Millions of non-producing oil and gas wells around the world are leaking methane and other contaminants, contributing to increased greenhouse gas emissions and polluting our water, soil, and air. Quantifying methane emissions and understanding the attributes driving these emissions are important for evaluating the scale of the environmental risks and informing mitigation strategies. With our national-scale direct measurement database of 494 non-producing wells across Canada, we find total annual methane emissions from non-producing wells in Canada to be 230 kt/year (51–560 kt/year) for 2023, which is 7 (1.5–16) times higher than estimated in Canada’s National Inventory Report (34 kt/year) and accounts for 13% of total fugitive emissions from oil and natural gas systems in Canada. We show that the role of well attributes in methane emissions is best evaluated by considering the emitting component (wellhead/surface casing vent) and the spatial scale (e.g., national, provincial, subprovincial). Large uncertainties in methane emissions from non-producing wells can be reduced not only with additional measurements but also with detailed well attribute analysis using direct measurements. Identifying attributes linked to high emitters can also be used to prioritize mitigation, thereby reducing methane emissions and broader environmental risks.