Light Bulb Base Sizes: E12 vs E17 vs E26 and Every Edison Screw Fitting Explained

Quick answer: The number in an Edison screw base is its diameter in millimeters, measured across the threads. E12 (12mm, "candelabra") is the small base on chandelier and night-light bulbs. E17 (17mm, "intermediate") is common in appliance and some desk-lamp bulbs. E26 (26mm, "medium" or "standard") is the everyday US household base. They are not interchangeable.

E12 vs E17 vs E26 (and E14, E27): quick comparison

Base	Diameter	Common names	Typical uses	Region	Closest equivalent elsewhere
E12	12mm (about ½ inch)	Candelabra, CES	Chandeliers, candle bulbs, night lights, C7 holiday bulbs	North America	E14 in Europe (not interchangeable)
E14	14mm	Small Edison Screw, SES	Candle bulbs, small lamps, chandeliers	Europe, UK, Asia, Australia	E12 in North America (not interchangeable)
E17	17mm (about ⅔ inch)	Intermediate	Microwave and appliance bulbs, some desk lamps and ceiling fans, C9 holiday bulbs	North America	No common European equivalent
E26	26mm (about 1 inch)	Medium, standard	Most household lamps and ceiling fixtures	North America	E27 (physically fits, but check voltage)
E27	27mm	Edison Screw, ES	Most household lamps	Europe, Asia, Australia	E26 (physically fits, but check voltage)

How do I tell which base I have?

1. Check the old bulb. The base code (E12, E17, E26…) is often printed on the glass or the metal base, or on the packaging.
2. Measure across the threads. Use a ruler or calipers on the widest part of the bulb's threaded base and round to the nearest standard size: about 12mm means E12, about 17mm means E17, about 26mm means E26.
3. No ruler? Use a US coin as a rough check. An E12 base is clearly smaller than a dime. An E17 base is roughly dime-sized (a dime is 17.9mm). An E26 base is slightly wider than a quarter (a quarter is 24.3mm).
4. Only have the socket? Switch off the power, then measure the inside opening of the socket. It will be close to the base size, so round to the nearest standard.

Important: the base size tells you nothing about voltage. An E26 base can be a 120V bulb, a 230V bulb or a 12V bulb. A 12V bulb screwed into a 120V lamp will fail, and a 120V bulb on a 12V system won't light properly. Always match the voltage as well as the base. (More on that in Are There 12V Light Bulbs With a Standard E26 Base?)

What do the E-numbers mean?

Screw-in base names start with the letter E, for Edison Screw. The number after it is the diameter of the bulb's threaded base in millimeters, measured at the crest of the threads. So an E12 base is about 12mm across, an E17 base about 17mm, and so on. Bulbs and sockets with the same code are designed to fit together.

What is Edison Screw?

Edison Screw is the standard screw-in light bulb base used on most household bulbs worldwide. Screw-base lamps developed by Thomas Edison's company date back to around 1880 (Spark Museum). The bulb has a right-hand threaded metal base that screws into a matching threaded socket.

On mains-powered (AC) lamps, the threaded shell is connected to neutral and the small contact at the bottom tip to the live (hot) wire. Edison screw bases are standard across North America and continental Europe. In the UK, the B22 bayonet cap (a push-and-twist fitting, not a screw) is one of the most common household fittings, alongside E27 and E14.

5 Common Thread Size Groups

• Candelabra: E12 in North America (12mm diameter)
• Candelabra: E14 in Europe (14mm diameter)
• Intermediate: E17 in North America (17mm diameter)
• Medium or standard: E26 in North America and E27 in Europe (26–27mm diameter)
• Mogul: E39 in North America and E40 in Europe (39–40mm diameter)

E12

E12 is the small candelabra base, about 12mm across. It's used for chandelier and candle bulbs, night lights, bathroom vanity and decorative bulbs, and C7 holiday bulbs. It's less common in everyday stores, but specialist suppliers carry a wide range, including low-voltage and decorative E12 bulbs.

E17

E17 is the intermediate base, between E12 and E26 in size. In North America it's commonly found in microwave, range hood and other appliance bulbs, as well as some desk lamps, ceiling fans and C9 holiday bulbs. If your microwave bulb has a screw base, there's a good chance it's E17, but check the old bulb, because appliances vary.

Other Edison screw sizes you may come across

E10

E10 is the Miniature Edison Screw (MES). It's used in flashlights, bicycle lamps, indicator lights, model and toy lighting and some holiday decorations. We carry E10 miniature screw bulbs for many of these uses.

E11

E11 is the mini candelabra base, about 11mm across, so slightly smaller than E12. It's mostly used for small 120V halogen bulbs in track lights, pendants and some appliances. E11 and E12 are not interchangeable, although adapters exist.

E14

E14 is the Small Edison Screw (SES), about 14mm across. It's very common across Europe, the UK, Asia and Australia for smaller household bulbs: bedside lamps, wall lights and chandeliers. E14 bulbs come in candle, golf-ball, spotlight and filament shapes. E14 is close in size to the North American E12, but the two are not interchangeable.

E26 and E27

E26 is the medium or standard base used in most household lamps and ceiling fixtures in the United States and Canada. A huge range of bulb shapes use it, from A19 household bulbs to reflector floods such as a PAR20 halogen replacement bulb.

E27 is the European, Asian and Australian equivalent. E26 is 26mm and E27 is 27mm, so the bulbs usually fit each other's sockets physically. But fitting is not the same as working. E26 bulbs are typically made for 120V and E27 bulbs for 220–240V. A 120V bulb in a 230V socket will burn out quickly or fail, and a 230V bulb in a 120V socket may run dim or not light at all. Always match the bulb's voltage to your supply.

E39 and EX39

E39 is the mogul base, about 39mm across, used for high-wattage lamps such as streetlights, warehouse and high-bay lighting, and metal halide and other high-intensity discharge lamps. E40, its 40mm European equivalent, is generally interchangeable with it.

EX39 is a variant with a longer center tip. EX39 bulbs have a protective shield and can be used in either E39 or EX39 sockets, in open or enclosed fixtures. Standard E39 bulbs fit E39 sockets only, not EX39 sockets, and unshielded E39 metal halide bulbs are meant for enclosed fixtures only.

Base size also limits wattage. Under the US National Electrical Code (NEC Section 410.103), general-use incandescent lamps may not have a medium (E26) base if rated over 300 watts, or a mogul base if rated over 1,500 watts. Local code adoption varies, so check with an electrician for fixed installations.

Rare sizes: E5 and E29

E5 (Lilliput Edison Screw) is a tiny base still used in some indicator lights and small decorative lamps. E29 ("admedium") is an older North American size that's now uncommon. If you think you have one of these, measure carefully before ordering.

Frequently asked questions

What is the difference between E12 and E26?

E12 is the small 12mm candelabra base used in chandeliers, candle bulbs and night lights. E26 is the 26mm standard base used in most US household lamps. They are not interchangeable, but E12-to-E26 adapters exist.

Is E12 the same as E14?

No. E12 (12mm) is the North American candelabra base and E14 (14mm) is the European small Edison screw. They look similar, but an E14 bulb will not screw properly into an E12 socket.

What is an E17 bulb used for?

E17, the intermediate base, is commonly used in microwave and appliance bulbs, some desk lamps and ceiling fans, and C9 holiday bulbs.

Can I use an E27 bulb in an E26 socket?

Usually yes, physically, but only if the voltage matches your supply. In North America, use a bulb rated for 120V. In Europe, the UK and Australia, use a bulb rated for 220–240V.

What is the standard light bulb base size in the US?

E26, often called the medium or standard base.

How do I measure a light bulb base?

Measure across the widest part of the threads in millimeters and round to the nearest standard size: about 12mm is E12, about 17mm is E17, about 26mm is E26.

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This guide is general information. Always match the base, voltage and wattage to your fixture, and switch off the power before checking or changing a bulb.