Measuring a rectangle lampshade is easy. All you need is a ruler or tape measure, and measure the six dimensions shows in the diagram.
Measurement Tip – For each measurement, you take, make sure you are measuring rim-to-rim or point-to-point. For the width across the top and bottom of the lampshade, your ruler or measuring tape should pass through the center of the lampshade to ensure the maximum measurement is taken.
Six Dimensions
Lampshades are measured and sized using the diameters of the top and bottom as well as a side measurement:
A1 – the width across the top of the shade in one direction
A2 – the widths across the top of the shade in the other direction
B1 – the widths across the bottom of the shade in one direction
B2 – the widths across the bottom of the shade in the other direction
C – the slant height down the side of the shade
D – the height of the shade
Note:
The width across the bottom is what is referred to as the size of the lampshade. The top dimension is only important to the shape of the shade – it does not contribute to how the lampshade will fit the lamp. The side dimension is important in fitting the shade to the lamp. However, with the availability of harps in different heights, shades of varying heights can be adjusted / changed to allow for a proper fit to the lamp.