Fillet Weld Size is the size of the weld of approximately triangular cross section joining two surfaces approximately at right angles to each other in a lap joint, T-joint, or corner joint. And is denoted by Sf. Fillet Weld Size is usually measured using the Millimeter for Length. Note that the value of Fillet Weld Size is always positive. Typically, the value of Fillet Weld Size is greater than 0.