Summary: We develop a model, which we call the quality index (QI) model, for measuring how good a mathematics question is. Based on the literature on mathematics assessment, we develop a theoretical framework, using three measuring parameters: discrimination index, confidence index and expert expectation. The theoretical framework forms the foundation on which we form an opinion of the qualities and characteristics of a good mathematics question. The QI assigns a quantitative value to the quality of a question. We also give a visual representation of the nature of a question in terms of a radar chart using the mentioned criteria and illustrate the use of the QI model by applying the measure to question examples, given in each of two formats ‒ provided response questions (PRQs) and constructed response questions (CRQs). A greater knowledge of the quality of mathematics questions can assist mathematics educators and assessors to improve their assessment programmes and enhance student learning in mathematics.