1
Introduction

Pharmacists provide the final safety check before medicines reach the patient. During your time at university as a pharmacy student and during your preregistration year, you will develop many skills to help you minimise the risk of the medicine you supply harming a patient. The ability to accurately carry out pharmaceutical calculations is one of these skills.

As a student, it is easy to become engrossed in the mathematical side of the calculation and forget that the purpose of the calculation is to ensure the patient receives the correct product at the appropriate strength with appropriate dosing instructions. Throughout this book we have therefore tried to relate the chapters back to the circumstances in practice in which you may need to use the particular type of calculation.

We have also observed students over many years trying to overcomplicate pharmaceutical calculations when they carry them out. We understand that while you are a student, complicated mathematics are required in other parts of your course, for example in statistics and pharmaceutics. The mathematics needed for pharmaceutical calculations is of a much more basic level and primarily requires you to be able to add, subtract, multiply and divide. This is one of the reasons you are encouraged or expected to be able to carry out these calculations without the aid of a calculator. The General Pharmaceutical Council supports this recommendation. It may have been a long time since you attempted mathematical problems without a calculator and you may have forgotten some of the methods, in particular, when asked to multiply or divide a number. As a reminder we have produced Appendix 3, which reminds you how to carry out both multiplication and long division.

We understand that for some of you, calculations are never going to be a topic of choice. If you feel like this, you need to consider whether you could think about numbers on a regular basis. Being comfortable with basic numeracy is a skill that can, like many skills, be developed through practice. Like any new skill, try to develop it slowly but regularly:

• add up your bill in a café while standing in the queue;
  
• if you carry out some exercise, keep working out how far you are towards your target as a percentage, decimal or fraction of that target;
  
• keep a check on how much money you have got left in your bank account by keeping a running balance in your head.

You will need to do these activities without the aid of a calculator in order to get any benefit.

The other reason you are required to work without a calculator is to encourage you to sense-check your calculation and final answer. We encourage you to do this because it will help you think about the calculation in real rather than abstract terms. Consider the following:

You are asked to express the strength of salbutamol syrup 2 mg/5 mL as a percentage.

Percentage calculations require you to work only in grams and millilitres. If you didn’t remember this rule, it would be very easy to tap numbers into a calculator and decide that the answer is a percentage strength of 40%, i.e.