OBJECTIVES: To develop an algorithm mapping the Functional Assessment of Cancer Therapy – Breast (FACT-B), to the 5-level EuroQoL Group’s 5-dimension questionnaire (EQ-5D-5L) index. METHODS: A baseline survey of 238 self-administered breast cancer patients in Singapore was conducted. Regression models using ordinary least square, Tobit and censored least absolute deviation were fitted to predict the EQ-5D-5L index based on the five subscale (physical, social/family, emotional, functional well-beings, additional concerns for breast cancer) scores of the FACT-B. Data from a follow-up survey of these patients were used to validate the results. RESULTS: A model that maps the physical, emotional, function well-being subscales and the breast cancer concerns of the FACT-B to the EQ-5D-5L index was derived. The social/family well-being subscale was not associated to the EQ-5D-5L index. Although theoretic assumptions may not hold, ordinary least square showed better goodness-of-fit measures than other regression methods. The mean predicted EQ-5D-5L index within each performance status level deviated from the observed mean. However, the magnitudes of the deviations were smaller than the minimally important difference in EQ-5D for cancer patients. CONCLUSIONS: The mapping algorithm converts the FACT-B to the EQ-5D-5L index, which is essential in health economic evaluation. This enables oncologists and clinical researchers to obtain a quantitative utility summary of a patient’s health status when only the FACT-B is assessed.