As urbanisation and climate change progress, the frequency of flooding will increase. Each flood event causes damage to infrastructure and the environment. It is thus important to minimise the damage caused, which can be done through planning for events, real-time control of networks and risk management. To perform these actions, many different simulations of network behaviour are required involving complex and computationally expensive model runs. This makes fast (i.e. real-time or repetitive) simulations very difficult to carry out using traditional methods, thus there is a requirement to develop computationally efficient and accurate conceptual sewer simulators. A new Cellular Automata (CA) based sewer model is presented which is both fast and accurate. The CA model is Lagrangian in nature in that it represents the flow as blocks, and movement of the blocks through the system is simulated. To determine the number of blocks which should be moved it uses either the Manning's or Hazen–Williams equation depending on the flow conditions to calculate the permitted discharge. A case study of the sewer network in Keighley, Yorkshire, is carried out showing its performance in comparison to traditional sewer simulators. The benchmarks used to verify the results are SIPSON and SWMM5.