How do I know the size of my light bulb?

How do I know the size of my light bulb?

Light bulbs are identified using a code similar to the code used for the bulb, so identifying the size of a light bulb is simple. 

The letters in a bulb code indicate its shape and special features such as reflector type.

 The numbers of the bulb indicate its size, which is based on its diameter at its widest point, measured in eighths of an inch (⅛”). 

For example: An A19 bulb indicates that the bulb comes in a standard household shape (code A), and the ‘19’ indicates that the bulb is 19/8ths inches in size. 

Similarly, A15 bulbs are smaller than A19 bulbs, though both fit in the standard household light bulb base (E26).