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You'll have a much easier time measuring the shade if the lamp isn't in the way. Carefully remove the shade from the lamp and set in on a table for the moment. If the lamp has a clip-on lamp shade fitter, you can simply pull it off the light bulb. Be gentle, though, to avoid damaging the bulb. If the lamp has a spider lamp fitter with a lamp harp, unscrew the finial that holds the shade onto the harp and then lift the shade off. If your lamp has an UNO lamp shade fitter, unscrew the bulb and carefully lift the shade off. Hold your tape measure at one end of the top edge of the lamp shade, and pull it across to the opposite edge in the right in the middle. If your shade has a square shape, measure one side of the square from corner to corner for the proper measurement. Have a piece of paper handy to jot down the first measurement. Mark it as the "top diameter" so you know exactly what size replacement shade you need.
Take the shade off the lamp. Place the measuring tape across the top diameter in the center. Write down the measurement.