INTRODUCTION: To ensure the efficient use of chemotherapy drugs, chemotherapy wastage is an area that can be investigated. This study aims to quantify current parenteral chemotherapy wastage and estimate parenteral chemotherapy wastage when dose banding is executed, using a chemotherapy wastage calculator in an ambulatory cancer centre. The study also examines the variables that significantly predict the total cost of chemotherapy wastage, investigates the reasons for wastage, and explores opportunities to reduce wastage. METHODS: Data were collected from the pharmacy in National Cancer Centre Singapore over 9 months retrospectively. Chemotherapy wastage is the sum of wastage in the preparation phase and potential wastage in the administration phase. The calculator was created using Microsoft Excel and generated chemotherapy wastage in terms of cost and amount (mg) and analysed the reasons for potential wastage. RESULTS: The calculator reported a total of 2.22 million mg of chemotherapy wastage generated over 9 months, amounting to $2.05 million (Singapore Dollars, SGD). Regression analysis found that the cost of drug was the only independent variable that significantly predicted the total cost of chemotherapy wastage (P = 0.004). The study also identified low blood count (625 [29.06%]) as the top reason for potential wastage and no-show ($128,715.94 [15.97%]) as the reason that incurred the highest cost of potential wastage. CONCLUSION: The pharmacy has generated a considerable amount of chemotherapy wastage over 9 months. Interventions in both the preparation and administration phases are required to reduce chemotherapy wastage. The use of the chemotherapy wastage calculator in pharmacy operations could guide efforts to reduce chemotherapy wastage.