LED Base Types Explained

There is a whole other side to the LED.

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 aluminium 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 Diameter

This 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

These 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 throughout the world.


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.

"E" or Edison base types, sizes and names












Base or Socket Name

Lilliput Edison Screw Base

Miniature Edison Screw Base

Mini-Candelabra Edison Screw Base

Candelabra Edison Screw Base

Small Edison Screw Base (supports 240V)

Intermediate Edison Screw Base

Medium Edison Screw Base

Medium Edison Screw Base (supports 240VAC)

Mogul or Giant Edison Screw Base

Mogul or Giant Edison Screw Base (supports 240VAC)












G or Bi-pin light bulb base and sockets 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 centre 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.








G5.3 - 4.8



















Typical Usage

MR11 and other small halogens of 5/10/20 watt and 6/12 volt




T4 and T6 fluorescent tubes




MR16 and other small halogens of 20/35/50 watt and 12/24 volt



Halogen various Wattage (eg. 50W/100W), various Voltage (eg.12/24V), Common for task lighting, landscape lighting.


Halogen 100W 120V

Halogen 100W 120V

Halogen 120V  (US)  / 230V (EU)

Common for theater use, several variants

Twist-lock 120/230-volt MR16 halogen lighting of 20/35/50 watt

Used in theatre and single-end metal halide lamps

T8 and T12 fluorescent tubes


Twist-lock for self-ballasted compact fluorescents, since 2000s

Mostly used for high-wattage theatre lamps

Twist-lock for puck-shaped under-cabinet compact fluorescents, since 2000s

Pin center to center

4.0 mm

4.0 mm

4.0 mm

4.0 mm

5.0 mm

5.33 mm


5.33 mm

5.33 mm

6.35 mm

6.35 mm

6.35 mm


6.35 mm

8.0 mm

8.6 mm

9.0 mm

9.5 mm

10 mm

12 mm

12.7 mm

23 mm

24 mm

38 mm

53 mm

Pin Diameter

0.65 - 0.75 mm

0.95 - 1.05 mm

0.62 - 0.75 mm

0.95 - 1.05 mm


1.47 - 1.65 mm



1.45 - 1.6 mm

(Flat Pins)

0.95 - 1.05 mm

0.95 - 1.05 mm


1.2 - 1.3 mm

0.95 - 1.05 mm



3.10 - 3.25 mm


2.35 mm


2 mm


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