A method for determining object's size, including: providing a calibration card having a picture printed thereon; providing a first real size of the picture; setting a camera of a computing device to a fixed focus; capturing a first image of the calibration card when the picture is in focus; measuring a first virtual size of the at least one dimension in the first image in pixels; calculating a pixel density of the first image, by dividing the first virtual size by the first real size; capturing a second image of an object, when the object is in focus; measuring a second virtual size of at least one distance between two virtual points located on the second image; calculating a second real size of at least one distance between two real points of the object corresponding to the two virtual points, by dividing the second virtual size by the pixel density.