Light bulb base codes can look like a secret language, especially when you are standing in a store aisle holding a burned-out bulb and staring at labels like E26, GU10, or B22. Those short combinations are not random. They tell you exactly how a bulb connects to a fixture, and matching them correctly is the difference between a quick replacement and a frustrating return. Once you understand the logic behind light bulb base codes, choosing the right bulb becomes much faster and far less intimidating.
This guide breaks down the letters, numbers, and regional differences that define common bulb bases. You will learn how to identify Edison screw bases, bayonet caps, pin bases, and twist-lock styles, plus how to avoid dangerous mismatches. Whether you are upgrading to LED or simply replacing a bulb in a chandelier, recessed light, or desk lamp, the base code is the first detail to check.
The Anatomy of Light Bulb Base Codes
A light bulb base code is a compact identifier made from letters and numbers. The letters usually describe the connection family, such as screw, bayonet, or pin. The numbers typically indicate a critical measurement in millimeters, most often the diameter of a screw base or the spacing between pins. This system helps manufacturers, electricians, and shoppers communicate fit without needing to see the bulb in person.
It is important to separate the base code from the bulb shape code. A label like A19 E26 tells you two different things: A19 describes the bulb’s shape and approximate diameter, while E26 describes the base. Similarly, MR16 GU10 means an MR16 reflector shape with a GU10 twist-lock pin base. Mixing up shape and base is one of the most common reasons people buy the wrong replacement.
Letter Prefixes: Screw, Bayonet, Pin, and Twist-Lock
The letter prefix is your first clue. E stands for Edison screw, the familiar threaded base found on most household bulbs. B indicates a bayonet base, which pushes in and locks with a twist. G generally indicates a pin-based connection, while GU often appears on pin bases with a twist-lock or recessed pin design. Other letters can indicate specialty configurations, but these four families cover the vast majority of residential lighting.
For example, E26 is a medium Edison screw base used in North America, while B22 is a large bayonet base common in the United Kingdom and parts of Asia. G4 and G9 are small pin bases used in under-cabinet and decorative fixtures, and GU10 is a twist-lock pin base often found in track and recessed lighting. Recognizing the prefix immediately narrows down what will physically fit.
Numbers and Pin Spacing: What Measurements Mean
For Edison screw bases, the number after E is the base diameter in millimeters. E12 is about 12 mm across, E14 is 14 mm, E17 is 17 mm, E26 is 26 mm, and E27 is 27 mm. E26 and E27 are close enough that they can sometimes fit the same socket, but they are designed for different regional voltages. A bulb that screws in may still be electrically wrong if the voltage does not match.
For pin bases, the number usually refers to the distance between pin centers. G4 has 4 mm pin spacing, G9 has 9 mm spacing, GU10 has 10 mm spacing, and G13 has 13 mm spacing for many linear fluorescent tubes. Bayonet numbers generally refer to the base diameter, so B15 is a small bayonet and B22 is a large bayonet. Additional letters can indicate pin length, contact type, or locking style, so the full code matters.
Common Light Bulb Base Codes and Their Applications
Most homes rely on a surprisingly small set of base codes. In North America, E26 is the standard medium screw base for table lamps, ceiling fixtures, and floor lamps. E12 is the smaller candelabra base used in chandeliers and night lights. GU10 and GU24 appear in track, recessed, and code-compliant fixtures, while G4 and G9 show up in cabinets, pendants, and decorative sconces.
In Europe, Asia, and other regions, E27 and B22 are far more common, with E14 serving as the small screw alternative to E12. GU10 is also widespread for mains-voltage spotlights. Knowing where a fixture was manufactured or where it will be used can help you predict which base code you are likely to encounter, but the only reliable method is to check the existing bulb or the fixture label.
Edison Screw Bases: E12, E14, E17, E26, E27, E39, E40
Edison screw bases are named for their threaded design, and their codes are straightforward. E12 is candelabra, often used in chandeliers and decorative lamps. E14 is the European small screw, similar in purpose but slightly wider. E17 is an intermediate base that falls between candelabra and medium. E26 is the North American medium base, and E27 is the European medium base. E39 and E40 are mogul bases for high-output fixtures like streetlights and high bays.
When replacing an Edison screw bulb, check both the base and the electrical rating. An E12 bulb will not fit an E26 socket without an adapter, and an E26 bulb will not fit an E12 socket. E26 and E27 may physically interchange, but a 120-volt bulb should not be used in a 230-volt socket, and vice versa. Wattage, heat tolerance, and dimmer compatibility also matter, especially in enclosed fixtures.
Bayonet and Pin Bases: B15, B22, G4, G9, GU10, GU24
Bayonet bases use a push-and-twist motion instead of threads. B22 is the large bayonet base common in the UK, Australia, India, and other regions. B15 is the smaller bayonet version, often used in appliances and smaller fixtures. Pin bases use two or more pins to make contact. G4 is a tiny 4 mm pin base for under-cabinet lights, G9 is a 9 mm looped-pin base for decorative bulbs, and GU10 is a 10 mm twist-lock base for spotlights and track heads.
GU24 is another twist-lock pin base, but it is wider and often required by energy codes in North America for certain fixtures. Although G9 and GU10 both have two pins, they are not interchangeable: G9 pins are typically looped and 9 mm apart, while GU10 pins are cylindrical, 10 mm apart, and lock into place. Always match pin spacing, pin shape, and locking mechanism exactly.
How to Match a Light Bulb Base Code to Your Fixture
Start by turning off power to the fixture and letting the old bulb cool. Remove it carefully, then inspect the base and the socket. Look for a label on the fixture or a stamp on the old bulb. If the label is faded, measure the threaded diameter or the distance between pins with a ruler or calipers. Compare your measurement to common base codes to narrow down the possibilities.
Physical fit is only part of the equation. You also need to match voltage, wattage, bulb shape, and size. A bulb that fits the socket but exceeds the fixture’s wattage rating can overheat and create a fire hazard. LED bulbs run cooler than incandescent, but they still need to be compatible with dimmers, electronic transformers, and enclosed fixtures. When in doubt, choose a bulb with the same base code and equal or lower wattage than the original.
Step-by-Step Identification Tips
For screw bases, measure across the threaded section in millimeters. A 26 mm measurement points to E26, 27 mm to E27, 12 mm to E12, and 14 mm to E14. For bayonet bases, measure the diameter of the metal cap where it meets the glass or housing. For pin bases, measure the distance between the centers of the pins, then note whether the pins are round, flat, or looped. Also check whether the base locks with a quarter turn.
If you are still unsure, take a photo of the old base next to a ruler and bring it to the store. Write down any text printed on the bulb, including voltage and wattage. When ordering online, search by the exact base code rather than a generic description like “small screw” or “two-pin.” This avoids confusion between similar codes such as E12 and E14 or G9 and GU10.
Adapters, Converters, and Safety Considerations
Adapters can convert one base type to another, such as E12 to E26 or B22 to E26. They are useful in a pinch, but they add length and can affect fit, heat dissipation, and mechanical stability. Never use an adapter to change voltage. A 120-volt bulb in a 230-volt socket will fail immediately, while a 230-volt bulb in a 120-volt socket will produce weak, dim light and may not work reliably.
Respect the fixture’s wattage and temperature limits. Enclosed fixtures trap heat, so choose LED bulbs rated for enclosed use. Dimmers require dimmable bulbs, and some older dimmers are not compatible with modern LED drivers. If you are unsure about wiring, voltage, or a specialty base, consult a qualified electrician. Safety should always come before convenience when dealing with electrical fixtures.
With a little practice, light bulb base codes become a quick reference rather than a puzzle. Keep a photo of your most-used bases on your phone, note the voltage and wattage on the fixture, and you can replace or upgrade bulbs with confidence. The next time you need a bulb, you will know exactly what to look for, and you can skip the frustration of a base that almost fits.