Diameter of the Shade = Distance from Bottom of Lamp Base to the Bottom of the Bulb Socket
We just tested it out on lamp in our house and yup, sure enough, that seems to be a good way to estimate the size. One to remember!
Now, we KNOW that the amazing hive mind that is the Apartment Therapy readership must know some more of these "rules of thumb" for decorating. Please pass them on in the comments...
Thanks to Fine Diving for the lampshade tip!
Image: Janel Laban