Remove the lampshade from the Stiffel lamp. Set the lampshade on a clean work surface.
Measure the width at the top of the lampshade in inches. Stretch a measuring tape from one side of the lampshade to the other at the widest point. Jot down the measurement.
Flip the lampshade over, bottom facing up. Measure the width of the bottom of the lampshade at its widest point using the measuring tape. Jot down the result.
Turn the lampshade upside-right and measure one of the sides, or slant, of the lampshade. Stretch the measuring tape from top to bottom and record the measurement.