Pipe tank sizing calculator was prepared together with Amiblu Poland Sp. z o.o., a producer and sup-plier of GRP pipe systems. This tool allows for a quick selection of an underground horizontal storm and melt water retention basin, using the data given by the user. As a result, the user gets an illustra-tive design of the basin together with its parameters such as basin’s diameter, length and capacity. All calculations connected with basin’s capacity are done in accordance with DWA-A117 guidelines (for catchment areas of ≤ 200 ha with sewage systems or systems with flow time of ≤ 30 min). The calculator makes work easier and reduces time needed to prepare necessary calculations when selecting the appropriate retention basin.