One of the major causes of landslide triggering of sandy slopes is heavy rainfall occurrences. The present study (a) clarifies the mechanism of rainfall-induced failures of sandy slopes and (b) based on this mechanism proposes a simplified expression predicting the instability. The proposed expression in some aspects has a form similar to others previously proposed, but the novelty of the present study is that, by analysis, it is specified (a) in which type of soil it can be applied exclusively, (b) which is the depth of the sliding surface and (c) which drainage condition corresponds to the strength parameter and how to measure this parameter. In order to achieve this, first a simplified expression is proposed assessing the stability, based on the assumptions that (i) the problem can be treated as one-dimensional under drained conditions and (ii) temporary water flow exists just above a relatively impermeable inclined layer. Furthermore, the paper gives typical values of model parameters and predictions of the proposed equation. Next, it discusses the drained assumption used to obtain the proposed equation and the manner in which to measure the soil friction angle, needed in the proposed equation. Partial validation of the proposed equations is also performed at six well-documented landslides at two sites: Jiuzhaigou Park, China and Sifnos island, Greece, where geotechnical investigations are performed in order to establish the soil properties. Finally, it proposes, based on the developed equation, (i) a methodology assessing the stability of sandy slopes under rainfall conditions and (ii) mitigation measures.