Background Loneliness is a pressing public health concern with wide-ranging impacts on mental, physical and social wellbeing. Building on the INTERACT Study which represents one of the largest volunteer-based studies of loneliness and social disconnection conducted in the UK, this paper explores demographic, social and health-related predictors of loneliness and social capital, using multiple validated measures.Methods We analysed cross-sectional data from 135,725 community-dwelling adults across England. Loneliness was assessed using both the UCLA 3-item Loneliness Scale and the ONS Direct Measure of Loneliness (DMOL). Social capital was measured using a composite scale of neighbourhood trust, cohesion and reciprocity. Multivariable ordinal logistic regression was used to examine factors associated with loneliness; binary logistic regression was used to analyse correlates of high versus low social capital.Results Younger age (particularly 16–25), being single, unemployed or living with disability were consistently associated with higher loneliness across both scales. In contrast, greater social contact (having nine or more friends or relatives) was strongly protective (UCLA: aOR 0.09; DMOL: aOR 0.16). University education was associated with higher loneliness on the UCLA scale but lower loneliness on the DMOL. High social capital was more prevalent among older, married and retired individuals and strongly predicted lower loneliness. Respondents with long-term conditions or disability had reduced odds of high social capital (aORs 0.82 and 0.77 respectively).Conclusions This study highlights consistent sociodemographic and social factors associated with loneliness, as well as the protective role of social capital. Findings highlight population subgroups that may warrant prioritisation in future intervention research and policy prescriptions that address social connection among young adults, single people, the unemployed and individuals in poor health. Given the non-probability sampling design, findings are not intended to estimate population parameters, but support further evaluation of strategies that strengthen neighbourhood cohesion and social infrastructure to mitigate loneliness and strengthen community wellbeing.