Guide to Light Bulb Sizes and Shapes

Different sizes and shapes of light bulbs can fit in different fixtures. Have you gotten confused when purchasing? You may feel overwhelmed to navigate endless options. Therefore, we here provide a ​light bulb size ​guide to help you make a better decision. Read on.​

light bulb size

Part 1: What Are the Different Light Bulb Shapes?

To start, let’s learn the different shapes of light bulbs. 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.

That's all shapes of light bulbs, and the most common we see in our life are Type A, C, R, MR, PAR, G and T. The light bulb codes are represented by a letter or a series of letters with a number. Take A21 for example, letter ‘A’ indicates standard household incandescent shape, and ‘21’ indicates a diameter of 21 divided by 8 inches, or approximately 2.6 inches.

light bulb shapes 

There are a variety of light bulb bases sizes also. Same with light bulb shapes, they use light bulb base codes, letters for the shape of the base and the number for the size. Find details in the next part.

Part 2: What are the Light Bulb Base Sizes Used Today?

Not like the ready-to-install LED options, light bulbs must screw into a fixture to operate. The industry has standardized six base types to use with light bulbs today. Here are the six base sizes for light bulbs.

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: screw and pin bases.

What are Light Bulb Screw Bases?​

Screw bases for light bulbs are the most common design today. They're sometimes called “Edisons" because of the concept's 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. The number found in the light bulb base reflects its diameter in millimeters. For example, 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.

Part 3: Diameter and Length Designations for Light Bulbs

Each type of light bulb gets separated into diameter and length categories. And 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 short. If you saw a 38L bulb, that indicates the circumference would be 4.75 inches long.

light bulb shapes 

Bonus Tips: What is the Light Bulb Filament?

The light bulb filament is responsible for producing the illumination when sending voltage to the unit. And it has 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. The “CC" light bulb 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. The electrical filament for the classic incandescent light bulb 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.

As for LED bulbs, they also have a filament, but the design is different. Instead of heating the wire to produce light, this technology uses a metal strip to line it with diodes. Although not as thin as a tungsten filament, the result is similar.​

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.

In Conclusion

Now that you're armed with this information, how to pick up the bulb? First, review the shape and size requirements of each fixture, along with the wattage minimum or maximum that the setup supports. Then thinking about space. If the space doesn't need a bright, glaring light, a dimmable, recessed fixture would be a better option. For example, in the warehouse, the LED light with the highest Kelvin rating can help workers see things clear.

If it is still hard to make a choice, contact the LEDVANCE sales team, we'll contact you as soon as we can.​​​​​​​​​​


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