Strombolian eruptions are cyclic, mildly-explosive events that are driven by the rise andvburst of discrete gas slugs which ascend rapidly through a almost stagnant column ofvlow-viscosity magma. Slug’s burst overpressure with respect to the ambient pressure isvthe main factor controlling explosion dynamics and vigor. We present a physical model for the calculation of overpressure within a bursting gas slug. The model neglects inertial and viscous effects and demonstrates that significant overpressure may develop even in their absence. Results show that the thickness of the magma film draining around the rising slug exerts a primary control on the development of overpressure: a thicker liquid film (resulting form, e.g., a more viscous magma) results in larger overpressure. Film thickness is represented through the geometrical parameter A’, which is the fraction of the conduit section occupied by draining magma in the slug region. A number of existing models relating A’ to conduit diameter and liquid viscosity were tested by performing scaled laboratory experiments on air slugs ascending in cylindrical pipes filled with liquids with a range of viscosities. The best-fit model is used to calculate A’ for the range of nondimensional flow conditions expected in volcanic conduits. We apply our model to calculate burst overpressure of strombolian eruptions using appropriate volcano-scale parameters, and compare results with previously published estimates of bursting overpressure derived from a broad dataset of eruptions at Stromboli: our results show that magma-static load and geometrical factors alone can account for the observed overpressures during slug-driven explosions.