How Light Bulb Sizes Are Measured and What the Numbers Mean

Light bulb size refers to the maximum diameter of the bulb, not its length or wattage. The sizing system differs slightly between regions, but the underlying concept remains consistent. In the United States, bulb size numbers represent eighths of an inch, while in most other regions, the size is expressed directly in millimeters.

For bulbs measured in eighths of an inch, the number following the shape code must be divided by eight to determine the actual diameter. An A19 bulb, for example, measures 19 eighths of an inch, which equals 2⅜ inches. In metric regions, an A60 bulb indicates a diameter of 60 millimeters. Although A19 and A60 differ in naming, they represent comparable bulb sizes in different measurement systems.

Common A-series sizes include A15, A19, A21, and A25, all widely used in residential lighting. Decorative bulbs such as B, C, and CA series tend to have smaller diameters, while globe-shaped G-series bulbs range from compact sizes like G11 to larger decorative formats such as G25 and G30. Reflector bulbs are identified by their face diameter, making sizes like PAR16, PAR30, and PAR38 easy to measure and compare.

Tubular fluorescent lamps use the same eighth-inch system to define tube diameter. A T8 lamp measures one inch in diameter, while a T12 lamp measures one and a half inches. Understanding these size codes ensures that replacement bulbs fit properly within existing fixtures and maintain the intended lighting performance.