The best way to purchase a lampshade is to bring the lamp base into the shop. Like buying a pair of shoes, you need your feet to fit into them. Should that not be possible, measure:
- A) Top diameter
- B) Bottom Diameter
- C) Side/slant (from top to bottom)
However, just knowing these measurements might not be enough. A new harp might solve your fitting problems.
You do not want to see the bottom of the socket when you are at eye level with the bottom of the shade.