A calibration system for a dimensioner is described. The calibration system uses a reference pattern with multiple optically-perceptible elements. The optically-perceptible elements have a parameter associated with size and are separated from each other by predefined distance. The dimensioner captures an image of an object and calculates physical dimensions of the object. The dimensioner captures an image of the reference pattern. The dimensioner measures the parameter associated with size for a pair of optically-perceptible elements and measures the distance between the pair of optically-perceptible elements from the captured image. The dimensioner then calculates a ratio between measured parameter for the pair of the optically-perceptible elements and of a predefined distance between the pair of the optically-perceptible elements. The calculated ratio is compared with a actual reference ratio to determine if calibration is needed. The dimensioner is calibrated based on the calculated ratio.