The article reveals the fundamentals of engineering and designing mobile weir with rectangular opening for farmlands, applicable for measuring discharge in interfarm and temporary canals. It includes the fundamentals of choosing most efficient structural solutions for the weir, choosing material for fabrication of its main elements, performing the static design for weir horizontal beams according to marginal states of providing reliability and stability of weir main elements. Results of calculations of diameter of steel bar for rectangular weir with B=25cm showed that the diameter needed is 6mm. The diameter is small enough for the weir to be called a thin plate weir. Thus, using 6mm bar is reasonable enough from the point of view of stability and the by the requirements for a thin plate weir.