The present study is a large-scale quantitative analysis of intra-individual variation (linked to type of interlocutor) and inter-individual variation (linked to multilingualism, sociobiographical variables and three personality traits) in self-reported frequency of code-switching (CS) among 2116 multilinguals. We found a significant effect of interlocutor (friends, family, colleagues and strangers) on self-reported CS. Participants who grew up and work in a multilingual environment, know many different languages, which they learnt from a young age, have advanced proficiency in various languages reported more frequent CS. Sex, extraversion and cognitive empathy, but not tolerance of ambiguity, are linked with higher self-reported CS. We conclude that the frequency of self-reported CS depends not just on situational, complex sociobiographical and environmental factors, but it is also mediated by the personality of the multilingual.