A method of operating an internal combustion engine. With reference to FIG. 1, fuel is supplied to charge air using an injector (116) which in each operation delivers a set amount of fuel. The amount of fuel supplied to the charge air in each engine cycle is controlled by how many times the injector (116) operates in each cycle. A desired fuel demand is calculated as a number of operations of the injector per cycle, calculated to at least one decimal place. The desired fuel demand is rounded to a near integer to provide an output fuel demand for the injector as a number of operations of the injector for the next operating cycle in varying operating conditions of the engine. The controller calculates an aggregate number of operations for a plurality of engine cycles which is closer to an aggregated desired fuel demand for the plurality of cycles than if for each cycle of the plurality of output cycles the output fuel demand is calculated independently.