The present disclosure describes methods and systems, including computer-implemented methods, computer program products, and computer systems, for measuring a water cut for hydrocarbon fluid in a production pipe. One method includes transmitting a microwave through a first waveguide attached to a production pipe, wherein the microwave is directed to the hydrocarbon fluid in the production pipe; and obtaining, measurement results based on reflection or propagation of the microwave, wherein the measurement results are used to determine a water cut of the hydrocarbon fluid.