Gender data gaps in the United Kingdom are political choices that reflect deeper assumptions about whose labour, lives, and experiences are deemed worth measuring. This paper examines the strengths and limitations of the UK census for bridging these gaps, drawing on feminist participatory action research with 170 participants across 12 workshops in England, Wales, Scotland, and Northern Ireland. Participants comprised representatives from local government, women’s and non-profit organisations, academia, and unaffiliated individuals with varying statistical skills. Through co-produced inquiry, participants identified critical gender data gaps and questioned the political and epistemological assumptions embedded in census design. We find that while the UK census provide comprehensive disaggregated multivariate data at the local level, they systematically omit critical dimensions of gendered life, including income, unpaid childcare, and occupational segregation. Drawing on Fricker’s (2007) concept of epistemic injustice, we argue that these omissions are not incidental but structural: the census lacks the conceptual framework to recognise them as dimensions worth counting. Realising the census’s potential for feminist analysis, therefore, requires deliberate reshaping of data systems and practices.Crucially, we demonstrate that involving those affected by gender data gaps as co-analysts, rather than consultation subjects, surfaces blind spots and analytical priorities that academic or statistical institutions are unlikely to identify from within, making the case for embedding participatory mechanisms in how official statistics are designed, produced, and used. The paper concludes with four user-informed recommendations for census reform and the wider reimagining of UK gender data systems.