LED Bulb Bases and Sockets Explained

Lamps can be described in a number of ways:


    • LED bulb shape


     LED bulb diameter


    • LED bulb length


    • LED bulb base type


LED Light Bulb Shape

Light bulb shapes describe the general physical characteristics of the light bulb. LED light bulbs vary in shape from tube shaped bulbs to funnel or pear shaped light bulbs. These LED light bulb shapes each have a code. For example: T designates a ‘tube’, PAR signifies a ‘parabolic aluminum reflector’ and MR is used for ‘multifaceted reflector’.


LED Bulb Diameter

The diameter of a light bulb is measured in two ways, in metric it is the number of millimeters and in imperial measure it is the number of 1/8 inch increments. The diameter for light bulb shape is measured at the point of maximum size. Examples are: T12 is 12 eights of an inch or 12/8 = 1.5 inches in diameter, PAR20 is 20 eighths of an inch in diameter or 2.5 inches, MR16 is 16 eighths = 16/8 = two inches in diameter.


LED Bulb Length

LED light bulb length is the overall length measured, typically from electrical connection, or base, to the tip of the bulb, or in the case of T bulbs from end to end. This can be in metric or in inches or feet. In some cases the stated length may differ from the actual length like in the case of some four foot (1200mm) T8 fluorescent replacements; these may actually be slightly less in length.


Bulb Base Types

LED light bulb base types are typically measured by type and diameter in millimeters. For example a typical Edison medium screw-in base like E27 is 27 millimeters in diameter. LED bi-pin bases, like a fluorescent tube replacement or a bi-pin MR16, are measured between the centers of each pin.


Naming Conventions of Light Bulb Sockets and Base Types

Light bulb bases and sockets are normally defined by a letter-number-letter format, with the last letter optional. The first letter designates the shape or form of the base, the numbers represent either the width of the base or the distance between the pins. The second letter designates the number of pins or contacts on the lamp. The numbers are normally in millimeters. LED light bulb sockets and bases are manufactured to the same standards as halogen, incandescent and other traditional lights.


For example a standard European screw-in incandescent bulb has a base type of E27. E stands for Edison screw-in and the 27 means that the base is 27 millimeters.


LED light bulbs and LED lamps use the same socket designations as traditional light bulbs:


    • (B) Bayonet Collar Base

    • (E) Edison Screw Light Base

    • (F) Single Pin Type Base

    • (G) Multiple Pin Type Light Bulb Base K Cable Connections

    • (P) Pre-focused Light Base

    • (R) Recessed Contact(s) Base

    • (S) Shell-type Light Bulb Base

    • (T) Telephone Slide Base

    • (W) Wedge Base

    • (X) Special Type Light Bulb Base


Lamp Number of Pin or Contacts


    • (s) Single Pin

    • (d) Double or Bi-Pin Base Type

    • (t) Triple Pin Base or Tri-Pin Base or 3 Pin Base q Quadruple Pin Base or 4 Pin Base



Examples of common bayonet bases in the UK are: B15d, B22

Screw-in bases, also known as Edison bases and light bulb sockets are used

The naming conventions include terms like medium base and candelabra base. Often they are referred to with a letter-number designation that starts with an “E” followed by the number of millimeters in diameter of the base. If your base ends in a “d” this indicates a double contact base, this is typically used for 3-way bulbs.

The “G” base type of lamp and light bulb is used to designate a “pinned” base. This can include several pin types and different numbers of pins.


Designation is by letter(s)-numbers-optional letter: The first letters include “G” followed by an optional U, X, Y or Z. Next is a number that designates the number of millimeters between the center of each pin. Finally the optional final letter designates the number of pins, no letter implies 2 pins as does a d for double. Pin number designations include: s : Single, d : Double, t : Triple for 3 pins and q : Quadruple for 4 pins.


Examples include: GU10 base type, G24q base type, or for T8 tubes G13 bi-pin.


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