Most new lamp shades list three to four measurements. If you want a direct replacement for your old shade, it's important that you know what to measure. You'll need a measuring tape (or yardstick), along with a pen or pencil to accurately document the specifications:
Top diameter - If it's a round shade, hold the tape on one edge of the circular top and pull it across the open shade to the direct opposite side. For square shades, simply measure from one flat edge to the opposite one.
Bottom diameter - Not a shocker here. Just repeat the same steps, but do it on the bottom of the shade.
Slant or slope - Hold the tape at the bottom edge and measure along the side to the top edge. Make sure that you are holding it straight and not tilted to the left or right. This is an important measurement for tapered lamp shades.
Height - Set the shade on a flat surface and lower the tape through the opening from the top. Place one end on the tabletop and measure the distance to the top edge. Make sure that the tape is perfectly vertical. For drum or cube-shaped shades, the height can be taken by simply measuring the side.
Here are a few "rules of thumb" if you want something different than your current shade:
Shades should be roughly 2/3 the height of the visible portion of the lamp base.
Lamp shades should have a width that is roughly equal to the height of the visible lamp base.
Shades should be wider than the widest part of the base, but narrower than the table on which it sits.