Replacing a light bulb requires understanding the standardized codes that define its physical dimensions. A bulb’s size is defined by two components: the base, which connects the bulb to the electrical socket, and the bulb’s body, which dictates its shape and overall diameter. Understanding this two-part code ensures the bulb fits correctly into the fixture and provides the intended light distribution. This article explains how the base and the bulb’s body are measured and coded.
Identifying the Base Size
The base is the foundational element of the light bulb sizing system, as it determines compatibility with the electrical socket. The most common type is the screw-in base, referred to as the Edison screw or E-series, prevalent in household fixtures across North America and Europe. The letter ‘E’ signifies the Edison design, and the following number specifies the diameter of the screw base in millimeters. This direct measurement system makes identifying the correct base straightforward.
The standard size for most household fixtures in the U.S. is the E26 base (26 mm), often called the Medium or Standard base. Smaller fixtures, such as those found in chandeliers, often utilize the E12 base (12 mm), known as the Candelabra size. The E17 base (17 mm) is used in some specialized applications.
Another major category is the pin-type, designated by ‘G’ or ‘GU’, used in track lighting and low-voltage applications. The number following the letter indicates the distance between the two contact pins, measured in millimeters. This design relies on friction or a twist-and-lock mechanism rather than a screw thread.
A common pin base is the GU10, featuring pins spaced 10 millimeters apart. For smaller, low-voltage applications, codes like G4 or G9 are common, where the number represents the pin spacing. The precise measurement of the base diameter or pin spacing determines whether the bulb connects to the fixture.
Understanding the Bulb Shape and Diameter
The second part of the sizing code defines the physical characteristics of the bulb’s body. This code consists of a letter specifying the general shape, followed by a number indicating the maximum diameter. This system ensures the bulb fits within the fixture’s housing and that its light distribution pattern is appropriate for the intended use.
The letter designation references the bulb’s form factor:
- ‘A’ denotes the standard household shape, the familiar pear-like form.
- ‘C’ is used for Candle bulbs, which are slender and often tapered for decorative fixtures like chandeliers.
- ‘G’ designates Globe-shaped bulbs, featuring a large, spherical body used in vanity strips or exposed fixtures.
Specialized lighting applications use different letter codes for reflector designs. The ‘PAR’ designation stands for Parabolic Aluminized Reflector, which uses a hard glass shell and an internal reflector to project a focused beam of light. The ‘MR’ code, meaning Multifaceted Reflector, refers to smaller, low-voltage bulbs that use a pressed glass reflector with multiple facets to control the beam angle.
The number immediately following the shape letter represents the diameter of the bulb at its widest point, measured in increments of one-eighth of an inch. To determine the actual diameter in inches, the code number must be divided by eight.
For instance, the common A19 bulb has a diameter of 19/8ths of an inch (2.375 inches). A PAR38 bulb, frequently used in outdoor floodlights, measures 38/8ths of an inch (4.75 inches). The MR16 bulb measures 16/8ths of an inch (2 inches). This standardized fraction-based measurement system is consistently applied across all shape types to provide a precise physical dimension for the bulb’s body.
Practical Tips for Finding the Code
When a light bulb burns out, the first step in finding a replacement is locating the complete size code, which combines both the shape/diameter and the base designation, such as A19/E26. The most reliable place to find this information is typically printed directly onto the base of the bulb itself, often near the threads or pins. If the bulb is still functional or the code is worn away, the original packaging is the next best source for the full specification.
If the code is completely illegible, a simple measurement can be taken to determine the necessary replacement size. The base diameter should be measured in millimeters using a ruler or caliper to determine the E-series or G-series number. For the bulb’s body, measure the maximum diameter in inches, then multiply that measurement by eight to derive the correct code number for the shape designation. This practical approach ensures the correct fit even when the printed information is unavailable.