Most lamp shades that go back to the store were the right size. They just didn’t fit. Measuring a lamp shade takes three numbers and two minutes, and nearly every guide stops there. The part that decides whether the new shade actually sits on your lamp is a small piece of metal most people never look at: the fitter.
Short answer: To measure a lamp shade, measure three things in inches: the top diameter, the bottom diameter, and the height, straight up inside the shade rather than along the slanted side. Shades are listed as top × bottom × height, so a 10 × 12 × 8 shade is 10 inches across the top, 12 across the bottom and 8 inches tall. Then check the fitter, the part that attaches the shade to the lamp: spider, uno or clip-on. A shade in the right size with the wrong fitter will not go on your lamp.
Disclosure: this post contains affiliate links, and as an Amazon Associate I earn from qualifying purchases. If you buy through them I may earn a small commission at no extra cost to you. I only recommend things I would use in my own home.