Background: Failure to understand the rules and procedures regarding self-isolation when symptomatic with Covid-19 may have contributed to the low level of adherence found in the UK. We investigated knowledge of these rules and factors associated with knowledge.Methods: Data were drawn from repeated weekly/fortnightly online surveys of nationally representative cross-sectional samples of approximately 2,000 UK adults each from 9 November 2020 to 2 June 2021 (40,544 responses from 33,568 participants). We computed a composite measure of knowledge of what self-isolation means by combining item responses. We investigated associations between knowledge of self-isolation rules, with survey wave, socio-demographic characteristics (age, gender, UK nation, rural/urban area, index of multiple deprivation), trust in government, and participants’ belief that they had received enough information about self-isolation.Results: 87.0% (95% CI 86.7% to 87.4%, n=33,541/38,533) of participants knew that if they had symptoms of COVID-19 they should ‘self-isolate’. However, only 58.3% (n=22,451/38,533, 95% CI 57.8% to 58.8%) knew about all the main rules regarding what that meant. Younger people had less knowledge than older people, and men had less knowledge than women. Knowledge was lower in people living in England versus those living in Scotland, Wales, and Northern Ireland. The pattern of association with trust in government was unclear. Knowledge was lower in people living in a more deprived area and those who did not believe they had enough information about self-isolation. Knowledge was lower in December 2020-January 2021, compared with before and after this period.Conclusions: Fewer than 60% of adults in the UK between November 2020 and June 2021 appeared to know all the main rules regarding self-isolation if symptomatic with Covid-19. Knowledge was lower in younger than older people, men than women, those living in England compared with Scotland, Wales or Northern Ireland, and those living in more deprived areas.