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Light Bulb Shapes & Sizes Explained

Naming Conventions for Light Bulb Shapes

Lighting lamps and light bulbs are described by a shape and diameter and length designation. This normally is in the form of letter(s) followed by a number followed by an optional letter. The first letter(s) indicate the shape, the numbers indicate the diameter and the final, optional letter can designate the length.

Lightbulb shape examples include: PAR for Parabolic Aluminum Reflector light bulb or T for Tube. Many household halogen light bulbs are a MR16 for Metallic Reflective.
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Sizes are referenced by a number that follows the shape. In the US these numbers indicate the maximum diameter in eighths of an inch. In other parts of the world this number will designate the diameter in millimeters. An example of a US bulb is the R20, this is 20/8 or 2.5" in diameter. An example of a metric measure is the A19 which is the typical household incandescent light.

Sample LED Light Bulb Shape Designations:

A Standard household incandescent light bulb shape
B Bulged bulb shape
BT Bulged or Blown Tubular bulb shape
BR Short reflector style light bulb
C Candle or Conical bulb shape
CP Crystalline Pear bulb shape
E Ellipsoidal bulb shape
ER Extended reflector light bulb shape
F Flame style candelabra bulb shape
G Globe bulb or circular bulb
HK Hexagonal Candle
K Krypton or narrow reflector light bulb shape
MB Halogen bulb
MR Quartz reflector lamp (low wattage)
P Pear bulb shape
PAR Parabolic Aluminized Reflector (PAR) lamp
PS Elongated standard incandescent bulb
R Reflector light bulb style
S Strait Sided bulb style
T Tube lamp shape

Here are some standard diameter sizes:

8 = 1 inch Example: T8
11 = 1.375 inches Example: MR11
16 = 2 inches Example: MR16
20 = 2.5 inches Example: R20
30 = 3.75 inches Example: PAR30
38 = 4.75 inches Example: PAR38

Length designations include:

S for short
L for long

A Shaped Light Bulbs

"A" type bulb shapes are standard house hold light bulbs. The number after the bulb shape is the number of eighths of and inch in diameter. For non-imperial measured bulbs the number following the bulb shape is the number of centimeters at the widest point of the bulb.
Standard bulb sizes include A19 for imperial measurements and A60 for a slightly smaller metric measured bulb.

B and C Shaped Light Bulbs

B shaped bulbs are also know as Candelabra Light Bulbs. The have a bulged base that tapers to a rounded or pointed tip. In some bulbs the tip is bent giving the slight look of a licking flame. C shaped bulbs are similar to holiday lamps.

These bulbs are common in chandeliers, night lights and lower wattage applications such as holiday string lights.
Typical bases for B and C shaped bulbs are E12 in North America.

PAR Light Bulbs: PAR16, PAR20, PAR30 and PAR38

PAR light bulbs utilize a parabolic mirror or reflective surface to focus the light. For LEDs the PAR designation is used to define the shape, as most do not have a reflective lower surface due to the directive nature of LEDs.

Common PAR lamps include the PAR20, PAR30, PAR38 and PAR42, though many other sizes exist. These bulbs come in various beam angles that result in spot and flood lamps.

This shape is very similar to the R shape and in some cases the PAR and R are interchangeable as shapes for LED replacements.