The disconnection of sewered impervious areas is a measure to reduce the overflow frequency and hydraulic load of both combined and improved separate sewer systems. In this article the sizing of disconnection facilities is elaborated. The usual procedures for determining the required dimensions of the facilities contain some spurious assumptions. These assumptions have been removed, resulting in a more realistic design procedure for disconnection facilities to make sure that the discharge of stormwater is guaranteed. With the use of simulations various design graphs have been constructed. In the graphs the permeability of the soil (K value) is plotted against the required construction length of the facility (L value) scaled to the amount of connected impervious area. For a known K value, a known amount of impervious area and a chosen cross-section of the facility, the required construction length can be assessed directly from the graphs. The effect of a discharge drain at the bottom of a percolation trench has also been investigated. It appeared that, compared to a facility without a drain, the required dimensions of a facility with a drain decrease significantly. Consequently, soils with low permeability allow the application of percolation facilities, provided that a discharge drain is installed at the bottom of the facility. As the drain prevents groundwater filling the facility, the facility with a drain can be applied in areas with relatively high groundwater levels, as occur in many cities in the delta and polder areas of The Netherlands.