This tool is to approximately estimate the thickness of a layer or the diameter of particles from the Kiessig fringe in SANS/NR data, and uses the Kiessig relation; thickness = 2*Pi/fringe_width.
To get a rough thickness or particle size, just type the Kiessig fringe width (in the 1/A unit) and click on the 'Compute' button. Then the output value will be show up in the 'Thickness' text box.