Informal care plays a significant role in the care system for older people in the United Kingdom, and this is projected to increase considerably in the next three decades as the population ages. Understanding these trends requires a good quality measurement of informal care. In this study, the authors compare care-givers’ responses to different informal care questions from the English Longitudinal Study of Ageing (ELSA) to investigate the influence of question design on the self-reporting of informal care. They also analyse spousal care dyads in order to model discrepancies in the reporting of care provision between spouses to provide an insight into the reliability of informal care measurements. The authors find that the most common measures used are likely to be under-estimating both the scale and scope of informal care, and they recommend careful consideration of the content of informal care survey questions in order to operationalise the measures of informal care activities.