How to Identify a Light Bulb Base Size at Home

Standing in a shop aisle holding a dead bulb and squinting at a wall of packaging is a familiar frustration. Wattage and brightness get all the attention, but the number that decides whether a bulb physically fits your fixture is the base size, and it is printed in a code most people have never learned to read. The good news is that identifying a light bulb base takes a ruler and about thirty seconds once you know what to measure. This guide shows you how to read the letter-and-number codes, measure a base accurately at home, tell screw bases from pin and bayonet types, and avoid the adapters and mismatches that cause flicker and loose connections.

How Bulb Base Codes Actually Work

Nearly every base code follows the same pattern: a letter or letters identifying the base type, followed by a number giving a key dimension in millimetres. Once you internalise that, the codes stop looking cryptic.

• E stands for Edison screw. The number is the outside diameter of the screw thread in millimetres. E26 is a 26 mm screw base.
• G stands for a pin or bi-pin base. The number is the spacing between the pin centres in millimetres. GU10 has pins 10 mm apart.
• B or BA stands for bayonet, the push-and-twist base with side lugs. The number again gives the diameter.
• U after a letter, as in GU, means the pins twist and lock rather than simply pushing in.

That single rule handles the overwhelming majority of household bulbs. Where it gets confusing is regional naming, because the same physical base carries different common names in different markets.

The Base Sizes You Will Actually Meet at Home

• E26 (medium or standard) — 26 mm screw. The default for lamps, ceiling fixtures, and recessed cans across North America.
• E27 — 27 mm screw. The European standard. Physically close enough to E26 that bulbs often interchange, though the voltage rating will not.
• E12 (candelabra) — 12 mm screw. Chandeliers, sconces, decorative fixtures, night lights.
• E14 (small Edison screw) — 14 mm screw. The European candelabra equivalent, common in appliance and decorative fittings.
• E17 (intermediate) — 17 mm screw. Appliance bulbs, some ceiling fan lights, and older desk lamps.
• E39 / E40 (mogul) — large screw base for high-output commercial and yard lighting.
• GU10 — twist-lock pins 10 mm apart, with distinctive fat pin tips. Track and recessed spotlights.
• GU5.3 / MR16 — two straight thin pins 5.33 mm apart, usually low-voltage 12V.
• G9 — a looped wire base, common in small decorative and under-cabinet fittings.
• G4 — tiny 4 mm bi-pin used in low-voltage accent and cabinet lighting.
• GU24 — twist-lock 24 mm pins, used on efficiency-mandated fixtures.
• BA15d / B22 — bayonet bases found in the UK, Australia, and in some automotive and appliance uses.

Step by Step: Identifying Your Bulb Base at Home

1. Switch off the power and let the bulb cool. Turn off the wall switch, or better, the breaker, and unplug lamps. A halogen bulb stays hot enough to burn for several minutes after switch-off.
2. Remove the old bulb carefully. Grip the base rather than the glass and turn counter-clockwise for screw bases. For pin and bayonet bases, push in gently and twist, or pull straight out for straight-pin types.
3. Look for printing on the bulb itself. Most bulbs print the base code on the glass shoulder or on the metal or plastic base — often alongside wattage and voltage. If you can read E26, E12, or GU10 there, you are already done.
4. Check the fixture and the packaging. Many fixtures carry a label inside the canopy or near the socket listing the accepted base and maximum wattage. Old boxes and manuals also list it.
5. Measure a screw base across the threads. Place a ruler or calipers across the widest point of the metal thread, at its outside diameter, not inside the socket. Read the figure in millimetres. Roughly 26 mm means E26, 12 mm means E12, 17 mm means E17, and 14 mm means E14.
6. Measure a pin base between pin centres. For two-pin bulbs, measure from the centre of one pin to the centre of the other. About 10 mm with thick, blunt pin tips is GU10. Around 5.3 mm with thin straight pins is GU5.3 or MR16. About 4 mm is G4.
7. Look at the pin shape, not just the spacing. GU10 pins are notably fatter at the tip because they twist and lock into a slot. GU5.3 pins are uniformly thin and simply push in. Confusing these two is the single most common mistake, since the spacing figures alone do not tell them apart.
8. Identify bayonet bases by the side lugs. Two small metal nubs on opposite sides of a smooth cylindrical base means a bayonet fitting. Count the contacts on the bottom: one is single-contact, two is double-contact, and they are not interchangeable.
9. Photograph the base before you shop. A clear photo of the base and any printing on it is far more useful in a shop than a memory of “the medium-ish one”.
10. Cross-check the shape code too. Base size and bulb shape are separate. A code such as A19, B11, or PAR30 describes the envelope shape and diameter, and a fixture may accept your base but not your bulb’s physical size. Measure the available space inside the shade or can.

Measuring Without Calipers

You do not need precision tools. Lay the base against a millimetre ruler and read across the widest thread point. If the ruler is imperial, remember that E26 is very close to one inch and E12 is a little under half an inch, which is usually enough to tell them apart at a glance.

A useful trick when you have no ruler at all: an E26 base is roughly the diameter of a standard AA battery plus a little, while an E12 base is roughly the diameter of a pencil. If a bulb feels distinctly slender and comes from a chandelier or a wall sconce, it is almost certainly E12 or E14. If it is the fat base from a table lamp or a ceiling fixture, it is E26 or E27.

Matching the Base to Your Fixture Type

Fixture type is a strong predictor of base size, so it works as a sanity check. Chandeliers, candle sconces, and decorative fittings almost always take E12 or E14 candelabra bases; if you are replacing several at once, the best LED candelabra bulbs and the wider best chandelier bulbs lists show which shapes suit exposed-bulb fixtures where the glass is on display.

Table lamps, floor lamps, ceiling fixtures, and recessed cans are overwhelmingly E26 or E27. Decorative filament and vintage-look bulbs usually come in that same standard base, which makes them a straight swap into most existing fixtures; browsing the best Edison bulbs is a quick way to see the shape options available on a standard screw base. Track lighting and directional spots are the main GU10 territory, and under-cabinet or cabinet accent fittings are where G4 and G9 turn up. When you are stocking up across several fixture types at once, a general comparison of the best bulbs helps you standardise on as few base types as your house allows.

Adapters, Mismatches, and When Not to Force It

Screw-base adapters exist and can convert E12 to E26 or the reverse. They work, but they come with real caveats. Each adapter adds length, so the bulb may no longer fit inside a shade or a recessed can. They also add a mechanical joint that can loosen, and they let you exceed a fixture’s intended wattage by fitting a much larger bulb into a small socket.

Never force a bulb that does not thread in easily. Cross-threading a base damages both the bulb and the socket, and the resulting poor contact causes flicker, arcing, and heat. If a bulb spins loosely without making contact, the base is undersized for that socket and you have the wrong bulb. Similarly, a GU10 bulb pushed into a GU5.3 socket will not lock and may make intermittent contact.

Common Mistakes When Identifying Bases

• Measuring the socket instead of the bulb. Screw base codes refer to the bulb’s outside thread diameter.
• Confusing base code with shape code. A19 and E26 describe different things and appear together on the same bulb.
• Assuming wattage implies base size. A 60W-equivalent LED exists in E26, E12, and GU10 forms.
• Mixing up GU10 and GU5.3. Check pin thickness and whether the base twists to lock.
• Ignoring voltage. Many pin-base bulbs run on 12V through a transformer. Fitting a mains bulb there, or the reverse, will not work and may be dangerous.
• Overlooking single versus double contact on bayonet bases. They look identical from the side.

Frequently Asked Questions

Are E26 and E27 interchangeable?

Physically they usually thread together, since the difference is one millimetre. The important difference is electrical: E26 bulbs are built for 120V systems and E27 for 220 to 240V. Match the voltage of your region, not just the thread.

What is the most common household bulb base?

E26 in North America and E27 across most of Europe and much of Asia. Between them they cover the majority of lamps and ceiling fixtures. E12 candelabra is a distant second, concentrated in decorative fittings.

How do I identify a base if the printing has worn off?

Measure it. A ruler across the widest thread point gives the number, and the physical form — screw, pins, or bayonet lugs — gives the letter. Those two together are the full code.

Can I use an LED bulb in any base my old bulb fitted?

Generally yes, provided the base and voltage match and the fixture is suitable. Check for enclosed-fixture rating and dimmer compatibility, since those two limitations catch people out far more often than base size does.

Final Thoughts

Identifying a light bulb base size is a two-part question every time: what shape is the base, and how big is it in millimetres. Read the printing first, and if it has worn away, put a ruler across the threads or between the pin centres and match the number to the code. Take a photo of the base before you shop, note the voltage as well as the size, and check that the bulb’s overall shape will actually fit the fixture. Get those details right and the replacement goes in first try, every time.