Calculations topic – Dosage and unit conversions

Dosage conversions

Dosages are sometimes written on prescriptions in a way that is unhelpful to the patient and as a pharmacist, we have to convert them into a more patient friendly version.

As an example:
Folic acid solution, 3 mg each day
Since the medicine is a liquid, it would be inappropriate for us to label the medicine as 3 mg each day.
We need to know the strength of the liquid, in this case 2.5 mg/5mL, and then calculate the volume to give.

You can use a stepwise process:

2.5 mg in 5 mL Divide each side by 5
0.5 mg in 1 mL Multiply each side by 6
3 mg in 6 mL

Or by a simple spotting technique where you can see that 3 mg is 20% (or 1/5) bigger than 2.5 mg so you will need to increase the volume in the same way

5 mL + 20% = 6 ml

You would follow a similar process in other situations, for example

When the strength of a product is in units/ mL, the dose is prescribed in units but the patient will be measuring out a volume

Unit conversions

We tend in this country to use metric units in medicine.

Where this is the case, it is rare to need to convert except between different scales of units:

Weight – units getting smaller
x 1000x1000x1000x1000
Kg—–>g —–> mg —–> microgram —–> nanogram
Weight – units getting bigger
nanogran—–>microgram —–> mg —–> g —–> kg

So for the same weight of product, the larger the unit the smaller the number


This works just the same way, as the weights but we only tend to change from:

  • L to mL, multiply the number by 1000
  • mL to L, divide the number by 1000

A person’s height and weight – you may need to do this to calculate BMI

Many people still measure their height and weight in imperial measurements, so

There are:

  • 12 inches in one foot
  • 3 feet in one yard
  • 1 inch is equivalent to 2.54 cm (to two decimal places)

So take an imperial height measurement, convert it to inches and then multiply the number of inches to get the measurement in cm

There are also:

  • 16 ounces in one pound
  • 14 pounds in one stone
  • 1 pound is equivalent to 0.453592 kg (to six decimal places)

So take an imperial weight measurement, convert it to pounds and then multiply the number of pounds by 0.453592 to get the measurement in kg