In liquid-metal-cooled fast breeder reactors (LMFBRs), electromagnetic flowmeters (EMFs) have been extensively used to measure the coolant flow rate. Because the coolant flow rate is one of the most important parameters, a high accuracy and a fast response are required for the flow rate measurement. However, it was thought that the response might become slower when the pipe diameter of the EMF was increased. Therefore, a quantitative evaluation of the response was needed. To evaluate the response time of EMFs, an equation of the transient response was derived based on the realistic approximation that the EMF pipe is made of nonconductive material. The response is expressed as a function of the reciprocal of the square of the pipe radius a and of the length L of the external magnetic field along the pipe axis. However, when the aspect ratio L/2a is larger than two, the length of the external magnetic field has an almost insignificant effect on the response, and the response time increases with increasing a2. The transient response can be calculated with an uncertainty of less than a few percent. A first-order approximation of the derived equation is given by the first lag term with the time constant of µσa2/ 3.832 with permeability µ and conductivity a of the coolant. Even though the EMF has a diameter as large as 30 in., the response time is 45 ms and sufficiently fast compared with other sensors used in LMFBRs.