Calculations

Last updated: Monday, July 31, 2017

This page is aimed at pre-registration hospital pharmacists.

There are many situations in which hospital pharmacists need to perform calculations, so you will have to be up to the required standard in order to practise. This page provides links to free resources that may assist you.

Pre-reg exam 

The pre-registration framework shows the common types of calculation that pharmacists might need to do in practice. It states that the pre-reg exam is likely to include at least one calculation question involving each of the following in part 1:

  • doses and dose regimens 
  • dosage and unit conversions 
  • estimations of kidney function (see Renal tutorial)
  • displacement volumes and values 
  • concentrations (e.g. expressed as w/v, % or 1 in x) 
  • dilutions 
  • molecular weight 
  • using provided formulae 
  • infusion rates 
  • pharmacokinetics (see Drug handling tutorial) 
  • health economics 
  • quantities to supply 

In addition, up to ten questions in part 2 will require some calculation.

Feedback from the 2017 pre-reg exam showed that candidates did less well in calculations involving infusion rates, and where dose rounding was required. Examiners also stressed that candidates should ask themselves if their answer is realistic and practical. For example, you should review your answer if a calculation results in a dose requiring 100 ampoules, or suggests a dose of 403.28mg when there are 400mg tablets available.

The basics 

Not everyone likes maths. But are you worried that you don’t really understand some of the basics? If so, then please take time to tackle this because pharmacists in hospital need to do calculations all the time. BBC Bitesize has a series of maths revision tools and worked examples aimed at GCSE students. This will help you with fundamental skills such as percentages, decimals, equations, and approximating.

Calculations in the healthcare setting 

The NHS Standardised Numeracy Assessment Process (‘SNAP’) website provides free interactive training videos about drug calculations for staff who prescribe, dispense or administer medicines. You have to register with the site first to get in (top right-hand corner). If your employer is not listed in the drop-down list of Trusts offered when registering, then just choose ‘Other’. In the section called Clinical Numeracy Videos, the site offers tuition and self-testing exercises on:

- Overview of drug calculations (including units and percentages)
- Common calculation types (including dose by body weight, infusion rates and dilutions)
- The ‘WIG’ method of calculating (‘what you Want, what it’s In, what you’ve Got’)
- Infusions and drip rate
- More complex calculations (including ten multiple-step drug calculations)

The ResourcePharm website has a large collection of materials concerned with drug calculations in practice. You'll find example clinical problems involving calculations, worked examples, and maths tuition in a variety of healthcare settings. There’s a lot of material here ranging from basic to more specialist.

Our quiz


The best way to improve your calculation skills is to keep practising them. In addition to the other resources here, we've created a practice-based quiz that may help you. Please select 'practice' as your game mode then click Start Game. You can look things up if you need to.

For your mobile 

Health Education Thames Valley and Health Education Wessex have developed an Adult Drug Calculations app for use on Apple and Android smartphones. It’s designed for nurses, and looks at converting between metric units; percentages and concentrations; infusion dosing and rate; and there is a quiz at the end. Type ‘Adult Drug Calculations UK’ into your app store or get the download address from here.

US websites 

A number of American sites provide tuition on calculations involving medicines and you may like to look at these, but bear in mind that they may not represent UK or NHS practice. For example, PharmaFactz has example worked calculations including displacement values, concentrations, dilutions, and moles.