Lampshades are grouped in general sizes of Small, Medium, Large and Jumbo. The table below gives specifics: the top and bottom diameters, the slope, and the height of individual frames within those general groupings.
*refers to the maximum recommended incandescent or halogen bulb wattage
With a measuring stick and a few guidelines, you’ll have no trouble getting the proportions right. Sizing questions start from one of these three conditions, in which you already have:
a well-proportioned lampshade
best to simply replace with a similar size and shape, and confirm your choice using the table and drawings below
a well-proportioned harp
measure the harp and find a lampshade of similar height, using the table and drawings below to confirm your choice
just the lamp base
First match your lamp base to one of the following four general shapes…
… and then measure it from the table top to the base of the neck, or, if there is no neck, to the base of the socket. Use the height and shape, in the table below, to determine what size lampshade will be the best fit: S, M, L or J (for Jumbo). Any of the frames within that general size grouping should be a good fit.
The drawings for the general sizes can be a helpful guide as well, showing individual frames on different shaped lamp bases.
Remember that you will need to buy a harp*: use the height of the lampshade as a guide and buy a harp that is an inch or so shorter.
*Grand Brass Lamp Parts will have what you need
A Few More Tips
Stick lamps are exactly what they sound like: a very slender column with a small base. They are exceptionally versatile and allow a bit more wiggle room when shopping for shades.
Assuming that they are not meant to have very small shades perched on top (like some buffet lamps), just about anything smaller in diameter than the height of the base will work, and look best when the stick is a bit less than twice the height of the shade. It is unlikely that you would ever find yourself needing a lampshade larger than 16″ in diameter for a tabletop stick lamp, unless you’re going for a flamingo effect. Empire shapes seem to work best, as the taper at the top balances the bottom.
Floor lamps are universally tall and skinny, but they don’t take a universal lampshade size.
A good rule of thumb is that the smaller the bottom diameter, the more flared they should be, like a coolie hat. As the bottom diameter increases, less tapered and more drum-like frames can work. I have made both: diminutive shades that are like little hats, and large and jumbo shades, up to 24″ in diameter.
Chandelier lampshades are all more or less similarly sized: they have a torpedo clip fitting, and are typically about 4″ high, and under 5″ in diameter, too. And because they are often more diminutive in scale, or at least in wall projection, vintage wall lights are often better paired with chandelier shades.
Larger sconces or wall lights are a bit trickier as the key measurement is the projection from the wall, and if they are double or triple light sconces you must also take into account the distance between the lights. Remember to use half the diameter measurement when determining whether a shade will fit: a sconce with a 6″ wall projection to the light will easily take a 7″ diameter lampshade, as the shade will extend only 3.5″ towards the wall, leaving a good bit of space.
Finally, swing arm lamps of the sort you’d find above a sofa or reading chair often take a 12″ diameter small empire lampshade with washer fitting at top.