Light Bulb Base Sizes Explained by Lighting Expert

A light bulb base is the part of a light bulb that connects it to the fixture or bulb socket.

Light bulb bases and sockets are designed to hold the bulb securely and ensure electrical contact. Light bulb base sizes vary based on lighting technology, applications, and power requirements.

Light Bulb
Light Bulb Base
Light Bulb Socket

Luckily, there are standards for light bulb base sizes in the lighting industry, and parts are made to meet those standards. To fit the bulb socket, light bulb bases are made to fulfill these same specifications.

This article discusses the different types of light bulb bases. It broadly covers what the different light or lamp bases mean, why different lamp bases are used for different lighting applications, and where those light bulb bases are used. Additionally, it provides a light bulb base sizes chart in shareable image format.

Light Bulb Base Sizes

Light bulb base sizes vary with the type of bulb and application. The base of a light bulb also contains information about the bulb type, and wattage, which may also indicate the maximum overall length of the bulb.

Base sizes are determined by a simpler rule known as Type-Size.

The most common type of base is the E type. The E stands for Edison and indicates that the base is a screw type. The size is a simple millimeter measurement of the width.

An E26 base (the most common type) is 26 millimeters wide and features an Edison thread interior. Additionally, this base is referred to as a Medium socket.

The three most often used bases in the United States of America for traditional lighting are the E12 (candelabra), the E26 (medium), and the E39 (mogul).

Light Bulb Base Sizes Chart

Here is a light bulb base chart that includes pictures of conventional light bulb bases, such as fluorescent bases, LED light bases, automotive light bulb bases, halogen bases, etc., to help you find the light bulb base you have in your hand or need for your light bulb socket.

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Light Bulb Base Sizes
Light Bulb Base Sizes

Why do lights have different bases?

Light bulbs have to fit snugly into light fixtures, so they are designed with different bases. The light bulb base has to match the socket in the light fixture.

If light bulb bases were standardized, light bulbs could be interchangeable across many fixtures.

However, light bulb bases are not standardized. This is to prevent dangerous light bulbs from being used in light fixtures that aren’t designed for them.

Light fixture sockets have a grip on the inside of the socket, and light bulb bases have a matching metal tab on the outside of the base. A light bulb with no grip can be inserted into any kind of light fixture, but it won’t stay in place because the grip and tab don’t match up.

This is why light bulb bases aren’t standardized: if they were interchangeable across light fixtures, people would risk inserting a light bulb into sockets that weren’t designed for them.

Light Bulb Base Types Explained

There are dozens of different light bulb bases. The most common is the Edison screw base, which may be found on most incandescent bulbs and many halogens, compact fluorescents, HIDs, and now LED lights.

Edison Screw Bases

The Edison screw (ES) is a type of light bulb base that is universally used with electric light bulbs. Thomas Edison invented it and patented it in 1881.

The bulbs have metal bases (caps) that are threaded in the right direction and screwed into matching threaded sockets (lamp holders).

In most cases, when bulbs are powered by alternating current, the thread is linked to neutral, and the contact on the bottom tip of the base is connected to the “live” phase.

In North America and continental Europe, Edison screws took the place of other types of bases for general lighting.

In the early days of electricity, the only standard connector was an Edison screw, and all appliances except light bulbs were hooked up to AC power through lamp sockets. Edison screw sockets are currently compliant with international standards.

The name “Exx” is made up of the letters “E” for “Edison” and “xx” for the millimeter diameter measured across the peaks of the thread on the base (male).

For example, E12 has a diameter of 12 mm.

DesignationBase major diameter (thread external)NameApplication
E5 5 mmLilliput Edison Screw (LES)Indicator lights, decorative lights
E1010 mmMiniature Edison Screw (MES)Flashlights, bicycle lights

11 mmMini-Candelabra Edison Screw (mini-can)120 V halogen mini-candelabra
E12 12 mmCandelabra Edison Screw (CES), C7120 V candelabra/night lamp
E14 14 mmSmall Edison Screw (SES)230 V candelabra/chandelier, night lamps, and some pendant lights.
E1717 mmIntermediate Edison Screw (IES), C9 120 V appliance
E2626 mmMedium Edison Screw (ES or MES) Standard120 V lamps
E2727 mm[Medium] Edison Screw (ES) Standard 230 V lamps
E2929 mm [Admedium] Edison Screw (ES)
E3939 mmSingle-contact (Mogul- in America) Goliath Edison Screw (GES)120 V 250+ W industrial
E4040 mm (Mogul) Goliath Edison Screw (GES)230 V 250+ W industrial
Source: Wikipedia: Edison Screw

E10 Base or Miniature Screw Base

In the lamp base designation “E-10” (sometimes called Miniature Edison Screw (MES), “E” stands for “Edison” and “10” indicates the diameter in millimeters as measured across the peaks of the thread on the base, e.g., E-10 base bulbs have a diameter of 10 mm. E-10 base light bulbs have a single contact on the bottom and are used in flashlights, indicators, and toys.

E11 Base or Miniature Candelabra Base

E11 (mini candelabra) is 11mm (0.433 in) in diameter and is a bit smaller than E12 (candelabra) which is 12mm in diameter. Its applications are very limited. E11’s are most commonly used for small, halogen (hot!) bulbs. Another most obvious application is obviously for 120-volt chandelier/candelabra. Low voltage models are used on Lionel toy trains and other toys. The E11 base is sometimes used for 50/75/100-watt halogen lamps in North America, where it is called the “mini-can”.

E11 Mini Candelabra Base Identification

E12 Base or Candelabra Base

As the name suggests, the fitting is 12 millimeters in diameter size and is often deemed the Candelabra Edison Screw (CES). It is the home of C7-sized lamps, so if you bought a C7 lamp before, it utilized the E12 fitting.

Bulbs with E12 bases are commonly used for decorative applications, such as chandeliers, wall sconces, Christmas lights, restaurant pendant lights, ornamental lights, residential or commercial string lights, and night lights. Many bulbs with these bases are designed to mimic the look of a candle flame.

Common bulbs with E12 bases:

  • Small tube-shaped bulbs: T8, T6, T22
  • Candle-flame-shaped bulbs: C7, C9, CA10, B10
  • Small globe-shaped bulbs: G14, G16, G50

E14 Base or European Base

European Or SES (Small Edison Screw) is a very common size used throughout Europe and the UK. An E14 bulb is one of the most-used bulbs in our homes found in many types of small light fittings around the home from bedside table lamps to wall lights and chandeliers . It is also called the bulb with the small screw socket. The bulb is called E14 because it refers to the socket: 14-millimeter diameter.

Common bulbs with E14 bases:

  • A-shaped bulbs: A15
  • Small tube-shaped bulbs: T8, T6, T22
  • Candle-flame-shaped bulbs: C7, C9, CA10, B10
  • Small globe-shaped bulbs: G14, G16, G50
  • Marquee and sign bulbs: S11

E12 vs E14

Name Candelabra Edison Screw (CES), C7 Small Edison Screw (SES)
Size12 mm14 mm
Application 120 V candelabra/night lamp 230 V candelabra/chandelier, night lamps, and some pendant lights.

E17 Base or Intermediate Edison Screw Base

Intermediate Screw Base (E17) Bulbs are sometimes called Intermediate Edison Screw (IES), “E” stands for “Edison” and “17” indicates the diameter in millimeters as measured across the peaks of the thread on the base

Like candelabra bulbs with E12 bases, bulbs with E17 bases are used for decorative applications, such as chandeliers, wall sconces, Christmas lights, restaurant pendant lights, ornamental lights, and sign or marquee lights. Bulbs with these bases are commonly used for appliance lights, such as microwave lights, refrigerator lights, and range hood lights. They’re also found in ceiling fan light fixtures.

Common bulbs with E17 bases:

  • A-shaped bulbs: A15
  • Small tube-shaped bulbs: T8, T6, T22
  • Candle-flame-shaped bulbs: C7, C9, CA10, B10
  • Small globe-shaped bulbs: G14, G16, G50
  • Marquee and sign bulbs: S11

E26 Base or Medium Edison Screw (MES) Base

E26 is the size of most light bulbs used in the U.S. It’s referred to as having a “medium” or “standard” base.

The letter “E” in E26 refers to Edison Screw, the most standard of lamp bases, while the numerical “26” refers to its diameter in millimeters (approximately 1 inch).

E26 is also commonly referred to as a Medium Edison Screw (MES).

They are commonly used for general lighting in standard light fixtures, such as ceiling lights, lamps, can lights, outdoor spotlight and flood light fixtures, and vanity lights.

Common bulbs with E26/E27 bases:

  • A-shaped bulbs: A19, A15, A21
  • Globe-shaped bulbs: G30, G25, G16.5
  • PAR (parabolic aluminized reflector) spotlight and floodlight bulbs: PAR16, PAR20, PAR30, PAR38
  • BR (bulged reflector) spotlight and floodlight bulbs: BR20, BR30, BR40

E39 Base or Mogul Base

E39 is known as the large base or mogul base. Its typically what is used on older metal halides and high pressure sodium from 100 watts and up. The E is for Edison screw base and the 39 is 39 mm across. Larger than the standard E26 or 26 mm across.

The large E39 “mogul” base is used on street lights, and high-wattage lamps (such as a 100/200/300 Watt three-way). Chances are you won’t have a need for these guys.

Common bulbs with E39/E40 bases:

  • PAR (parabolic aluminized reflector) bulbs: PAR20, PAR38, PAR40
  • Blown tubular bulbs: BT37, BT56
  • Elliptical dome bulbs: ED17, ED28, ED37


BaseSocket TypeSpacingApplication
Miniature Bi-PinG5 Bi-Pin base5 mmT4/T5 / T16 Tubelights
Medium Bi-PinG13 base13 mmT8 / T26 Tubelights
Recessed double contactR17d base17mmT17 / PG17
4-PinG10 quad pinT9 /T10/ T29
Single pinFa6, Fa86 mm, 8 mmLess common
Single-Ended, 4-Pin

Miniature Bi-Pin

Miniature BiPin, also called mini-BiPin, sockets have pins placed closer together. Not only are they smaller in size, but they’re also used in a smaller type of light too.

The miniature bi-pin sockets are used in linear fluorescent lighting, like T5 lights.

Medium Bi-Pin

When you’re wondering what the difference between a medium and a miniature bi-pin socket is, it doesn’t just involve the size of the part itself. The actual socket is what helps to generate the correct amount of electricity to the light. If the bi-pin is too small or large for the light, it could cause a significant electrical problem.

Medium bi-pin sockets are generally used in a variety of linear fluorescent bulbs, like the T8 and T10 lights.

Single Pin

Some linear fluorescents have a single pin Fa8, but they are not very common.

Recessed Double Contact

Some linear fluorescents have double contact R17d, but they are not very common.


  • GU10
  • GX10
  • GU24


A bipin or bi-pin (sometimes referred to as two-pin, bipin cap or bipin socket) is a type of lamp fitting. They are included in the IEC standard “IEC 60061 Lamp caps and holders together with gauges for the control of interchangeability and safety”. They are used on many small incandescent light bulbs (especially halogen lamps), and for starters on some types of fluorescent lights.

The bi-pin base was invented by Reginald Fessenden for the 1893 World’s Fair in Chicago.

  • G4 (smallest light bulb base)
  • GU4
  • GZ4
  • G5.3
  • G5
  • GU5.3
  • GX5.3
  • GY5.3
  • G6.35
  • GY6.35
  • GZ6.35
  • GY8
  • G8.5
  • G9
  • G9 Pin Blade
  • G9.5
  • GX9.5
  • GZ9.5
  • G12


Bayonet bases are a type of electrical connector that uses a bayonet-style locking mechanism to secure the connection. They are commonly used in various applications, such as lighting, electrical power distribution, and telecommunications.

The bayonet base consists of a base and a mating pin, which are designed to fit together in a specific way. The base has two or more protruding tabs that fit into corresponding slots in the pin, and the pin is then rotated to lock the connection in place. This design allows the connection to be made and broken quickly and easily, while also providing a secure connection.

There are several different types of bayonet bases:

  • Miniature Bayonet
  • Double Contact Bayonet (BA15d)
  • Single Contact Bayonet (BA15s)
  • D. C. Bayonet, Medium (BY22D)
  • Index Double Contact Bayonet

Each type has a specific number of pins and is designed to fit a particular type of lamp or device. For example, a bi-pin base is commonly used for halogen and LED lamps, while a double contact base is used for fluorescent lamps.


Wedge bases are a type of electrical connector that are commonly used in automotive lighting and other applications. They consist of a base and a mating pin, which are designed to fit together in a specific way. The base has a wedge-shaped groove that fits over a corresponding protrusion on the pin, and the pin is then pushed into the base to lock the connection in place.

Wedge bases are designed to be easy to use and provide a secure connection. They can be found in a variety of sizes and configurations, depending on the specific application.

  • Wedge
  • Wedge D.F.
  • Wedge S.F.
  • RX7s
  • Slide
  • Wedge Subminiature

For example, a T10 wedge base is commonly used in automotive taillights, while a T15 wedge base is used in some turn signal and brake light applications.

In addition to automotive lighting, wedge bases are also used in other applications such as outdoor lighting, commercial lighting, and theatrical lighting. They are a popular choice because of their simplicity, reliability, and versatility.


  • 2G7
  • 2GX-7
  • 2G11
  • G23
  • G24d-2
  • G24d-3
  • G24q-1
  • G24q-2
  • G24q-3
  • GX23
  • GX23-2
  • GX24q-1
  • GX24q-2
  • GX24q-3
  • GX24q-4
  • GX24q-5


  • Prefocus
  • Right Angle Prefocus
  • Axial Prefocus
  • Straight
  • Midget Flanged Single Contact
  • Miniature Flanged Single Contact
  • Miniature Single Contact
  • P15S
  • P28S
  • P32d2
  • P43T
  • P43T-38
  • PK22s
  • PX26d


  • G53
  • Festoon
  • 2 Contact Lugs
  • 3 Contact Lugs
  • Screw Terminal (Multi-Purpose)
  • Mogul End Prong (GX16d)
  • Metal Clip
  • Rigid Loop
  • Side Prong
  • Lead wires
  • S14s


  • PK30d


  • GU6.5

How do I know what base my light bulb is?

The light bulb base is usually indicated by a series of numbers that look like E-26, B22d or E-27.

These markings are found on the outside of light bulbs and indicate what type they are.

For example, this marking indicates an Edison screw light bulb: E-26. This marking means that Edison screw base is used. The number 26 indicates the diameter of the light bulb base in millimeters – so you can see this would fit into a standard E26 socket.

Which light bulb base types are interchangeable?

The screw-in bases, Edison E26 and the larger-sized E27 are usually interchangeable. The medium-sized B22d is also often used with a bayonet fitting.

The smaller S14s size may not be compatible with all fixtures because of its small diameter which can prevent it from being screwed into the light fixture socket.

E11 and E12 are not interchangeable because of their different size.

Does the Bulbs Base affect voltage and performance?

The light bulb base affects the voltage and wattage that can be used on a light fixture.

In North America, there are three different types of electrical outlets: A-type, B-Type, and C-Type sockets. The type of socket dictates how much power you can safely put through it without overloading or shorting out your home’s wiring.

You can put more power through a B-type socket than an A-type one, and you can put even more power through a C-Type socket than a B type – but for this reason, it is recommended to not use light bulbs with higher wattage in your home’s fixtures unless they are specifically designed for that voltage or higher.

You should always check your light fixture’s label to see what type of voltage and wattage it will support.

When you buy light bulbs for a home with C-type outlets, make sure they can handle the higher operating voltages – otherwise, you risk having them blow out or catch fire if used in such fixtures.

If these safety risks don’t concern you, then by all means go for it – but be sure to read the fine print on the light bulb packaging.

Which is the largest light bulb base?

The biggest light bulb base is one with a diameter between 40mm and 50 mm (E39 or E40).

Which is the smallest light bulb base?

The smallest light bulb base is G4 with a diameter of 4 mm.

Final Verdict on Light Bulb Base Sizes

In this article, we tried to discuss different light bulb base types. There are many different types of light bulb bases, each of which is designed to fit a specific type of socket or fixture. It is important to choose the right type of light bulb base for your specific application, as using the wrong base can cause problems such as flickering or improper electrical connections.

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