How are glass shades measured? The fitter is the outer diameter of the lip which is inserted into the fixture’s shade holder. This will measure a little less than the fitter size so it can fit into the shade holder of the same measurement. The height of a glass shade is taken from the base of the fitter lip to the bottom of the shade. The width is measured across the widest portion of the shade. How to install it? This glass is held in place with 3 screws on the original fixture. Just twist the screw into the groove on the light fixture (by hand). Screws not included.