Bulb Shapes & Bases
Bulb Shapes
A shape code is a letter or letters describing the form of the glass, followed by a number. That number is the diameter of the lamp in eighths of an inch — so an A19 measures 19/8", or 2-3/8", across its widest point. Once you know the rule you can read the physical size of any bulb straight off the code.
A Series
Arbitrary Spherical
A Shape lamps are the most common. The number is the diameter of the widest part of the lamp in eighths. In the case of an A19, it would be 19/8's, or 2-3/8" in diameter.
B Series
Bulged Lamps
B is short for Bulged. The number is the diameter of the widest part of the lamp in eighths. For instance, a B10 would be 10/8's, or 1-1/4" in diameter.
BR / ER Series
Bulged Reflector
BR is short for Bulged Reflector. The number is the diameter of the end of the lamp in eighths. For instance, a BR30 would be 30/8's, or 3-3/4" in diameter.
C Series
Candelabra Lamps
C is short for Candelabra. The number is the diameter of the widest part of the lamp in eighths. For instance, a C7 would be 7/8's in diameter.
CA Series
Candle Angular
CA is short for Candle Angular. The number is the diameter of the widest part of the lamp in eighths. For instance, a CA5 is 5/8's in diameter.
CFL Screw-In
Compact Fluorescent Lamp, Screw-In
The number refers to the size of the coil; T2 is a smaller coil (as illustrated) while T6 is larger.
CFL Plug-In
Compact Fluorescent Lamp, Plug-In
The number refers to the number of tubes on the CFL.
ET / Prism Series
ET-16 and Prism Candelabra Base
ET-16 lamps are available in both incandescent and LED. Prism lamps use a candelabra base.
G Series
Globe Shape
G is short for Globe. The number is the diameter of the widest part of the lamp in eighths. For instance, a G40 would be 40/8's, or 5" in diameter.
HID
High Intensity Discharge
B is short for Bulged. BT is short for Bulged Tubular. E is short for Ellipsoidal. ED is short for Ellipsoidal Dimple. The number is the diameter of the widest part of the lamp in eighths. For instance, a B17 would be 17/8's, or 2-1/8" in diameter.
LFL
Linear Fluorescent
LFL is short for Linear Fluorescent Lamp. The number is the diameter of the lamp in eighths. For instance, a T8 would be 8/8's, or 1" in diameter.
MR Series
Multifaceted Reflector
MR is short for Multifaceted Reflector. These lamps are generally used in spot lamps and track lights. The numerical part of the shape is the diameter of the end of the lamp in eighths. For instance, an MR16 would be 16/8's, or 2" in diameter.
PAR Series
PAR Lamps
PAR is short for Parabolic Aluminized Reflector. The number is the diameter of the widest part of the lamp in eighths. For instance, a PAR16 would be 16/8's, or 2" in diameter.
PS Series
Pear Shaped Lamps
PS is short for Pear Shaped. The number is the diameter of the widest part of the lamp in eighths. For instance, a PS25 would be 25/8's, or 3-1/8" in diameter.
R Series
Reflector
R is short for Reflector. The numerical part of the shape is the diameter of the end of the lamp in eighths. For instance, an R30 would be 30/8's, or 3-3/4" in diameter.
RP + S Series
RP Miniature and S Series Specialty
The numbers refer to the diameters of the glass in 1/8 inch increments.
Specialty
Specialty Shapes
Specialty shapes cover formats that fall outside the standard series, including appliance, showcase, sign and instrument lamps.
T Series
Tubular Light Bulbs
T is short for Tubular. The number is the diameter of the lamp in eighths. For instance, a T10 would be 10/8's, or 1-1/4" in diameter.
Bulb Bases & Sockets
There are two main types of light bulb bases: screw-in bases and pin bases. Each works in a different way to connect your light bulb to electrical voltage.
Screw bases come in several sizes and are referred to by both a descriptive name and their size. Unlike light bulb shapes, the number in the light bulb base measures the base diameter in millimeters. For instance, the candelabra E12 has a diameter of 12 millimeters, and the "E" indicates that it has an Edison base.
Besides screw-in bases, there are also plug-in fluorescent bases, GU series, bi-pin bases and specialty bases. If you don't see the specific base you're looking for, contact us and we'll help answer your questions.
E Series
Edison Screw
Screw-in E type bases are the most common. The numerical part is the diameter of the base in millimeters. For instance, an E27 is 27 mm in diameter. E26 is the North American standard medium base; E12 is candelabra and E39 is mogul.
Fluorescent Bases
Linear Pin Bases
Linear fluorescent lamps use pin bases sized by the distance between the pins. G13 medium bi-pin is used on T8 and T12; G5 mini bi-pin is used on T5.
Plug-In Fluorescent Bases
2-Pin and 4-Pin
Plug-In fluorescent light bulbs have either 2 pins or 4 pins. 2-pin lamps have an internal starter and are designed for preheat, magnetic operation. 4-pin lamps are designed for electronic ballast operation and are dimmable.
Bi-Pin Bases
Push-In Pin Bases
Bi-Pin bases are push-in type pin bases. The numerical part of the base is the distance between the pins in millimeters. For instance, a G12 base would be 12 mm from pin to pin.
GU Series
Twist-Lock Pin Bases
GU bases have pins with locking ends on them, and twist into the socket. The numerical part of the base is the distance between the pins in millimeters. For instance, a GU10 is 10 mm from pin to pin.
Specialty
Bayonet, Wedge and Prefocus
Specialty bases include bayonet types such as BA15s and BA9s, wedge bases, and prefocus bases like P28s used in projection and marine applications.
Still have questions? Call or text us at (847) 675-3538, or send the fixture make and model — or a photo of the bulb and socket — through our contact page and we'll identify the replacement.