Calculating medication doses

Chapter 4

Calculating medication doses

Learning outcomes

At the end of this chapter, the practitioner will be able to:

imageUnderstand the importance of calculating medication doses accurately

imageUnderstand the principles that underpin safe, accurate medicine calculation

imageDemonstrate competency when calculating medication doses.


The National Patient Safety Agency (NPSA) reported a total of nearly 73,000 medication incidents in 2007 (NPSA 2009). While most of these incidents resulted in little or no harm to patients, the NPSA (2009) found that 41% resulted in serious harm or death and these were caused during the medication administration stage. In total, 5% of the incidents were directly attributable to the wrong dose of medication being administered (NPSA 2009).

Medication administration is a task that a healthcare practitioner performs on a daily basis, so it is vital to be able to calculate medication doses correctly. The healthcare practitioner needs to understand what they are doing, and be familiar with some of the commonly used dosage formulas. This knowledge will help the practitioner maintain safe practice with medicines administration.

Many of the errors reported by the National Patient Safety Agency (2009) could have been avoided if the calculation had been checked, and checked again. More often than not, it is human error that leads to mistakes being made.

What are the main points to remember when calculating medication?

Points for your practice

imageIs the amount correct? Often it is not the number that is wrong, but the amount given (O’Brien et al. 2011).

imageAsk yourself, is this a sensible answer?

imageHave you used the correct dose (e.g. milligrams, not micrograms)?

Points for your practice

Remember, the weight of a medicine (using tablets or capsules) is usually measured in grams, milligrams or micrograms:

1000 MICROgrams make a MILLIgram (mg)

1000 MILLIgrams (mg) make a GRAM (g)

1000 GRAMs (g) make up a KILOgram (Kg) (the weight of a bag of sugar).

The British National Formulary (BNF) recommends that micrograms are written in full, rather than using the abbreviation mcg. This is to avoid confusion, and potential accidental medication overdose.

The ‘conversion’ factor using these types of weights is 1000. Look at the medication that you are preparing to administer and check how it is measured. If a medication is to be dissolved or suspended in a liquid, the liquid format is measured in LITRES (L) and MILLIlitres (mL). Here, the conversion factor of liquid is also 1000. You will need to calculate how much liquid to administer to your patient. Some medicines state that the medication needs to be prepared using a weight in a specific volume of fluid (e.g. 1mg per mL). This means that 1 MILLigram (mg) of medication will be delivered per MILLIlitre (mL) of fluid administered, if it is prepared correctly.

Non-liquid forms of medication

Medications are manufactured in many different forms, such as liquid, tablets and powders. As this is a book about the administration of intravenous (IV) therapy, the examples provided will relate to liquid, in order to calculate doses that are appropriate for this route.

As a reminder, for medicines that are to be administered in a solid form (such as tablets or capsules), the basic formula for calculation is:


For instance, paracetamol is dispensed as 500mg tablets. If a patient is prescribed 1 gram, the formula would be applied as:


Medications in liquid form

Medications are often measured in grams. However, for IV administration, they need to be dissolved in a liquid or suspension format. Therefore, you need to calculate what amount of liquid to administer.

When calculating the quantity of medicine needed in a liquid form, the following formula can be used:




Example 4.1

Here is an example to help you apply the formula:

A patient is prescribed 80mg of Drug A. The label on the medicine says there are 40mg in 2mL. How many mL do you think you need to administer?

First set the problem out, using the formula above:

Want: 80mg (what you want)

Got: 40mg (what you have got)

In: 2mL (the volume it’s in)

Applying the formula, your calculation will be:


Your answer will be:


Activity 4.1

Think about some of the common medications that are used in your clinical area. How are these prepared and administered? Work out the calculations for the common doses that are used.

When you have worked out each calculation, check that it makes sense! Always stop and think – does it look as if there are too many tablets or too large a volume of liquid? Equally, is there a very small amount? Or do you need to split tablets? Always check your calculation again, and use a second checker if something does not look quite right.

Mar 25, 2020 | Posted by in NURSING | Comments Off on Calculating medication doses
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