How to Tell What Kind of Light Bulb I Have

It seems like an easy repair to manage when a light bulb stops functioning. Although that's the case in most situations, there can be times when you don't know what product is providing illumination.


Some light bulbs come with an odd shape, while others might have a base you've never seen before.


If you're unsure of what you have, here are some steps to help you know how to tell what kind of light bulb you have.


1. Know the Source

You can find hundreds of different LED bulb designs that duplicate the look and feel of other technologies. There might be an old incandescent bulb to replace, a unique fluorescent design, or something else.


Since many light bulbs use the same letter and number designations, such as A19, to tell what they are, the result is often more confusion.


Here are some tips to help you know what you've got so that you can find a suitable replacement for the light bulb.


Light Bulb Type

Features of This Light Bulb Type

This light bulb design uses thin wires and filaments to support illumination. It produces a lot of heat while operating, so a bulb that is hot to the touch when the switch is on is likely this type. These products tend to be the cheapest, so they're found most often. The surface can be frosted or transparent.
HalogenThese bulbs use a coiled wire filament within a small glass casing. You'll see a second casing to protect the gas used to create illumination. They're sometimes small enough to fit in your hand. If you have track or landscape lighting that burned out, it's likely this option.
FluorescentWith a compact fluorescent light (CFL), you'll see a standard light bulb shape with curved glass tubes inside. Other bulbs in this category are tubes that come in circular, U-shaped, or straight designs that deliver illumination. These are often found in kitchens, garages, and workshops. A CFL could be in any standard fixture.
HIDHigh-intensity discharge light bulbs use high-pressure sodium or metal halide to create illumination. This option isn't found in most homes because they produce a significant amount of light. They're used in outdoor applications, such as for street lights or stadium lighting, but could be included in some security systems.
LEDThis light bulb type is defined by its diodes. The outer casing has the same variations as other household lamps, but it doesn't use a filament to create illumination. Most designs use a grid or strip shape to fit within the bulb.


2. Understand the Shape

Several light bulb shapes are available today, with many coming in multiple sizes to consider. Most homes use an A-style bulb with a size designation of 19 or 21. You'll see this information included in the product packaging.


Decorative bulbs have their own letter and number designation. You'll want to be careful when replacing candelabra products because the base sizes can be significantly different.


Some fixtures use R, MR, BR, or PAR light bulbs. These have a flared casing that makes them distinctive.


If you have chandelier bulbs to replace, the tips can be different even when the letter and number designation are the same.


It's always better to try a direct replacement of the same light bulb from the same manufacturer. Check the size and shape for comparison when that isn't possible, or if you want to upgrade to a more energy-efficient product.


Here's a closer look at each shape that could be found in the home.


Bulb DesignationBulb ShapeInformation About the Light Bulb Type
Standard household incandescent shapeThis light bulb shape is used for most standard types of residential lighting. It comes in several sizes, although the most popular options are A19 and A60. The number reflects the diameter.
BBullet tipA Light bulb with this shape uses a bulged base that comes to a rounded end.
BTBlown or bulged tubular shapeThese light bulbs are similar to what you'll find with other B-type forms, but the end tends to be blunted and less round.
BRShort, reflector-style light bulbThis bulb is typically used for security applications, heat lamps, or workshops. They're longer and broader than most other B-type shapes.
CConical or candle-style shapeThese bulbs are like a holiday lamp. The most common usage is in low-wattage applications, such as night lights and chandeliers, because it looks a lot like a candle's flame.
FFlame-style candelabra light bulbThis light bulb replicates the candle-style appearance while creating simulated “flames" that appear on the outside of the bulb. It's typically reserved for chandeliers and candelabras.
GCircular or globe-style shapeThis option is similar to the A-type bulb. The outer form is broader and more rounded to create a more circular viewing experience.
KNarrow reflector light bulb (Krypton)These light bulbs are an incandescent design with a few modifications. The light tends to burn whiter and purer, reducing the yellowish colors in the environment.
MBHalogenThis light bulb is an incandescent-style lamp. The difference is that the interior uses halogen gas to increase its rated lifespan and total light output.
MRQuartz reflector lamp, often low-wattageThis light bulb type is typically used outdoors because of how much heat it generates. Skin burns around the same temperature as boiling water, which means a 500W product could cause problems if you were in the same room with it.
PPear bulbThese lights are very similar to what the circular design offers with the circular and globe products. The primary difference is the lack of an extension between the base and the outer shape.
PARParabolic aluminized reflector lampThis light bulb design incorporates a parabolic mirror to focus the light. Since most LEDs lack a reflective lower surface, this design element is necessary.
PSElongated standard incandescent light bulbWhen you have an extended fixture, the PS-type bulb delivers a valuable result. The base is longer and skinnier while maintaining the traditional shape at the end, allowing it to work for most home-based installations.
RReflector-style light bulbAn R-type bulb encompasses any design that isn't included in the other choices. It delivers unrivaled brightness and color when you need it the most.
TTube lampThe T-type light bulb is commonly seen with fluorescent technologies. More options for LEDs and similar designs are becoming available. This option can use the screw-in base or work from pin connectors.


3. Recognize the Base Size

Most residential light bulbs use a screw-in base. There are different widths, but you can visually see the differences in size.


Some light fixtures use a pin base, especially if you have fluorescent lights to replace.


Here's a closer look at the different designations. It helps to look at the bulb to see if any markings are on it to indicate what you need.


Base Shape or Style

Design Options Available for Each Bulb Base Shape or Style

Screw-in Base
E10, E11, E12, E14, E17, E26, E27, E39, EX39, and E40. Some designs use specific names instead of a number designation in this category, including candelabra, European, mini-screw, mini-candelabra, intermediate, medium standard, medium non-standard, mogul, and extended mogul.
Twist and Lock Base10 mm GU10 and 24 mm GU24
Specialty and Custom BaseMedium side prong, end prong, M-P screw terminal, S14, ridged loop cap, festoon, wedge, recessed single contact (R7), double contact bayonet (BA15D), single contact bayonet (BA15S), and single contact (SC).
Bi-pin Base4 mm (G4 or GU4), 5.3 mm (GU5.3), 6.35 mm (GY6.35), 8 mm (GU8 or GY8), 8.6 mm (GY8.6), 9 mm (G9), or 12 mm (G12).
Fluorescent Pin BaseMini bi-pin, recessed D.C., G10q 4-pin, medium bi-pin, single bi-pin, 2GX13, and Axial.
Compact Fluorescent Plug-in BaseG24q-1, G24q-2, G24q-3, GX24q-2, GX24q-3, GX24q-4, Gx24q-5, G-23, G23-2, GX23, GX23-2, G24d-1, G24d-2, G24d-3, GX32d-2, GX32d-3, 2G7, 2GX7, 2G11, and GX10q-4.


4. Measure the Base Circumference

If you're unsure of the number designation to use for a light bulb that requires replacement, you can measure the base to find the correct product.


All light bulb number designations are based on standard sizing from the product's base circumference.


Each number represents 1/18-inch. That means you can multiply that fixture by the number of lengths found to find the designation you need.


Some light bulb bases are listed in millimeters instead of inches.


If you have an A19 light bulb in the United States, it likely has an E26 base. That indicates it is a screw-type Edison product with a 26 mm diameter. Here are a few examples of what you can find in the marketplace today.


Light Bulb Number

Diameter and Length Measurement of the Light Bulb

1 Inch
111.375 Inches
192.375 Inches
202.5 Inches
303.75 Inches
384.75 Inches


When you're unsure of the bulb size because a direct comparison is difficult, you can measure the circumference. This step is helpful if you don't know if the product requires a 19 or a 20 base in inches since the difference is only 1/8 inches.


5. Ask for Professional Help

If you've followed these steps and are still unsure what light bulb you have, contact our customer service department to receive assistance. Our team will work to get you the answer you need to replace the product that isn't working.


It can be frustrating to need to replace a light bulb and not know the product you need. With the information found in this guide, you'll be able to select the correct item. 


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