This is a revision guide in calculations for pharmacy students and registration trainees. This text is part of the RPS "Tomorrow's Pharmacist" series. The pharmaceutical societies of Great Britain and Northern Ireland (RPSGB and PSNI) have introduced compulsory calculations elements into the registration examinations. These sections must be passed independently of the rest of the examination with a mark of 70 per cent. The authors of this text actually set the calculation questions for the PSNI preregistration exam and are therefore aware of the standards required and the type of questions asked; making this an essential revision textbook for students taking this exam. This text is part of the RPS "Tomorrow's Pharmacist" series following the same style/design as these texts, which are all aimed at graduating students and preregistration trainees (titles currently include: "Registration Exam Questions", "The Preregistration Interview" and "Hospital Preregistration Training").