An apparatus for determining a fluid's kinematic viscosity from ultrasonic energy that has passed through the fluid of unknown viscosity along an acoustic path of known length. A computer of the apparatus determines a characteristic frequency of a received electrical signal associated with the ultrasonic energy and measures the fluid's velocity of sound. The kinematic viscosity of the fluid is determined by the computer on a continuous basis based on the characteristic frequency and the sound velocity. A method for determining a fluid's kinematic viscosity.