The light bulb size is determined by the maximum overall diameter (MOD) of the bulb's shell. It is represented in eighths of an inch (1/8").
It's also important to keep in mind the bulb's length (or height) that is defined as the maximum overall length (MOL) expressed in inches.
Will my Bulb fit?
An A19 light bulb is 19-eighths of an inch or 19/8" in diameter. To determine a light bulb size, use the full size scale shown (calibrated in eighths). Place on edge of the light bulb to be identified on the top end of the scale and measure the approximate diameter by reading the measurement at the opposite edge of the light bulb.
Sizes of the more popular light bulb types are indicated on the scale above.