A centrifugal fluid pump assembly includes a body section and a control mechanism 6. The body section includes a housing, an impeller rotatable within the housing, and a motor for rotating the impeller. The pump assembly includes a motor current flow rate-related data storage section for storing, for a plurality of different predetermined viscosities, predetermined viscosity-related flow rate data formed from a plurality of motor current flow rate relation data at a plurality of different impeller rotational speeds at predetermined fluid viscosity, a fluid measurement data inputting section, an impeller rotational speed measurement function, a motor current measurement function, and a flow rate calculating section 58 for using a fluid viscosity value, an impeller rotational speed value, a measured motor current and data stored in the motor current flow rate-related data storage section to calculate a fluid flow rate.