A19 vs. E26 vs. E27: Light Bulb Shapes and Base Types Explained

When shopping for a light bulb, you may encounter codes such as A19, A21, G25, E26, and E27. Although these labels often appear together on product packaging, they describe different parts of a light bulb. Confusing them can lead to buying a bulb that looks right but does not fit your fixture.

The key is to understand two separate specifications: light bulb shape and light bulb base type. One describes the bulb's outer form and approximate diameter, while the other identifies its connection to the socket. Once you understand how these specifications work together, comparing light bulbs becomes much easier.

Quick Answer: What Is the Difference Between Light Bulb Shapes and Bases?

A light bulb shape code, such as A19 or G25, describes the bulb's outer form and nominal size. A base code, such as E26 or E27, describes the connection at the bottom of the bulb.

For example, an A19 E26 light bulb has a standard pear-shaped envelope and a medium Edison screw base. The two codes are complementary, not alternatives.

The same principle applies to decorative globe bulbs, candle-shaped bulbs, and reflector bulbs. A compatible light bulb must have the correct base for the socket and a shape and size that fit the fixture. Voltage and other electrical requirements must also match.

1. What Do Light Bulb Shape Codes Mean?

Light bulb shape codes help identify the bulb's physical design. In many North American designations, a letter identifies the shape family, while the number indicates its nominal maximum diameter in eighths of an inch.

For example, A19 means an A-shaped bulb with a nominal diameter of 19/8 inches, or 2.375 inches (approximately 60 mm). G25 uses the same diameter convention: 25/8 inches, or 3.125 inches (approximately 79 mm).

This numbering system is useful when comparing bulb sizes, but it does not specify the base type. For a broader reference, see this guide to common light bulb shapes and sizes.

A-Series: A15, A19, and A21

A-series light bulbs have the familiar rounded, pear-like shape used in many homes. They are common in table lamps, floor lamps, ceiling fixtures, and other general-purpose lighting applications.

• A15: A smaller A-shaped light bulb, with a nominal diameter of 1.875 inches (47.6 mm). Its compact size makes it useful in smaller fixtures and certain appliance applications.

• A19: A standard household light bulb shape with a nominal diameter of 2.375 inches (60.3 mm). It is widely used in American table lamps, floor lamps, and ceiling fixtures.

• A21: A wider A-shaped light bulb with a nominal diameter of 2.625 inches (66.7 mm). It may be used when a larger bulb envelope is needed, but its dimensions must be checked against the fixture.

The main difference between A15, A19, and A21 is size, not base type. An A19 and an A21 may both use an E26 base, yet the larger A21 may not fit beneath a compact lampshade.

G-Series: G16, G25, and G30 Globe Bulbs

G-series light bulbs have a rounded, globe-like shape. Unlike standard A-series bulbs, their wider spherical envelopes are often intended to remain visible as part of the fixture's design.

• G16: A relatively compact globe bulb, with a nominal diameter of 2 inches (50.8 mm).

• G25: A globe bulb with a nominal diameter of 3.125 inches (79.4 mm). It is common in bathroom vanity lights, mirror lighting, and decorative fixtures.

• G30: A larger globe bulb, with a nominal diameter of 3.75 inches (95.3 mm). It is suitable for larger exposed-bulb fixtures where its dimensions and appearance work with the design.

Globe light bulbs can create a decorative effect in open fixtures, but their wider profiles require adequate clearance between neighboring bulbs and surrounding shades.

B-Series and C-Series: B11 and C11 Candle Bulbs

B-series and C-series bulbs are commonly associated with decorative, candle-like forms. They are often used in chandeliers, wall sconces, pendant lights, and other fixtures where the bulb itself is visible.

• B11: A narrow, bullet- or torpedo-shaped decorative bulb, with a nominal diameter of 1.375 inches (34.9 mm). It is commonly used in chandeliers and decorative wall lights.

• C11: A candle-shaped bulb with a nominal diameter of 1.375 inches (34.9 mm). Its conical outline suits decorative fixtures inspired by candlelight.

• CA10: An angular candle-shaped bulb, with a nominal diameter of 1.25 inches (31.8 mm). It can be used where a more tapered decorative profile is desired.

Although B11 and C11 have the same nominal diameter, their outlines differ. The number alone does not describe the entire shape.

These bulbs are frequently paired with smaller screw bases in decorative lighting, but the exact base must be confirmed separately. For more examples, see this light bulb shape guide for decorative and general-purpose bulbs.

BR-Series: BR20, BR30, and BR40 Reflector Bulbs

BR stands for bulged reflector. These light bulbs have a wider reflector-style envelope designed to direct light outward, making them common in recessed ceiling fixtures and floodlighting applications.

• BR20: A compact reflector bulb with a nominal diameter of 2.5 inches (63.5 mm).

• BR30: A wider reflector bulb with a nominal diameter of 3.75 inches (95.3 mm). It is frequently used in recessed ceiling lighting.

• BR40: A larger reflector bulb with a nominal diameter of 5 inches (127 mm). It is suitable for fixtures designed to accommodate a wider bulb.

The number identifies the nominal diameter, not the brightness or beam angle. Two BR30 light bulbs can have different lumen outputs and light distributions.

When choosing between reflector shapes, the bulb's diameter, length, beam characteristics, and fixture compatibility all matter. This light bulb shapes and sizes guide provides additional comparisons.

PAR-Series: PAR20, PAR30, and PAR38

PAR stands for parabolic aluminized reflector. PAR light bulbs use a reflector and lens design intended to produce more controlled directional light than a typical general-purpose A-series bulb.

• PAR20: A compact directional bulb, with a nominal diameter of 2.5 inches (63.5 mm).

• PAR30: A medium-sized directional bulb, with a nominal diameter of 3.75 inches (95.3 mm).

• PAR38: A larger directional bulb, with a nominal diameter of 4.75 inches (120.7 mm).

PAR bulbs are commonly used in track lighting, accent lighting, floodlights, and some outdoor fixtures. The appropriate size depends on the fixture opening and the desired light distribution.

Although BR and PAR bulbs can have similar diameters, their designs and optical characteristics are not identical. Check the fixture's requirements rather than treating them as interchangeable based on appearance alone.

MR-Series: MR11 and MR16

MR stands for multifaceted reflector. MR bulbs have compact reflector envelopes that direct light toward a particular area, making them useful for accent lighting, display lighting, and some track-lighting applications.

• MR11: A compact reflector bulb with a nominal diameter of 1.375 inches (34.9 mm).

• MR16: A reflector bulb with a nominal diameter of 2 inches (50.8 mm).

MR16 is particularly important because it describes the reflector form factor, not a single base type. An MR16 light bulb may use a GU5.3 pin base or a GU10 twist-lock base, depending on the product.

The distinction matters when replacing a spotlight: the shape may be correct while the connection is not. This comparison of MR16 and GU10 light bulbs explains the difference between reflector designations and base types.

T-Series: T8 and T12 Tubular Bulbs

T-series bulbs have a tubular form rather than a rounded or globe-shaped envelope.

• T8: A tube with a nominal diameter of 1 inch (25.4 mm).

• T12: A tube with a nominal diameter of 1.5 inches (38.1 mm).

These designations are common in linear fluorescent and LED tube lighting, including garages, utility rooms, workshops, and some kitchens.

For tubular bulbs, the complete replacement specification may depend on the tube's length, end-cap configuration, electrical design, and compatibility with the existing fixture. The T number alone does not identify the end connection.

What Do the Numbers in Light Bulb Shape Codes Mean?

For many North American bulb shapes, the number represents the nominal maximum diameter in eighths of an inch. A19 is 19/8 inches, while G25 is 25/8 inches.

This convention makes it easier to compare the sizes of bulbs within a shape family. However, it should not be applied indiscriminately to every code: base codes use different conventions, and some international designations use metric measurements or other standards.

2. What Do Light Bulb Base Codes Mean?

A light bulb base is the part that connects the bulb to the fixture's socket. It provides the physical connection and the electrical contacts needed to operate the bulb.

Unlike shape codes, base codes identify the connection style and its relevant dimensions. Common examples include E26, E27, E12, E14, GU10, GU5.3, and G9.

Base code	Connection type	Common applications
E26	Medium Edison screw	Many North American household fixtures
E27	Edison screw	Many European household fixtures
E12	Small Edison screw	North American chandeliers and decorative sconces
E14	Small Edison screw	Many European decorative fixtures
GU10	Two-pin twist-and-lock	Spotlights and recessed lighting
GU5.3	Two-pin connection	Many low-voltage MR16 spotlights
G9	Loop-pin connection	Compact decorative lights and wall lamps

The E in E26 and E27 refers to the Edison screw family. The number denotes the nominal screw-thread diameter in millimeters. Other base families use different connection designs and dimensional conventions.

For additional examples, see this guide to common light bulb base types and fittings.

LightsLux also explains the role of these connections in its guide to light bulb base sizes.

3. How Do Light Bulb Shapes and Bases Work Together?

Shape and base are separate specifications that describe different parts of the same light bulb. A complete bulb description may include both codes, such as A19 E26 or G25 E26.

Consider the following examples:

Complete bulb description	Shape specification	Base specification
A19 E26	Standard pear-shaped bulb	Medium Edison screw
A19 E27	Standard pear-shaped bulb	E27 Edison screw
G25 E26	Globe-shaped bulb	Medium Edison screw
B11 E12	Decorative torpedo shape	Small Edison screw
MR16 GU5.3	Multifaceted reflector	Two-pin base
MR16 GU10	Multifaceted reflector	Twist-lock base

The important point is that neither code replaces the other.

Can Two Light Bulbs Have the Same Base but Different Shapes?

Yes. An A19 E26 bulb and a G25 E26 bulb can both be designed for an E26 socket, but their outer shapes are different.

The A19 has a familiar pear-shaped profile, while the G25 is much wider and rounder. In an exposed fixture, this difference affects the overall appearance. In an enclosed fixture or narrow shade, it may determine whether the bulb physically fits.

The same principle applies to decorative candle bulbs and reflector bulbs: the base identifies the connection, while the shape helps determine physical clearance and the bulb's form.

Can Two Light Bulbs Have the Same Shape but Different Bases?

Yes. The same general bulb shape may be offered with different base configurations.

For example, an A19-shaped bulb may use an E26 base in a North American product or an E27 base in a product designed for a compatible fixture in another market. Similarly, MR16 reflector bulbs can use different base types.

A matching shape does not guarantee that the bulb will connect to the socket. A matching base does not guarantee that the bulb will fit inside the fixture.

Does the Shape of a Light Bulb Determine Its Brightness?

No. Shape and size do not directly determine brightness. Brightness is measured in lumens, while the bulb's design, reflector, diffuser, and beam angle influence how light is distributed.

For example, two A19 LED bulbs may have different lumen outputs, color temperatures, and dimming capabilities. Two PAR38 bulbs may also produce different beam patterns.

4. E26 vs. E27: Why Are They Different?

E26 and E27 are both Edison screw-base designations, and their physical dimensions are very similar. E26 has a nominal thread diameter of 26 mm, while E27 has a nominal thread diameter of 27 mm.

E26 is commonly associated with North American residential fixtures, while E27 is widely used in many European and other international markets.

The difference is not the shape of the glass envelope. An A19 bulb can be manufactured with either base designation, depending on the product and market.

It is also important not to infer voltage from the base code alone. Residential supply voltage differs between North America and many European countries, and bulbs must be rated for the fixture's electrical supply.

Before substituting an E26 bulb for an E27 bulb, or vice versa, follow the fixture manufacturer's specifications and check the bulb's voltage rating. Similar appearance does not guarantee suitability in every application.

5. Common Mistakes When Comparing Light Bulb Shapes and Bases

Mistake 1: Treating A19 and E26 as competing options.

A19 identifies the bulb shape and nominal diameter. E26 identifies the screw base. Both can appear in the description of one light bulb.

Mistake 2: Assuming the same base means the same physical size.

An A19 and a G25 may use the same E26 base, but the G25 globe is wider. The bulb still needs enough clearance inside the fixture.

Mistake 3: Assuming the same shape means the same connection.

MR16 describes a reflector form factor. The base may be GU5.3 or GU10, and the fixture must support the correct connection and electrical requirements.

Mistake 4: Assuming E26 and E27 are simply interchangeable labels.

Their dimensions are close, but compatibility depends on the socket design, electrical ratings, and manufacturer guidance.

Mistake 5: Using shape codes to estimate brightness.

The number in A19, G25, or PAR38 is not a lumen rating. Compare the actual lumen output and beam characteristics when evaluating brightness.

Final Thoughts

Understanding light bulb shapes and base types starts with separating two specifications that often appear together. A19, A21, G25, B11, BR30, PAR38, MR16, and T8 describe different bulb shapes or form factors. E26, E27, E12, E14, GU10, GU5.3, and G9 identify base connections.

A light bulb can have the right shape but the wrong base, or the right base but a shape that does not fit the fixture. Checking both specifications, along with voltage and fixture requirements, is the most reliable way to understand bulb labels and compare replacement options.

FAQs

Is A19 the same as E26?

No. A19 describes a light bulb's shape and nominal diameter, while E26 identifies its medium Edison screw base. An A19 bulb can have an E26 base, but the two codes refer to different specifications.

What is the difference between A19 and A21 light bulbs?

Both have a standard pear-like shape, but A21 is wider. A19 has a nominal diameter of 2.375 inches, while A21 has a nominal diameter of 2.625 inches.

What does G25 mean on a light bulb?

G25 identifies a globe-shaped light bulb with a nominal diameter of 25/8 inches, or 3.125 inches. It is commonly used in bathroom vanity lighting and decorative fixtures.

Can an MR16 light bulb have a GU10 base?

Yes. MR16 describes the reflector form factor, while GU10 describes a twist-lock base. MR16 bulbs may also be available with a GU5.3 base, so the connection must be checked separately.

Does E27 mean a light bulb is brighter than E26?

No. The number identifies the nominal screw-base diameter, not brightness. To compare light output, look at the bulb's lumen rating.

Why do light bulb packages list both shape and base?

The shape code identifies the bulb's form and nominal size, while the base code identifies its socket connection. Both help determine whether a light bulb is physically suitable for a fixture.