How do you measure for a new lampshade for a floor lamp?

The absolutely easiest way to buy a new shade for a floor lamp is to take the lamp with you to the store, and try on the shades. It is really the only way to know if a shade is going to work.

I agree. A good friend inherited a lot of table lamps and did just that. She hauled her lamp bases to a good lighting store. You not only see the scale, color and shape that best suits the base, but you'll have the aid of an associate trained in this. There may also be adapters that convert your harp to one that will accept the new shade. Of course you can always carry the old shade with you for reference, but you limit your choices that way.

Thank you. I never thought of doing that.