Owen Bradley There is a particular kind of frustration reserved for standing on a stepladder with a new bulb that will not go into the socket. It looked identical on the shelf, the brightness was right, the colour was right, and the threaded end is simply the wrong size. Base codes are the part of bulb shopping nobody explains, yet they are the one specification that makes a purchase either work or become a return trip. The codes themselves are logical once decoded, and there are only about a dozen you will ever meet in a house. This light bulb base sizes guide covers E26, E12, GU10, bi-pin types and the rest, which fixtures each belongs to, and how to identify what you already have before you buy in 2026.
How Base Codes Are Constructed
Almost every base code follows the same pattern: a letter or letters identifying the connection style, followed by a number giving a key dimension in millimetres. Once you know that, the codes stop looking arbitrary.
E stands for Edison, meaning a screw thread, and the number is the thread diameter in millimetres. So E26 is a screw base 26 millimetres across, and E12 is a much smaller 12-millimetre thread. GU and G denote pin bases, where the number is the spacing between the pin centres in millimetres. GU10 has pins 10 millimetres apart, G9 has 9 millimetres. B indicates a bayonet base, which pushes in and twists rather than screwing. Suffix letters refine the type: a U in GU means the pins are twist-and-lock, while d means a double contact.
Regional Differences Matter
Screw sizes vary by region, and the difference is small enough to be dangerous. North America standardises on E26 for general bulbs at 120 volts, while much of Europe uses E27 at 230 volts. The one-millimetre gap means an E27 bulb will often thread partially into an E26 socket and vice versa, but the fit is poor and the voltage mismatch is a genuine hazard. Buy for your region and treat imported fittings with caution.
Screw Bases: The Everyday Family
Screw bases cover the overwhelming majority of household sockets. Four sizes account for nearly everything you will encounter.
- E26 (medium or standard): the default North American household base. Table lamps, ceiling fixtures, floor lamps, most recessed cans, and outdoor porch fittings all use it. If a bulb is described without a base specification, assume E26.
- E12 (candelabra): the small thread found in chandeliers, wall sconces, decorative fixtures, some ceiling fans, and night lights. Roughly half the diameter of E26 and unmistakable side by side.
- E17 (intermediate): the awkward middle size, sitting between candelabra and standard. It appears in appliance lights, some ceiling fan light kits, display cabinets, and older decorative fixtures. Stocked less widely, so buy a spare when you find them.
- E39 (mogul): the oversized thread used for high-output lighting in warehouses, streetlights, and some large yard fixtures. Rare in homes outside a workshop or barn.
Candelabra bases cause the most confusion because so many decorative fixtures use them and the bulbs come in a wide variety of shapes. If you are replacing chandelier or sconce lamps, our comparison of the best LED candelabra bulbs covers the flame, torpedo, and globe shapes that share the E12 base, while the guide to the best chandelier bulbs addresses dimming behaviour and filament styling in exposed fittings.
Pin Bases: Spotlights and Compact Fixtures
Pin bases push straight in rather than screwing, and they dominate directional and low-profile lighting. The two numbers that matter are pin spacing and whether the pins lock.
- GU10: two thick pins 10 millimetres apart with widened tips. You push in and twist a quarter turn to lock. Standard for line-voltage track lighting and many recessed spotlights, requiring no transformer.
- GU5.3 and MR16: two thin straight pins 5.3 millimetres apart that push in without twisting. Usually low voltage, so the fixture includes a transformer. Common in accent lighting, display cases, and older downlights.
- G9: two looped wire pins 9 millimetres apart, used in compact decorative fixtures, some chandeliers, and small wall lights. Line voltage, no transformer needed.
- G4: two very fine pins 4 millimetres apart, low voltage, found in under-cabinet puck lights, caravan fittings, and small display lamps.
- GX53: a flat twist-lock base for shallow surface-mounted ceiling fittings and cabinet lighting where depth is limited.
- G13: the double-ended pin base of standard T8 fluorescent tubes and their LED replacements, with a pin at each end of the tube.
GU10 Versus GU5.3: The Classic Mix-Up
These two look similar at a glance and are constantly confused. GU10 pins are thick with bulbous ends and require a twist to seat. GU5.3 pins are thin, straight, and simply push in. Crucially, GU10 runs at mains voltage while GU5.3 typically runs at 12 volts through a transformer, so they are not interchangeable even with an adapter. Check whether your fixture has a transformer before ordering replacements.
Bayonet and Specialty Bases
Bayonet bases use two small side lugs that slot into a socket and lock with a push and twist. B22d is the common household size in the United Kingdom, Australia, and several other markets, while BA15s and BA15d appear in automotive, marine, and appliance lighting. The advantage is vibration resistance, which is why they persist in vehicles and in ceiling fixtures that shake.
Specialty bases fill niche roles. Wedge bases have no metal shell at all, with bare wire contacts pressed into a slot, and are typical in landscape lighting and some appliances. Double-ended R7s bases hold a slim linear tube between spring-loaded contacts, used in floodlights and torchiere floor lamps. Screw-terminal and hardwired modules are increasingly common in integrated LED fixtures, which brings its own consideration.
Integrated LEDs Have No Base at All
A growing share of fixtures ship with LEDs soldered directly to a board, with no replaceable bulb. There is no base code because there is no socket. These fixtures are slimmer and often more efficient, but when the LED fails you replace the entire fitting rather than a bulb. Worth knowing before you buy, particularly for recessed downlights where replacement means opening the ceiling.
How to Identify the Base You Have
The most reliable method is also the simplest: take the old bulb to the store, or photograph it clearly alongside a ruler. Failing that, work through these steps.
- Cut the power first. Switch off at the wall, and at the breaker for hardwired fixtures, before touching any socket.
- Check the bulb itself. Most bulbs have the base code printed on the metal shell or the glass near the base.
- Look at the connection style. Threaded means an E type, pins mean a G or GU type, side lugs mean a bayonet.
- Measure the diameter or spacing. For screw bases, measure the widest point of the thread in millimetres. For pin bases, measure centre to centre between the pins.
- Compare against a known reference. A standard E26 base is roughly the diameter of a coin around one inch across; E12 is visibly about half that.
- Check the fixture label. Many fittings state the accepted base type and maximum wattage on a sticker inside the housing.
Base Choice Interacts With Other Specifications
The base does not exist in isolation. A given base restricts the physical envelope of the bulb, which in turn limits brightness. E12 candelabra bulbs rarely exceed about 600 lumens because there is simply nowhere to put a larger heat sink. If a chandelier needs to light a dining room, you achieve it through several modest bulbs rather than one bright one.
Base type also affects dimming and heat. Compact bases such as G9 and E12 run hotter relative to their size, and enclosed decorative fixtures compound this, which shortens LED life. Check that any bulb going into a sealed fitting is explicitly rated for enclosed use. For recessed cans, where heat and beam control both matter, our guide to the best LED can light bulbs covers the base and trim combinations that fit common housings.
Adapters: When They Help and When They Do Not
Simple mechanical adapters exist for many combinations, most commonly E12 to E26 and E26 to GU10. They can be a legitimate fix when a fixture you love takes an uncommon base, or when you want to use a standard bulb in a candelabra socket temporarily.
The cautions are real, though. An adapter adds length, so the bulb may no longer sit inside a shade or below a trim. It adds a second electrical connection point that can loosen or overheat. It cannot bridge a voltage difference, so no adapter makes a low-voltage GU5.3 fitting accept a mains GU10 bulb safely. And it may exceed the fixture’s rated wattage if the larger bulb draws more power. Use adapters as a considered solution, not a habit.
Common Mistakes When Buying by Base
The biggest is assuming that if the base fits, the bulb fits. A standard A-shape bulb may thread perfectly into a small enclosed sconce and then refuse to let the glass shade close. Always check overall length and maximum diameter alongside the base code.
Another is buying a whole box of one base for a house with several. It is entirely normal to have E26 in the ceilings, E12 in the dining chandelier, GU10 in the kitchen spots, and G9 in a bathroom fitting. Inventory the house before bulk-buying. A third is ignoring voltage on pin bases, which produces a bulb that fits mechanically and fails immediately. A fourth is overlooking whether the fixture is dimmable and whether the bulb in that base size offers a dimmable version at all. For a broad overview across base types and applications, our roundup of the best bulbs is organised by fixture rather than by specification alone.
Frequently Asked Questions
What is the most common light bulb base?
E26, the medium screw base, in North America. It covers most lamps, ceiling fixtures, and recessed cans. In regions using 230-volt supply the equivalent everyday base is E27, which is one millimetre wider.
Can I use an E27 bulb in an E26 socket?
Mechanically it will often thread in, but it is not recommended. The fit is imperfect and, more importantly, the bulb is usually designed for a different supply voltage. Buy bulbs sold for your local electrical standard.
What base do ceiling fans use?
It varies more than any other fixture. Many use E12 candelabra, some use E26, older kits often use E17 intermediate, and a growing number have integrated LED panels with no bulb at all. Always check the specific model before ordering.
Are GU10 and MR16 interchangeable?
No. GU10 has thick twist-lock pins and runs on mains voltage, while MR16 with a GU5.3 base has thin push-in pins and usually runs at 12 volts through a transformer. Fitting the wrong one either will not seat or will fail immediately.
How do I know if my fixture takes a smaller bulb?
Look at the socket opening. A candelabra socket is roughly half the diameter of a standard one and obviously smaller by eye. Most fixtures also carry a label stating the accepted base and maximum wattage inside the housing or on the mounting plate.
Final Thoughts
Base codes look like technical clutter until you realise they are simply a letter for the connection style and a number for a measurement in millimetres. E26 for standard sockets, E12 for chandeliers and sconces, GU10 for mains spotlights, GU5.3 for low-voltage ones, G9 for compact decorative fittings, and a handful of specialty types for everything else covers virtually every home. Identify what you have before shopping, note the length and diameter limits of the fixture as well as the base, watch voltage carefully on pin types, and keep a small labelled stock of each base your house actually uses. Do that once and replacing a bulb in 2026 becomes a two-minute job rather than a second trip to the store.
