Guide to the Shapes and Sizes of Light Bulbs

What comes to mind when you think about the size and shape of a light bulb?


For most people, the image that comes to mind is called an A-type bulb. It's what the standard incandescent bulbs of the past looked like, with a rounded shape and a metal screw-in fastener for connecting it to a fixture.


Light bulbs have evolved significantly over the years, allowing for several new shapes and sizes to be available. Today, they're separated into several categories divided by base type, size, and shape.


Although the sheer number of options available today can make the identification of the different shapes and sizes of light bulbs to be confusing or overwhelming, the principles behind each category are straightforward and logical.


This guide takes you through the information you'll need to choose an appropriate light bulb style for your home.


What Are the Different Light Bulb Shapes?


The modern light bulb works in several different fixture options. Each one provides a specific set of requirements for shape and size that you'll need to meet. If you try to force an incompatible product into the installation, you could damage the items or hurt yourself.


When you see a specific light bulb shape requirement on a fixture, it indicates the product you should purchase. The table below provides an overview of each option.


Bulb Designation Bulb Shape Information About the Light Bulb Type
A Standard household incandescent shape This light bulb shape is used for all standard types of household lighting. They come in several different sizes, although the most popular options are A19 and A60. The number reflects the diameter.
B Bullet tip Light bulbs with this shape use a bulged base that comes to a rounded tip.
BT Blown or bulged tubular shape These light bulbs are similar to what you'll find with other B-type shapes, but the end tends to be blunted and less round.
BR Short, reflector-style light bulb The reflector bulbs are typically used for security applications, heat lamps, or workshops. They're longer and broader than most other B-type shapes.
C Conical or candle-style shape The C-type bulb is similar to a holiday lamp. It's the most common option used in low-wattage applications, night lights, and chandeliers because it looks a lot like a candle's flame.
CP Crystalline pear light bulb Although this option is a C-type bulb, it has more of an A-type shape. It uses several “panels" to distribute light in a more fractured manner.
E Ellipsoidal light bulb shape This light bulb shape is similar to the A-type bulb, but it has more of an oval than rounded appearance to it.
ER Extended reflector bulb shape When a fixture doesn't support the BR-type bulb, this shape serves as an effective alternative. It has the same length, but has less width to manage.
F Flame-style candelabra light bulb An F-type 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.
G Circular or globe-style shape This option is similar to the A-type bulb. The outer shape is broader and more rounded to create a more circular viewing experience.
HK Hexagonal candle light bulb style Similar to the C-type bulbs, this style replicates the visual aesthetic of a candle's flame while using a more angular definition. The screw-in base is typically smaller with this choice than a standard design.
K Narrow reflector light bulb (Krypton) The K-type light bulb is an incandescent design with a few modifications. The light tends to burn whiter and purer when connected to a power source, reducing the yellowish colors that some homeowners don't like for their spaces.
MB Halogen MB-type light bulbs are an incandescent-style lamp. The difference is that the interior uses halogen gas to increase its rated lifespan and total light output.
MR Quartz reflector lamp, often low-wattage This 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.
P Pear bulb The P-style light bulb is very similar to what the circular design offers with the G-style. Its primary difference is the lack of an extension between the base and the outer shape.
PAR Parabolic aluminized reflector lamp These light bulbs incorporate a parabolic mirror to focus the light. Since most LEDs lack a reflective lower surface, this design element is necessary. You can find PAR20 to PAR42 options available at most stores, and they're typically used for floodlights.
PS Elongated standard incandescent light bulb When you have an extended fixture to use, the PS-type bulb delivers a useful result. The base is more extended and skinnier while maintaining the traditional shape at the end, allowing it to work for most home-based installations.
R Reflector-style light bulb An 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.
S Straight-sided bulb This light bulb design provides a compact option with minimal width. It works well in recessed lighting applications, especially when working with low-wattage settings.
T Tube lamp The 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.


Diameter and Length Designations for Light Bulbs


When you start shopping for light bulbs, you'll notice that each type gets separated into a diameter and length category.


Although it can be a little confusing, each diameter designation is given a number that reflects the inches of the light bulb's measurement in this area.


Diameter and Length Number Diameter and Length Measurement
8 1 Inch
11 1.375 Inches
16 2 Inches
20 2.5 Inches
30 3.75 Inches
38 4.75 Inches


When you see the length measurement, you'll often see a short and a long category. Although the designation isn't always next to the number, an 8S bulb would be one inch and short.


If you saw a 38L bulb, that indicates the circumference would be 4.75 inches and long.


What Are the Light Bulb Bases Used Today?

Light bulbs must screw into a fixture to operate. They're not like the ready-to-install LED options that you wire directly to the installation point.


The industry has standardized six different base types to use with light bulbs today. You must match up what your fixture requires to the light bulb design selected. It is not possible to mix-and-match bases unless you have some sort of adapter to use.


Here are the six base types you can find.


  • Screw-in bases.
  • Twist and lock bases.
  • Specialty or custom bases.
  • Bi-pin bases.
  • Fluorescent pin bases.
  • Compact fluorescent lamp plug-in bases.


Each category has several specific base designs that might be necessary to find when shopping for new light bulbs. Different manufacturers can introduce new ones at any time, so the following list should be reviewed to ensure it is relevant for the fixture and bulbs you want at home or in the office.


Light Bulb Base Different Design Options Available for the Light Bulb Base
Screw-in Base E10, E11, E12, E14, E17, E26, E27, E39, EX39, and E40. Some designs go by specific names instead of number designations, such as mini-screw, mini-candelabra, candelabra, European, intermediate, medium (standard), medium (non-standard), mogul, and extended mogul.
Twist and Lock Base 10 mm GU10, and 24 mm GU24
Specialty and Custom Base Medium 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 Base 4 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 Base Mini bi-pin, recessed D.C., G10q 4-pin, medium bi-pin, single bi-pin, 2GX13, and Axial.
Compact Fluorescent Plug-in Base G24q-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.


Although all these screw bases exist for light bulbs today, only two of them are considered industry standard today: screw and pin bases.


Each base type works differently when looking at the main function, which is to connect the light bulb to an appropriate voltage to achieve illumination.


What to Know About Light Bulb Screw Bases?

Screw bases for light bulbs are the most common design you'll find in the marketplace today. They're sometimes called “Edisons" because of the concept originality.

When you have a halogen or incandescent screw base, two contact wires link the base to the filament where the electrical voltage reaches the bulb. Since they're made in several sizes, you'll need to see what specific type is offered by reviewing the item description or product packaging.


The number found in the light bulb base reflects its diameter in millimeters. That means an A-type bulb listed as an E11 would be a standard globular design with an 11 mm base.


What to Know About Light Bulb Pin Bases?

Most light bulbs with pin bases are MR16s, linear or plug-in compact fluorescents, or an HID design. The principle of this connection is significantly different from the screw-in style. The pins stick out of the base to connect the bulb to a voltage source.


Once the pin connection is established, the electrical current can pass through them to flow into the bulb. That process excites the filament to produce light.


Some fixtures allow you to plug in the pins, but then they require a lock and a twist to secure the bulb. You'll want to take that final step carefully if it is necessary to ensure the light stays where it should be. If you squeeze too hard during this process, there is a chance that the glass could shatter.


Information About Light Bulb Filaments


The light bulb filament is responsible for producing the illumination you receive when sending voltage to the unit. Today's filaments are broken into two categories: C or CC.


The “C" stands for “coiled," which means the filament wire is either deeply fluted or wound into a helical coil.


If you have a “CC" light bulb, that abbreviation stands for “coiled coil." The filament wire gets wound into a helical coil, and then that process is repeated.


Some light fixtures have specific filament shapes, designs, or voltage limitations that you'll need to consider.


When you think about the classic incandescent light bulb, the electrical filament is the small thin wire that sits between two longer ones that hold it up. The older designs use tungsten for this component because of the high melting point, although the first options were made from carbon.


If you upgrade to an LED bulb, they also have a filament. The design is different because instead of heating the wire to produce light, this technology uses the metal strip to line it with diodes. Although they're not as thin as a tungsten filament, the result it produces is quite similar.


Some LED bulbs don't have a filament replication. That's why evaluating what option you prefer is helpful when choosing the lighting you want for your home.


Here's a closer look at the differences between filament LEDs and traditional designs.


Advantages of Filament LEDs Disadvantages of Filament LEDs
The filament LED design requires a smaller heat sink in its design. That means more of the enclosure is glass, leading to nearly 360 degrees of light exposure. Most filament LEDs come with a single color choice. Although you can change the temperature of the emitted light through dimming, you won't get a wide Kelvin range.
The visual aesthetics of this technology create a vintage or retro style that contributes to any interior décor scheme. This design lasts up to three times less than what a traditional LED offers, although recent designs have reached a 40,000-hour guarantee.
Most light bulbs with this technology work with modern dimmer switches so that you can create unique mood lighting options. There aren't any smart lighting options that work well with this technology.


When you install a filament LED, the goal is to create a more decorative element than one that offers practical lighting. It's an option that delivers more ambiance to enjoy.


Advantages of Traditional LEDs Disadvantages of Traditional LEDs
The lifespan of a traditional LED bulb has reached 50,000 hours for most applications. Some products in this category have topped 200,000 hours. Once you install one of these products, you might not need to change it out ever again. You don't receive the vintage or retro style of the filament design. Most LED bulbs are shaped like their incandescent or fluorescent counterparts from the past.
You can use this light bulb option with many smart lighting options, including remote or app-based controls when you're away from home. The traditional LED design uses a focused, uni-directional output area instead of featuring multi-directional lighting. This issue occurs because of the larger heat sinks needed.
Traditional LED bulbs come in several RGB colors. Some models are capable of producing more than 16 million different options while working with a standard fixture. Older fixtures and switches might not support this technology. You may need to upgrade your connection point to ensure you receive the illumination you want at home.


How to Pick the Right Light Bulb


Now that you're armed with this information, you have everything you need to pick the best light bulb for your room. Please remember to review the shape and size requirements of each fixture, along with the wattage minimum or maximum that your current setup supports.


Once you know the technical requirements for your lighting, you can start thinking about your space. Choosing light bulbs that complement its purpose will maximize the value of your home.


If you sit on a sofa to watch TV, you don't need a bright, glaring light that shines right in your face. A dimmable, recessed fixture would be a better option. In the kitchen, you might want an LED light with the highest Kelvin rating you can find to let you see things clearly.


Some new fixtures come with LED bulbs that don't require replacement, which means you'll need to be happy with how the product looks once it's installed.


Cheaper bulbs don't always come with dimmers, so you'll want to review the box to see if this option is available. If this terminology isn't on the package, that feature isn't included.


The right lighting concept ensures that you can sleep better at night. Exposure to bright light during the day helps people experience improved rest at night, keeping your circadian rhythm synced to your schedule.


Smart bulbs are at the top of the price range today, but they come with several intelligent features that add value to your home. Think about what each room needs, and then select the appropriate light bulb for that situation.

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