A quinaria (plural: quinariae) is a Roman unit of area, roughly equal to 4.2 square centimetres (0.65 in2). Its primary use was to measure the cross-sectional area of pipes in Roman water distribution systems. A "one quinaria" pipe is 2.31 centimetres (0.91 in) in diameter. In Roman times, there was considerable ambiguity regarding the origin of the name, and the actual value of a quinaria. According to Frontinus: The importance of this measure was that water taxes in ancient Rome were based on the size of the supply pipe.