The Small Scale Gap Test is a relatively simple experimental tool to obtain a measure of the shock sensitivity of explosives. An evaluation of this test is carried out with use of a computer simulation. The simulated pressures in the water gap compare to experimental data and justify the obtained energy fluence in the shock wave versus distance in the water gap. The initiating shock pulse can be represented by an equivalent planar shock with an effective diameter of only 15 mm and a pulse duration of ≈1 µs. For the type of explosives that show some non-ideal behavior and that have a relatively large critical diameter, the initiation pressure might provide an inadequate value for the shock sensitivity. It is recommended that besides using a critical shock pressure criterion also a critical energy fluence criterion is applied to rank the shock sensitivity of explosives.