I don't believe most of the lamp shade size rules on the internet. They aren't universal.
I do believe that there's a list of don'ts that should be followed, though. Here are some of the big ones and the inevitable exceptions to them.
DON'T: Let the socket hardware show.
A shade should fully cover the socket, saddle, and in most cases half of the metal neck of the lamp. This applies when viewing the lamp at eye level.
Exceptions include: When the hardware is beautiful and compliments the form as a whole, then the bottom of a socket can peek out of the shade.
DON'T: Use a shade that is wider than the lamp base is tall.
Exceptions include: When the lamp is very short yet visually hefty. If it has enough visual volume, it can possibly handle a wide shade as long as the shade is also squat.
Or, if the shade has an extreme taper, like a coolie, then the shade volume won't overwhelm the lamp base.
DON'T: Use a shade that is narrower than the widest point of the lamp base.
DON'T: Assume you have to use the harp that came with the lamp.
Harps are inexpensive, available for purchase online, and come in 1/2" increments. They are easy to replace, and can transform your lamp by allowing you to use a different style shade than the original.
Exceptions include: In rare cases, you may come across vintage lamps with unusual or custom hardware. The solution to that dilemma would just be to rewire the lamp with standard hardware. A vintage lamp should be rewired for safety anyway.
DON'T: Believe the online lamp shade sizing rules over your own eye.
If you follow all the rules to a T, and you think the shade looks wrong - it's wrong. Balance in design is a subtle yet potent, and when you are the one to live in the space, you should always trust your gut.
Exceptions include: My rules. They are the correct ones. Just kidding! You don't need to listen to me either - you do you. Do you love it? Yes - then it's perfect.
I have sized lamp shades for over a hundred lamps, and I approach every one the same way. I look at the original lamp, ask questions about how the lamp will be used and where it will live, and I approach the project like a problem with an optimal answer. It should be a unique solution catered to the individual situation each time.
I've always believed that good design has guidelines, but no rules.
Curious to Learn More About How to Find the Right Shade For Your Lamp?
Visit my You Tube channel for a tutorial or discover more of my shade sizing blog posts here.
Have a question? If you have a question specific to getting the right shade on one of your lamps, please feel free to reach out through my Contact Page. I'll email you within a couple days. I'm happy to help.
-Krista