What Type of Light Bulb Is This?

What Type of Light Bulb Is This?

Mercury Vapor HID bulb

A light bulb goes out in your house and you think it should be an easy fix. Yet when you unscrew the burnt out bulb, you realize that you have never seen this kind of bulb in your life. Whether it’s the odd shape of the glass or the mysterious base that has you scratching your head, we know few tips and tricks to help you determine exactly what kind of bulb you’re holding.

Tip #1: Know Your Light Source

Today there are hundreds, if not thousands, of LED bulbs designed to mimic the look of incandescents, fluorescents, and everything in between. Sometimes the bulbs have similarly named bases or shape types, adding to the confusion. Some things to remember when determining what type of light source you have are:

Mercury Vapor HID bulb

Mercury Vapor HID bulb

  • Most incandescent light bulbs have thin wire filaments.

  • Halogen bulbs have spirals of coiled wire filaments usually in a small glass casing covered by a larger glass casing. Bulbs with coiled filaments that are small enough to fit several in the palm of your hand are halogen or xenon mini indicator bulbs. Halogens are most often found in track lighting, landscaping lighting, and under cabinet lighting.

  • Fluorescent bulbs have opaque frosted lenses and no filaments. Mostly tubular in shape, you are likely to find these bulbs in your kitchen or garage except in the case of spiraled CFL bulbs. Then you should check the nearest table lamp.

  • A high intensity discharge (HID) bulb, usually metal halide (MH) or high pressure sodium (HPS), is not often used in the home. Like halogens, HID bulbs contain a small interior envelope, full of gas instead a filament, supported by a wire frame and surrounded by a rather large glass outer casing. They produce a huge amount of light, white light for MH bulbs and an orange to yellow color for HPS bulbs, and are best suited for outdoor use on streets and roadways. However, you might find a MH bulb, HPS bulb, or possibly the newer ceramic metal halide (CMH) in your grow light fixture for indoor gardening.

  • LEDs are defined by their diodes. Regardless of what the outer casing may look like, LED light bulbs all have a series of small diodes inside. The diodes are usually round and set in rows, grids called arrays, or long strips depending on the bulb shape.


Tip #2: Know Your Bulb Shape

There are many bulb shapes, in various sizes, and we cover the most common light bulb shapes in the diagram below. Note that MR, BR, and PAR bulbs have a flared casing while the tips of decorative (i.e. chandelier) bulbs vary.

Pro Lighting Tip: Find more information about each lighting shape in one of our helpful Light Bulb Shape Guides on our blog.

Tip #3: Know Your Base Type

A large portion of light bulb bases are screw-in types of various widths but occasionally a pin base might throw you for a loop. Pin spacing matters quite a bit when replacing you bulb. If you come across a plug-in CFL then the pin placement can seem very similar but it is very important to choose the right base. We have added diameter or spacing for the bases in the table below.

Bulb Base Code Chart
Bulb Base Code Common name Base Diameter (mm) Bulb Base Code Alternate Bases/Names Pin Spacing (mm)
Screw Base Pin Base
E10 Miniature Screw 10 G4 GU4, GZ4 4
E12 Candelabra 12 G5.3 GU5.3, GX5.3, GY5.3 5.3
E14 European Candelabra 14 G6.35 GY6.35 6.35
E17 Intermediate 17 G8 GY8 8
E26 Medium 26 G9 (aka “Flat Pin”), G9 Pin Blade 9
E27 European Medium 27 GU7 Twist and Lock 7
E39 Mogul 39 GU10 Twist and Lock 10
Other Common Specialty Bases GU24 Twist and Lock 24
BA15s Single Contact Bayonet N/A Common Fluorescent Bases
BA15d Double Contact Bayonet N/A Axial for T2 lamps N/A
Wedge commonly for mini indicator bulbs N/A G5 Mini Bi-pin for T4 and T5 lamps 5
Festoon commonly for mini indicator bulbs N/A G13 Medium Bi-pin for T8 and T12 lamps 13
Rigid Loop commonly for mini indicator bulbs N/A FA8 Single Pin for T8 and T12 lamps N/A
R7s aka RX7s or Double Ended
Recessed Single Contact,
for stage and studio bulbs
N/A R17d Recessed Double Contact
for high output (HO) T8 and t12 lamps
P28s for stage and studio bulbs N/A 2GX-13 commonly for T5 circline lamps N/A
Screw Terminal multi-purpose N/A 4-pin commonly for T6 and T9 circline lamps N/A

Tip #4: Measure and Measure Again

Bulbs shapes have an alphanumeric code. The letter tells you the general shape of the bulb and number tells you the width or diameter of the largest part of the bulb in eighths of an inch.  For example, the common and well-known standard light bulb for a table lamp is also referred to as an “A19”. The “A” means “Arbitrary”, basically stating it’s a general use bulb. While the “19” in the code refers to 19 eighths of an inch or 19 divided by 8, which is 2.375 or 2 and 3/8ths inches. If you know the bulb shape code, then you know the bulb’s diameter. Conversely, if you know the bulb’s diameter (in decimals) then you can multiply it by 8 to learn the numeric part of bulb’s shape code.

Light bulb bases are most likely to be measured in millimeters. They also use an alphanumeric code as an identifier. Back to our standard A19 light bulb, it also has the accepted American standard base of E26. The “E” stands for “Edison” because this threaded base is the same one used for the first mass produced light bulb created by Thomas Edison. The “26” means the base is 26 millimeters in diameter. This rule is true for most household bulbs including the bi-pin bases of MR bulbs and fluorescent tubes or similar-but-smaller screw-in bases of decorative chandelier bulbs.  We have listed the common bulb shape codes along with their diameters in inches below.

Bulb Shape Codes
Bulb Shape Code Average Diameter
Bulb Shape Code Average Diameter
A Shape Globes
A15 1 7/8 G9 1 1/8
A17 2 1/8 G11 1 3/8
A19 2 3/8 G12 1 1/2
A21 2 5/8 G16 2
A23 2 7/8 G19 2 3/8
R Shape G25 3 1/8
R16 2 G30 3 3/4
R20 2 1/2 G40 5
R30 3 6/8 Decorative Chandelier
R40 5 B10* 1 2/8
BR Shape B11 1 3/8
BR20 2 1/2 C7* 7/8
BR30 3 6/8 C9 1 1/8
BR38 4 6/8 C11 1 3/8
BR40 5 C15 1 7/8
MR Shape CA10* 1 2/8
MR8 8/8 F10* 1 2/8
MR11 1 3/8 *Note:
B – Bullet or “Torpedo”
C – Candle or “Conical”
CA – Candle Angular
F – Flame Style
MR16 2
MR20 2 1/2 AR Shape
PAR Shape AR11 0.43 (11 mm)
PAR14 1 3/4 AR70 2.76 (70 mm)
PAR16 2 AR111 4.37 (111 mm)
PAR20 2 1/2 Fluorescents and CFLs
PAR30 3 3/4 T2 1/4
PAR30 Long Neck 3 3/4 T3 3/8
PAR30 Short Neck 3 3/4 T4 1/2
PAR36 4 1/2 T5 5/8
PAR38 4 3/4 T8 1
PAR46 5 3/4 T12 1 1/2
PAR56 7
PAR64 8

When in doubt, measure it out. By simply measuring your bulb’s length and width, and then measuring the width of the base, you can easily confirm what type and size of the light bulb you have.

Tip #5: Contact 1000Bulbs.com

If you’re still unsure of what kind of bulb you have or just don’t have ruler handy, feel free to contact us at 1000Bulbs.com. Call us at 1-800-624-4488 and any of our over 50 lighting experts would be happy to help to you figure out what kind of lighting you need. You can also contact us on Facebook, Twitter, Instagram, or email some snapshots of your bulb to socialmedia@1000bulbs.com.  


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