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The UltraSonic Thruster (UST) is an actuator which employs a piezoelectric transducer to generate a highly directive ultrasonic wave so as to produce bulk fluid movement. This streaming phenomenon can be utilized underwater for thrusting or maneuvering purposes in marine applications, and particularly at very small scale. We make a new connection between fluid flow and forces, establishing a specific formula for estimating overall thrust from the velocity field. Using Particle Image Velocimetry (PIV) and force measurements, we confirmed the result, with an error of about five percent in several tests. The use of PIV for characterizing UST properties is considerably richer than measuring thrust or acoustic intensity alone, and could lead to new insights during the design process.