An imaged-based measurement system using a single digital camera and a circular float to measure fill levels in liquid tanks is proposed in this paper. By choosing the float in a different color from that of the liquid in the tank, pixel counts of the float in the image captured by the camera can be calculated with the use of chrominance filtering and thresholding techniques. Based on an established relationship between the pixel counts of the diameter of the float in the image and the photographing distance, the measuring system can effectively measure the liquid level based on the images captured. Because pixel counts of the float in the image are first determined for calculating the diameter of the float, a subpixel resolution during the measurement can be achieved. As a result, measuring precision as well as accuracy via the proposed system can be significantly improved.