Measuring
7 Converting Units for Medication Amounts
Lesson
Learning Outcomes
By the end of this chapter, learners will be able to:
- identify when units require conversion when comparing between the medication order and medication supply, and
- convert between common units of measure.
Determining When to Convert Units
When an order for medication is supplied in an amount with a different unit of measure than the order, you will need to convert units so they match in order to ensure you are giving the correct dose of medication. Not all orders will require unit conversion.
Sample Exercise 7.1
Which of the following orders require unit conversion before medication administration?
Order A:
- Medication Order: prednisone 25 mg PO once daily
- Medication Supply: prednisone 5 mg tablets
Order B:
- Medication Order: acetaminophen 1 g PO QID prn
- Medication Supply: acetaminophen 500 mg tablets
Answer:
Order B requires unit conversion as the order is given in grams and the supply is provided in milligrams. Order A and the supply are both provided in milligrams.
Converting Units
To convert from one unit of measure to another, you need to know how many of a particular unit is equal to a single unit of the other type of measure. You can refer to the conversion table for quick reference if you are unfamiliar with how many of one unit would be in another for the units commonly used in medication administration. These amounts are called conversion factors. You will then set up an algebraic equation to convert between units, with the conversion factor written as a fraction.
Let’s say we need to give 0.5 grams (g) of a medication and the supply is in milligrams (mg). How many mg are equal to 0.5 g?
[latex]\text{? mg} = 0.5\text{ g}[/latex]
Start the equation with what you need to know, in this case, how many mg. We use “[latex]x[/latex]“ to represent the unknown amount of mg. Then, we need to use the conversion factor of 1000 mg = 1 g. When you set up the formula, put the type of units on top which matches the unit you are looking for. In this example, we are trying to find mg, so write in [latex]\dfrac{1000\text{ mg}}{1\text{ g}}[/latex]. Finally, we multiply by the known amount. The formula would look like this:
[latex]x\text{ mg}=\dfrac{1000\text{ mg}}{1\text{ g}}\times0.5\text{ g}[/latex]
To solve this equation, complete the calculation using the standard order of operations. Some people use the acronym BEDMAS to help them remember the order of operations: Brackets, Exponents, Division or Multiplication, Addition or Subtraction.
You can always check to see if you are ending up with the correct units by cancelling out units which match in the numerator and denominator of the equation. In this case, grams in the numerator and denominator cancel out, leaving us with just an amount of mgs, which is exactly what we want!
[latex]x\text{ mg}=\dfrac{1000 \text{mg}}{1\cancel{\text{g}}}\times0.5\cancel{\text{ g}}[/latex]
[latex]x=500\text{ mg}[/latex]
Sample Exercise