Bulb Shapes

Bulb Shapes: Everything You Need to Know

There are many types and varieties of light bulbs, each with its own specific usage. Even just for household lighting, the wide range of bulb shapes can be daunting. Fortunately, the designations for bulb shapes are ordered alphabetically for easy identification. There is a bulb shape for nearly every letter in the alphabet, some being more prevalent than others. For the purposes of this article, however, we will only cover the most common styles and explain their best usages. In addition to describing the shape designations, this article will also explain how to tell the dimensions of the bulb as well as relevant variants of the bulb shapes.

A-Shape – The “Arbitrary” Light Bulb

The “A” bulb shape, also known as the “arbitrary” bulb shape, is the most ubiquitous type of light bulb out there. However, there is nothing “arbitrary” about this mainstay of the lighting industry. As a testament to its universality, the design of A-Shape bulbs traces its origins to over a hundred years ago with Thomas Edison’s first light bulbs. Although the technology inside A-Shape bulbs have long since surpassed Edison’s incandescent bulbs, this style continues to stand the test of time. A-Shape bulbs can be found in a wide range of general lighting applications and are popular for both household indoor and outdoor lighting.

By far, the most common size of the A-Shape bulb is the A19. This number following the “A” designation refers to the bulb’s diameter at its widest point, measured in eighths of an inch. So, the A19 is an A-Shape bulb that has a diameter of 19/8 or 2.38 inches. This system of measuring in eighths of an inch is the rule of thumb for measuring the diameter of all lightbulbs. The number following a bulb's shape designations, such as B13, MR16, T8, etc., always refers to the diameter of the bulb measured in eighths of an inch. The common sizes of A-Shape bulbs range from the smallest, A15, to the largest, A23. Additionally, recent advancements in LED technology allows for RGB Color Selectable A-Shape bulbs as well as decorative, visible filament bulbs.

B/C-Shape – The “Candelabra” Light Bulbs

Both B and C-Shape bulbs fit into the same category with usage in decorative and low wattage applications. Both styles of bulb are designed in a teardrop shape to emulate a candle. The flair of B and C-Shape bulbs lends them well to decorative applications such as holiday and pendant lights, wall sconces, and chandeliers. Both shapes can also be used for low wattage lighting fixtures like night lights.

The B-Shape bulbs are known as “bullet” or “blunted” styles due to their rounded tip while C-Shape bulbs have a pointed tip and are known as “conical” light bulbs. Additionally, both shapes have an angularly tipped variant known as the BA and CA shapes. The sizes of B and C-Shape bulbs are typically smaller in diameter, ranging from B8 and C6 at the smallest to B13 and C15 at the largest. The size of the bulb required varies depending on the application. Chandeliers and wall sconces tend to use a medium size such as a B10 or CA10 while holiday lights and night lights typically use smaller bulbs like the B8 or C7.

E-Shape – The “Ellipsoidal” Light Bulb

Unlike B and C-Shape bulbs, E-Shape bulbs are large, bright lamps. Known as “ellipsoidal” bulbs due to their elliptical shape, these types of bulbs are typically used for metal halide lamps. Metal halide lamps, as well as other gas high-intensity discharge (HID) lighting systems, require larger bulbs in order to provide the highest lumen-to-wattage output. Another common bulb shape for HID lamps is the BT-Shape bulb. Due to their similar usage in gas-discharge lamps, BT-Shape and E-Shape bulbs tend to be grouped together.

E-Shape bulbs can be found in a variety of applications from outdoor sports and parking lot lighting to indoor overhead lights in warehouses and grocery stores. A “dimpled” variant of the E-Shape can also be found in similar HID applications. Known as ED-Shape bulbs, these are named after the dimple at the top of the bulb and can range in size from ED18 to ED37. The sizes of E-Shape bulbs have a similar range of sizes from E17 up to E25. The size of E-Shape bulbs tend to increase as the lamp’s wattage requirements increase. This correlates with the brightness of the bulbs. The brightest E-Shape bulbs are typically the largest with the highest wattage requirements.

G-Shape – The “Globe” Light Bulb

G-Shape bulbs are similar in shape to A-Shape bulbs, but have a much larger and rounder "globe" than A-Shape bulbs. This is where their “globe” designation comes from. And in a similar fashion to B and C-Shape bulbs, Globe bulbs are great for decorative lighting fixtures such as chandeliers and wall sconces. They are great for producing a similar amount of light to A-Shape bulbs while still having the decorative properties of B and C-Shape bulbs. The size of G-Shape bulbs can range widely, with the smallest being the G16.5 and the largest G40. The most common sizes of G-Shape bulbs, such as the G25, are typically found in both kitchen and foyer lights as well as in bathroom vanities.

R-Shape – The “Reflector” Light Bulb

The family of R-Shape or “reflector” bulbs encompasses multiple styles of floodlights which use a reflective material inside the lamp to project a wide beam of light. The three most common styles of R-Shape bulbs are BR, MR, and PAR. While most reflector bulbs were originally halogen or incandescent bulbs, energy efficient LED reflector retrofits have rapidly begun to replace conventional lamps for most applications.

BR-Shape bulbs, named for the visible "bulge" at the top of the bulb, have the widest beam range of the reflector shape bulbs with a beam range of 90 degrees or wider. This makes them perfect for indoor, high-ceiling applications where a wide coverage of light is required. The most common sizes of BR bulbs are R20BR30, and BR40. BR-Shape bulbs are similar in shape to regular R-Shape bulbs, so the two tend to be grouped together.

MR-Shape bulbs are known as “Multifaceted Reflector” bulbs due to the angled, reflective material inside the lamp which directs the light to one spot. This makes them perfect for spot lighting where a single focal point is desired. This type of bulb is typically found in display light applications in addition to general track and recessed lighting. The most common size of MR-Shape bulb is by far the MR16, but other variants can be found as well.

The last style of reflector bulb is the PAR-Shape, known as “Parabolic Aluminized Reflector” bulbs. This refers to the U-Shaped reflector found inside conventional PAR-Shaped bulbs, although newer LED PAR replacements do not have this reflector for which they’re named. PAR bulbs combine the wide angle of the BR-Shape and the spot lighting of the MR-Shape. This allows for PAR-Shape bulbs greater utilization on a wide range of both indoor and outdoor applications. PAR bulbs are typically found in the PAR30 and PAR38 sizes.

T-Shape – The “Tubular” Light Bulb

Tubular, or T-Shape, bulbs can refer to both linear tube lamps as well as short tube bulbs. Here, we will focus on short tube bulbs so as to not confuse them with fluorescent linear tubes since both use the same T-Shape designation. For more information specific to linear tubes, check out our blog post on LED tube retrofits. Short tube bulbs can be used in a wide range of applications from the bulbs inside a refrigerator to decorative fixtures. Some tubular bulbs are even designed with visible filaments to resemble the vacuum tubes found in vintage speakers and radios. Both linear tubes and short tube bulbs use the same rule of eighths to measure the diameter of the bulb. The size of T-Shape bulbs can vary greatly from T3 up to T20, with the most common sizes being T5, T8, and T10.

If you have any further questions, please don’t hesitate to contact us at 888-307-3700. We have qualified lighting specialists that will make sure that you get the product that best suits your needs. Stay tuned for an upcoming article which will similarly discuss the different types of bulb sockets and bases.