A coalescence model in the context of the ReDrop concept (Representative Drops) is proposed to design technical equipment for separating liquid-liquid dispersions in settlers or extraction columns. A fundamental study of drop interactions has been performed to obtain the complete picture of coalescence. The model proposed accounts for collision frequency of the drops, bouncing probability, and coalescence probability, for which the film-drainage approach is applied. The model proposed allows considering the appropriate formulation for different types of equipment. Especially noteworthy is that the coalescence probability fundamentally differs from the expression of Coulaloglou and Tavlarides, which is frequently used, but which shows inconsistencies at fundamental level.