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Succeed with maths: part 2
Succeed with maths: part 2

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3.2 Formulas in health care

The body mass index (BMI) is sometimes used to help determine whether an adult is underweight or overweight. It is calculated as follows:

Body mass index equation left hand side equals right hand side mass in kilograms divided by left parenthesis height in metres right parenthesis super two

Although care needs to be taken in interpreting the results – for example, the formula isn’t appropriate for children, old people or those with a very muscular physique – a BMI of less than 20 suggests the person is underweight and a BMI of over 25 suggests the person is overweight.

In this formula, the units have been included in the expression on the right-hand side of the equals sign. It is important to change any measurements into these units before you substitute the values into the formula. (No units have been included for the BMI on the left-hand side, in line with current health care practice.) This is true for any formula that you will come across, so it is very important to remember.

This time, instead of giving you an example first it is straight into an activity to apply the formula.

Activity 7 Body mass index

Timing: Allow approximately 5 minutes

If an adult man is 178 cm tall and weighs 84 kg, calculate his BMI and decide whether he is overweight.


The formula needs the mass in kg and the height in metres. You have the man’s height in centimetres, so this must first be converted into metres.

one m equals 100 cm

So, his height in metres equation left hand side equals right hand side open 178 division 100 close m equals 1.78 m

Substituting the mass and height into the formula for the BMI gives:

BMI equation sequence equals 84 division 1.78 squared equals 26.5 equals 27 (to the nearest whole number)

Since his body mass index is over 25, the man is probably overweight.

Hopefully, you can see that this relatively simple formula gives any health care professional a way to check the health status of any patient.

The next section example introduces a bit more complexity in a formula, but all the same principles apply that you have already used. These are, converting to required units before substituting into a formula and following BEDMAS as necessary.