<p>Small mafic explosive eruptions are a globally common and frequent eruption type, and understanding the size of past eruptions is key to preparing for future impacts. However, Canadian volcanoes have received relatively little investigation due in part to their location in remote and challenging terrain. Here, we measured tephra thicknesses from 96 locations for the ~ 1700 CE eruption of <i>Sii Aks</i> (Tseax) in the Northern Cordillera Volcanic Province, British Columbia, a region of active volcanism and rifting in northwest Canada. We used these data to produce isopach maps and estimate the volume of tephra fall during the eruption, using exponential, power law, and Weibull functions. We find the values are consistent across the different methods at 2.5 – 3.4 × 10⁶ m³, thus classifying the eruption as two on the Volcanic Explosivity Index. This provides the first field-based and ground-truthed estimate of tephra fall volume for a Canadian mafic eruption.</p>