/m2
14.3488
Round down
14.3 kg/m2
12.2139
Round down
12.2 kg/m2
15.0936
Round up
15.1 kg/m2
16.2683
Round up
16.3 kg/m2
15.4731
Round up
15.5 kg/m2
17.5537
Round up
17.6 kg/m2
If you want more rounding practice, do the exercises on page 18.
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Rounding practice
Complete the following table to practice rounding BMI. The first column provides numbers
shown on the calculator.
• Write the number that is important for the rounding in the second column.
• Write “up” or “down” to indicate the direction of the rounding in the third column.
• Write the final rounded BMI result in the forth column.
• Check to see if your answers are correct, the answers are provided on page 26.
The first example has been completed for you.
Calculator reading
Important number
for rounding
Rounding
(up or down)
BMI result
kg/m2
17.6384
3
Down
17.6
13.2056
_____
_______
_____
18.6507
_____
_______
_____
14.6983
_____
_______
_____
15.1728
_____
_______
_____
16.9983
_____
_______
_____
12.3497
_____
_______
_____
15.0097
_____
_______
_____
Check your answers with those supplied on page 26.
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© 2007
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Automatic BMI calculators
There are a number of automatic (computerised) BMI calculators available. Some health
care providers use their own automatic BMI calculators on systems such as desktop
computers or handheld devices. Automatic BMI calculators request and provide a selection
of information, which may include:
•
•
•
•
•
Information requested
Height
Weight
Age (or date of birth)
Gender
Date of measurement
•
•
•
•
•
Information provided
BMI
BMI percentile
BMI category
BMI z-score
Interpretation of BMI
Not all computerised BMI calculators request and provide all the above information. At the
very least they will provide the BMI which may then need to be manually plotted onto the
correct BMI chart and interpreted.
If you have access to the Internet, you can use the links below to view a couple of examples
of automatic BMI calculators.
• http://apps.nccd.cdc.gov/dnpabmi/Calculator.aspx?CalculatorType=Metric
• http://www.rch.org.au/genmed/clinical.cfm?doc_id=2603
If you use the CDC calculator, the units are entered in centimetres, and it gives the BMI and
BMI percentile, indicating whether it is the healthy range. Be aware – this is an American
site which uses “at risk of overweight” for overweight and “overweight” for children who are
obese.
Take a moment – reflect
• Is there a BMI calculator available from your health care service? If so, become familiar
with the system available, the information it requires and the information it provides, then
test it out using measurements from young children.
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© 2007
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Additional practice calculations
Despite a number of automatic BMI calculators available, there will often be occasions when
they are not practical to use. Being able to calculate BMI manually, using just a calculator,
will enable BMI to be calculated every time that height and weight is measured. Practice
calculating BMI by working out the BMI for the children below:
Calculation checklist
The following is a checklist for the correct calculation of BMI:
9 Weight is in kilograms and recorded to the nearest 0.1kg
9 Height is in metres, accurate to the nearest mm (or 0.1cm or 0.001m)
9 Weight is divided by the height twice
9 The BMI result is rounded to one decimal place
Practice the BMI calculation by working out the BMI for the following children:
Emma weighs 19.5kgs and is 1.156m tall; her BMI is:
_______
Greg weighs 24.6kg and is 1.198m tall; his BMI is:
_______
Sean weighs 14.3kg and is 97.2cm tall; his BMI is:
_______
Caroline weighs 25.2kg and is 114.4cm tall; her BMI is:
_______
Check your answers with those supplied on page 26.
Take a moment – reflect
• Do you feel confident in your ability to calculate BMI?
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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.
A large version of the individual BMI charts for boys and girls is available on the CD ROM in
the “Resources” section.
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© 2007
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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.
2.
3.
4.
5.
6.
7.
8.
Select the correct BMI chart (i.e. Girls’ chart or Boys’ chart).
Have the child’s calculated BMI ready.
Know the child’s exact age.
On the BMI chart, track along the X axis (bottom line) to find the age of the child.
Track up the Y axis (side line) to find the BMI for the child.
Join the lines for the child’s age and BMI.
Make a cross or dot at the point at which the age and BMI lines meet.
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:
9 The BMI is correct.
9 The correct chart is being used (BMI is gender specific).
9 The child is 2 or more years of age.
9 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.
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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
BMI range
Interpretation
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
This is the healthy BMI range.
percentile
On or above the 85th percentile This is the overweight BMI range.
but below the 95th percentile
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).
Ways to discuss a child’s BMI with their parents is covered later in Section 5: How to
discuss BMI with parents.
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Practice plotting and interpreting BMI
To complete this activity you will need to have a copy of the BMI percentiles for boys and
girls.
Select the correct chart for each child and using the following information, plot their BMI, fill in
the gaps regarding their BMI percentile and select their weight category from the options
provided.
Emma is 4 years and 9 months
old, her BMI is 14.6 kg/m2
Her BMI percentile is between the
____ and ____ percentile
She is:
† Healthy weight
† Overweight
† Obese
Sean is 3 years and 3 months
old, his BMI is 15.2 kg/m2
His BMI percentile is on the ____
percentile
Caroline is 2 years and 5 months
old, her BMI is 19.3 kg/m2
Her BMI percentile is above the
____ percentile
Greg is 4 years old, his BMI is
17.1 kg/m2
His BMI percentile is just above
the ____ percentile
He is:
† Healthy weight
† Overweight
† Obese
She is:
† Healthy weight
† Overweight
† Obese
He is:
† Healthy weight
† Overweight
† Obese
Check your answers with those provided on page 26.
Printed from “BMI Education” CD ROM
© 2007
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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 details
Calculation
BMI
Percentile
th
Classification
Actual BMI
18.5 ÷ 1.024 ÷ 1.024
17.6
90
Overweight
Weight overstated by 0.5kg
19 ÷ 1.024 ÷ 1.024
18.1
95th
Obese
Height understated by 1.2cm
18.5 ÷ 1.012 ÷ 1.012
18.1
95th
Obese
Weight understated by 1.5kg
17 ÷ 1.024 ÷ 1.024
16.2
75th
Normal
Height overstated by 2.5cm
18.5 ÷ 1.049 ÷ 1.049
16.8
75th
Normal
18.5 ÷ 1.024
18.1
95th
Obese
Weight divided by height
once
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.
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Section 3 Answers
Centimetres to metres practice
Height in cm
Height in m
105.3
1.053
113.7
1.137
96.4
0.964
84.0
0.84
110.0
1.100
Practicing calculating BMI
The answer is 15.1 kg/m2. You may have included more decimal places, if so, read the
information about rounding on page 17.
Rounding practice
Calculator reading
Important number
for rounding
Rounding
(up or down)
BMI result
kg/m2
17.6384
3
Down
17.6
13.2056
0
Down
13.2
18.6507
5
Up
18.7
14.6983
9
Up
14.7
15.1728
7
Up
15.2
16.9983
9
Up
17.0
12.3497
4
Down
12.3
15.0097
0
Down
15.0
BMI practice calculations
Emma weighs 19.5kgs and is 1.156m tall; her BMI is:
Greg weighs 24.6kg and is 1.198m tall; his BMI is:
Sean weighs 14.3kg and is 97.2cm tall; his BMI is:
Caroline weighs 25.2kg and is 114.4cm tall; her BMI is:
14.6 kg/m2
17.1 kg/m2
15.1 kg/m2
19.3 kg/m2
Practice plotting and interpreting BMI
Her BMI percentile is between the
Emma is 4 years and 9 months
25th and 50th percentile
old, her BMI is 14.6 kg/m2
She is:
Healthy weight
Sean is 3 years and 3 months
old, his BMI is 15.2 kg/m2
His BMI percentile is on the 25th
percentile
He is:
Healthy weight
Caroline is 2 years and 5 months
old, her BMI is 19.3 kg/m2
Her BMI percentile is above the
95th percentile
She is:
Obese
Greg is 4 years old, his BMI is
17.1 kg/m2
His BMI percentile is just above
the 85th percentile
He is:
Overweight
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© 2007
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