On a tanker for transporting oil and chemicals products, the calculation of the main ship's firefighting system with water deals with the calculation of the main ship's overall fire-fighting system. This paper is structured to initially deal with combustion, fire, fire classifications, and fire extinguishing agents. Fire detectors and fire extinguishing systems that can be installed on board on a ship, according to international regulations, are described. Water based fire extinguishing systems and foam fire extinguishing systems are described in more detail. In the chapter in which water and foam fire extinguishing systems are described in more detail, all the components of the system are listed, in addition to the requirements according to the international organization regulations, ie classification societies. The last part of the paper refers to the calculation of the foam fire extinguishing system on the main deck of oil and chemical tankers. The calculation was made according to the data obtained at the shipyard. The procedure is explained in detail, and the necessary equipment the for proper operation of the dimensioned system.