What Are The Different Light Bulb Bases? | Quick Clear Guide

Shopping for bulbs gets messy fast. Packages shout wattage, lumens, and shape codes, yet the part that decides fit is the base. Pick the wrong base and the lamp stays dark. This guide maps the common light bulb bases, what the letters mean, and where each one shows up. If you’re moving to LEDs, matching the base lets you upgrade without swapping fixtures. For a plain-English primer on efficiency and lifespan, see the Energy Saver page on LED lighting.

Different light bulb base types explained

Base codes look cryptic until you learn the pattern. The first letter signals the connection style: “E” for Edison screw, “B” for bayonet, “G” for bi-pin, “GU” for twist-lock bi-pin, and special codes such as “R7s” for double-ended linear lamps. The number that follows usually describes a size in millimeters: thread diameter for screw caps, pin spacing for many G bases. With that, you can read a box in seconds and know if a lamp fits your socket.

Quick chart: the bases you’ll meet most

How to read base codes without guesswork

Screw bases (E series)

Edison screw caps are everywhere. The letter “E” plus a number tells you the thread diameter. E26 is the North American workhorse for 120-volt fixtures; E27 fills the same role in 230-volt regions. The fit is close. In many cases an E27 bulb threads into an E26 socket, and the reverse sometimes works, though pitch and heat ratings differ by standard. For small fixtures you’ll meet E12 (candelabra) and E17 (intermediate). Large industrial fittings use E39 or E40, often labeled mogul sizes. A quick rule: the bigger the number after E, the wider the screw shell.

If you want the background and exact sizes, the article on Edison screw lists the common families and naming, including regional notes.

Bayonet bases (B series)

Bayonet caps lock with a push-and-twist move. The code starts with “B” then the diameter. The classic household size is B22d. You’ll see it in the UK, India, and several other regions. Pins on the base slide into keyways in the socket, then a short turn holds the lamp. This design resists vibration and keeps orientation fixed, which suits ceiling fittings that get bumped or rotated during cleaning.

Bi-pin and twist-lock pins (G and GU series)

Bi-pin caps use two pins set a specific distance apart. The “G” plus a number marks that pin spacing in millimeters. G13 is the two-pin end on T8 and T12 fluorescent tubes. G5 fits slimmer T5 tubes. Small capsule lamps use G4, G8, or G9. Reflector spots come in two families: GU5.3 (often called MR16) for low-voltage transformers, and GU10 for direct-mains fixtures. The “U” indicates a twist-lock feature that helps the lamp latch into the holder so it won’t work loose.

For pin spacing tables and the long list of variants, the bi-pin lamp base page is a solid reference. When replacing a halogen spot with an LED, match GU5.3 vs GU10 first, then match voltage.

Dedicated energy-efficient caps (GU24 and plug-ins)

Some fixtures use caps that nudge you toward efficient lamps. GU24 is a twist-lock two-pin base used with CFLs and many LEDs. It looks like a round puck with two posts that twist into place. Housing projects and code-driven jobs adopted it to prevent screw-in incandescents. You may also find plug-in CFL bases like G24q or 2G11 in commercial downlights, where the ballast lives in the fixture and the lamp clicks into keyed slots.

Picking an LED that matches your socket

Once you know the base, choosing the right LED gets simple. Start with the same base code as the bulb you’re replacing, then check three things. First, voltage: GU5.3 spots usually run on 12V, while GU10 runs on mains. Second, dimming: only use lamps labeled as dimmable with your existing dimmer type. Third, heat: enclosed fixtures and tiny shades need lamps rated for that environment. LEDs sip power and last a long time when heat is managed and the driver matches the circuit.

Taking the guesswork out of bulb bases at home

Here’s a simple routine that works in a kitchen drawer, garage, or studio.

Step 1: remove one lamp safely

Switch power off. Let the lamp cool. For screw caps, turn counterclockwise. For bayonet, push in slightly, then twist. For GU10, insert fingertips into the front grooves, turn a quarter-turn, and pull down. For tube lamps, rotate the tube a quarter-turn until the pins line up, then slide it out.

Step 2: read the tiny print

Most bulbs list the base on the neck or shell. You’ll see codes like E26, B22d, GU10, G13, or GU24. If the code is missing, measure. Thread diameter or pin spacing usually gives it away even when labels have faded.

Step 3: match shape, then lumen output

The base is only the start. Pick the same shape (A19, BR30, PAR38, T8, etc.), then choose brightness in lumens. For a table lamp that once used a 60W incandescent, an LED near 800 lumens hits a similar light level. If you prefer a softer look in shades, pick a frosted globe or a filament-style LED with a warm color temperature.

Close variations that cause mix-ups

Some bases look nearly the same yet don’t swap cleanly. These are the trouble spots people run into during a home upgrade.

E26 vs E27

They’re close in size and often thread together. The standards differ on lamp test voltage and temperature. If your fixture or lamp lists only one size, stick with that. If a lamp runs hot in a tight shade, pick the exact match rather than relying on “almost fits.”

E12 vs E11

Both are small screw caps. E12 is common in North America for chandeliers; E11 shows up on some miniature halogen lamps. Threads and electrical ratings differ, so they don’t swap even if the shells look similar at a glance.

GU10 vs GU5.3

GU10 is mains-voltage twist-lock. GU5.3 is a low-voltage two-pin. The pins aren’t the same, and the power gear behind the ceiling is different. Match what you have. Swapping between them needs a new socket and, for GU5.3, a transformer that suits the LED driver.

G9 vs E12

G9 is a looped-pin capsule that presses into spring contacts. E12 is a screw shell. They serve similar small fixtures yet connect in very different ways. If you’re changing the look of a chandelier, confirm the sockets before you order new decorative bulbs.

Pin and specialty base cheat sheet

Understanding the various light bulb base codes

Regional notes

Household sockets aren’t identical worldwide. North America leans on E26 and E12. Much of Europe uses E27 and E14. Many homes in the UK and India still use B22d bayonet fittings. Cars use bayonet or wedge styles rather than screw caps. Commercial ceilings mix G13, G5, GU10, and GU5.3 depending on age and design.

When adapters are a bad idea

Thread or pin adapters look handy online. They can add height, trap heat, and break wattage or certification limits inside a fixture. Dimmers and sensors also misbehave when the base or voltage changes. A safer plan is to buy the correct base, or swap the holder if you’re rewiring a fixture and need a different family of lamp.

Notes on linear double-ended lamps

Security floods and wall washers often use a thin tube lamp on an R7s base. LED versions exist, and they run cooler than the classic halogen types. Check length before ordering: many fixtures take 118 mm lamps, others take 78 mm. These lamps mount between spring contacts, so the base name describes a family rather than a screw or pin size, and the holder grabs the ends of the tube.

Mini guide to specialty and less common bases

Miniature Edison (E10)

E10 shows up in flashlights and models. It’s a tiny screw shell used on low-voltage circuits. Not the same as E12 candelabra, so don’t force a fit.

Small bayonet (BA15d and B15d)

These are two-contact bayonet styles used on signal lamps and some appliances. They’re common on vehicles and marine gear, where a positive lock matters when things shake.

Prefocus and stage caps

Theatre and projectors use specialty caps that lock focus and heat position. Codes include P28s, G9.5, and G38, each tied to a specific lamp family. If you’re replacing stage lamps with LED modules, follow the fixture maker’s parts list rather than mixing families by eye.

2-pin and 4-pin plug-in CFLs

Ballasted ceiling cans and wall sconces from past decades often take plug-in CFL lamps. The base keys the wattage and tube count, so match the letters exactly when converting to LED retrofit kits. Many retrofit kits keep the look of the old trim while giving you a standard screw-in or dedicated connector inside.

Quick recap: pick the base, then everything else

You don’t need a chart every time. Grab the old lamp, find the base code, and match it on the new box. If the print is gone, measure thread diameter or pin spacing. After that, match shape and lumens, then color temperature. For a long-term upgrade, pick quality LEDs with the same base; they save power and hold brightness while keeping your fixtures in service. For deeper background on screw sizes and pin spacing, the entries on Edison screw and the bi-pin base explain the naming you see on boxes and spec sheets.