Calculations

Last updated: Monday, September 13, 2021

This page is aimed at trainee 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 resources that may assist you.

Registration exam 

The pre-registration framework shows the common types of calculation that pharmacists might need to do in practice. It states that the registration 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, some questions in part 2 will require some calculation.

Feedback from the GPhC on which questions students find more difficult does vary from year to year. However from the 2021 March sitting candidates did less well in calculations involving concentrations and dilutions. Examiners frequently stress 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 capsules 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 

CPPE has a package entitled Clinical Calculations for Pharmacy Professionals which covers all areas of practice. It includes calculations related to ideal body weight, creatinine clearance, dose and dose equivalence, strengths, displacement values and more.

The Standardised Numeracy Assessment Process (‘SNAP’) website provides interactive training videos about drug calculations for staff who prescribe, dispense or administer medicines. 

Although a licence is required for full access to the site, you can view several helpful clinical numeracy videos without a subscription. You have to register with the site first (top right-hand corner). If your employer is not listed in the drop-down list of Trusts offered when registering, then just choose ‘Other’.  Once you are logged in, you can view the videos under the 'Dashboard' tab. These include;


- 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.


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.