© 2020, The Authors. Background: GPs prescribe multiple long-term treatments to their patients. For shared clinical decision-making, understanding of the absolute benefits and harms of individual treatments is needed. International evidence shows that doctors' knowledge of treatment effects is poor but, to the authors knowledge, this has not been researched among GPs in the UK. Aim: To measure the level and range of the quantitative understanding of the benefits and harms of treatments for common long-term conditions (LTCs) among GPs. Design & setting: An online cross-sectional survey was distributed to GPs in the UK. Method: Participants were asked to estimate the percentage absolute risk reduction or increase conferred by 13 interventions across 10 LTCs on 17 important outcomes. Responses were collated and presented in a novel graphic format to allow detailed visualisation of the findings. Descriptive statistical analysis was performed. Results: A total of 443 responders were included in the analysis. Most demonstrated poor (and in some cases very poor) knowledge of the absolute benefits and harms of treatments. Overall, an average of 10.9% of responses were correct allowing for ±1% margin in absolute risk estimates and 23.3% allowing a ±3% margin. Eighty-seven point seven per cent of responses overestimated and 8.9% of responses underestimated treatment effects. There was no tendency to differentially overestimate benefits and underestimate harms. Sixty-four point eight per cent of GPs self-reported 'low' to 'very low' confidence in their knowledge. Conclusion: GPs' knowledge of the absolute benefits and harms of treatments is poor, with inaccuracies of a magnitude likely to meaningfully affect clinical decision-making and impede conversations with patients regarding treatment choices.