Light Bulb Base Sizes

The Codes, Decoded

E stands for Edison, meaning a screw base. G means a pin base, where the number is the distance between the pins rather than a diameter. GU is a pin base that twists and locks. B is a bayonet, standard in the UK and much of the Commonwealth but rare in North America.

E26 and E27. Close Enough to Cause Trouble

One millimetre apart, and they will physically thread into each other. An E27 European bulb will screw into a North American E26 socket and generally works.

The problem is not the thread, it is the voltage. European bulbs are built for 220–240 V. Put one in a 120 V socket and it will run dim and disappointing. Bring a 120 V bulb home and put it in a 240 V European fitting and it will fail immediately, sometimes spectacularly.

For LED bulbs this matters less than it used to, because many are sold with universal 100–240 V drivers. Check the print on the base before assuming.

Why the Base Is Only Half the Question

A bulb that fits is not necessarily a bulb that belongs. Three other things have to match.

• Wattage limit. Every fixture has a maximum marked inside, and it is about heat, not electricity. With LEDs this rarely binds, a 100 W-equivalent LED draws maybe 15 W, but the limit still refers to actual watts, so an LED comfortably under it is fine.
• Enclosed-fixture rating. LEDs are electronics, and electronics dislike heat. A bulb in a sealed globe or a fully enclosed recessed can needs to be rated for enclosed fixtures, or its life will be a fraction of what the box claims.
• Dimmer compatibility. A non-dimmable LED on a dimmer will buzz, flicker or refuse to dim. Even dimmable LEDs can misbehave on older triac dimmers designed for incandescent loads, which frequently need replacing with an LED-rated dimmer.
• Bulb shape. The base can fit while the glass does not. A19 is the classic pear shape, A21 is larger, BR30 and PAR38 are reflector shapes for recessed cans, and a B10 candle shape is what most E12 chandeliers expect.

Adapters, and When Not to Use One

Screw adapters exist for most combinations, E12 to E26, E26 to GU10, and so on. They are inexpensive and fine for temporary or light-duty use.

The reasons to be cautious are physical rather than electrical. An adapter adds length, which can push a bulb past the edge of a shade or stop a fixture closing. It also adds a mechanical joint that can work loose in a fitting that gets vibration, such as a ceiling fan. And where the original base was small because the fixture is small, a larger bulb in an adapter often exceeds the heat the fitting was designed to dissipate.

Lumens, Not Watts, Now That Bulbs Are LED

Base size settles whether a bulb fits. Brightness is a separate question, and the habit of thinking in watts no longer works, because watts measure energy drawn rather than light produced. An LED making the same light as a 60 W incandescent draws around 9 W.

The number to read is lumens, printed on every bulb package in the Lighting Facts panel alongside the color temperature and the estimated life.

• Color temperature is measured in kelvin. 2700K is the warm yellow of an old incandescent, 3000K is a slightly cleaner warm white, 4000K reads neutral, and 5000K and above is a cool daylight tone that suits workshops and garages more than living rooms.
• Mixing color temperatures in one room is the most common lighting mistake. Bulbs from different batches or brands at the same nominal kelvin can still visibly differ, so buy a room in one go.
• Color rendering index (CRI) describes how accurately colors appear under the light. Above 90 is worth paying for anywhere you cook, dress or apply makeup.

Identifying a Base You Already Have

Three approaches, in order of reliability.

• Read the old bulb. The base code is usually printed on the metal or on the glass near it.
• Measure the base. Calipers across the widest part of the screw thread, in millimetres, gives you the number directly, 26 mm is E26, 12 mm is E12.
• Compare against a known bulb. A standard household bulb is E26. If yours is visibly about half that, it is E12. Noticeably between the two, E17.
• For pin bases, measure the center-to-center distance between the pins in millimetres. Two pins 10 mm apart with a twist action is GU10.

Common questions

What does E26 mean on a light bulb?

E means an Edison screw base and 26 is its diameter in millimetres. It is the standard household bulb base in North America.

Is E26 the same as E27?

Not quite. They are one millimetre apart and will thread into each other. The real difference is voltage: E27 bulbs are made for 220 to 240 volt European supplies, so they run dim on 120 volts, and a 120 volt bulb in a European fitting will fail immediately.

What is the difference between E12 and E26?

Size. E12 is a 12 mm candelabra base used in chandeliers, sconces and night lights. E26 is the 26 mm standard household base. They are not interchangeable without an adapter.

Can I use an adapter to fit a different bulb base?

Usually yes, and they are cheap. Watch for three things: the extra length can stop a shade or fixture closing, the joint can work loose in anything that vibrates, and a larger bulb in a small fitting can exceed the heat it was designed to handle.

How do I know what base my light bulb needs?

Read the code printed on the old bulb, or measure the screw thread across its widest point in millimetres, that number is the code. For pin bases, measure between the pin centers instead.