To measure a lamp for a correctly proportioned lampshade, begin by measuring the width of base of the lamp and multiplying that number by two to get the recommended width for the shade. Then, measure the height of the lamp base, and divide that number by three, and then multiply it by two. The resulting number is the suggested height for the lampshade.
When measuring the width of the lamp base, measure diagonally for square and rectangular lamp bases and use the diameter measurement for round bases. Also, make sure that the width of the shade is at least 1/2 inch wider than the lamp base at any position. Before purchasing any shade, check that the shade adequately covers the harp of the lamp.
Additionally, consider the location of the lamp. Narrower shades might be the best choice for hallways and other areas where space is tight. The location of the lamp should also determine whether you want an opaque or translucent shade. Translucent shades add ambient light and are great for reading, while opaque shades create vertical beams of light that highlight specific areas in a room.
Finally, pick a lampshade that has the proper hardware to fit the existing base. A shade with an uno fits a lamp with a harp, and a shade with a clip perches directly over the bulb.