Ultrasonic level sensors are commonly used to measure the motion of the free surface in fluid sloshing. They are used to measure the elevation of the free surface at a single point. The sloshing forces are generally measured with load sensors, which require two sets of measurements, with and without the fluid in the tank. This paper develops a method, which tracks the free surface motion during sloshing with a camera and uses the captured images to estimate the forces due to sloshing in a rectangular tank. One of the major assumptions is that the displacement input which causes sloshing is one dimensional and the resulting sloshing motion is two dimensional. For the method to correctly estimate the sloshing forces along the displacement input direction, sloshing should be around the resonant sloshing frequency. This new method can track the motion of the complete free surface rather than a single point. It estimates the sloshing forces using image processing and potential flow theory, without the need for a load cell measurement. Free surface shapes and sloshing force estimates obtained by image processing are compared with those measured by the sensors. Good agreement is observed for low amplitude sloshing around fundamental resonance frequency. (C) 2017 Elsevier Inc. All rights reserved.