Lamp size is determined by the maximum diameter as expressed in eighths of an inch(1/8''). For example: an A19 lamp is 19-eighths of an inch of 2 1/8'' in diameter. To determine a lamp size, use the full size scale shown above (calibrated in eighths). Place one edge of the lamp to be identified on the top edge of the scale and measure the approximate diameter by reading the scale at the opposite edge of the lamp. Size of the more popular lamp types are indicated on the scale.