Whether a reduction in overpressure across a hydrocarbon column and its seal will reduce the column height that the seal is capable of supporting has been much debated. Recent studies have focused on measuring water relative permeability at high capillary pressure, and have concluded that hydrocarbon columns may be sufficiently permeable to water for the seal capacity to be unaffected by hydrodynamic flow in most situations. A new method for estimating the capillary seal capacity is developed which properly takes into account viscous as well as gravity and capillary forces. A reassessment of existing analyses indicates that hydrodynamic flow has a greater influence on the seal capacity than previously thought. Sets of sensitivity models, including a range of reservoir and seal geometrical and petrophysical characteristics, indicate that – as a general rule – hydrodynamic flow should not be neglected when assessing capillary seal capacity. The sealing capacity of thick top seals above reservoirs with a basal aquifer are least likely to be affected by hydrodynamic flow. The sealing capacity of membrane fault seals, irrespective of whether they have a basal or edge aquifer, may be influenced by realistic overpressure differences. Simple end-member equations define the possible influence of hydrodynamic flow in a particular situation, and only if this is significant should a more comprehensive evaluation be considered.