The competence framework for prescribers states that they should be able to accurately complete and routinely check calculations relevant to prescribing and practical dosing. Prescribers should know about common types of medication error and how to prevent them. The incorrect application of dosing equations is considered a major contributor to preventable adverse events associated with the prescribing of medicines. Anyone required to check calculations performed by others must be competent to perform such calculations independently. A complex calculation is any process requiring more than one step in the preparation and/or administration of a medicine to a patient. Throughout the chapter there are useful tips on how to reduce the risks associated with complex calculations. This chapter includes information on units of measurement, units of amount and various methods used to calculate drug concentration, dose and rate of drug administration. Also included are simple pharmacokinetic concepts and calculations specific to palliative care. At the end of each section there are sample calculations (with answers) allowing readers to test their calculation abilities