Look at the metal end of the old bulb. A screw base is an E base: E26 is the standard US size, E12 (candelabra) is much smaller, and E17 falls between them.
What does the base look like?
With the power off and the bulb cool, compare its base with a bulb you know.
You most likely have
E26 medium base
The standard US screw base. Europe’s E27 looks alike but is used mainly on 220-240 volt systems.
On the package: E26 (medium)
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E12 candelabra base
The small US screw base of chandeliers, nightlights and decorative fixtures.
On the package: E12 (candelabra)
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E17 intermediate base
Between E12 and E26 in size, used in microwaves, refrigerators and other appliances.
On the package: E17 (intermediate)
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GU10 or GU24
Both push in and twist. GU10 is mostly on spotlights; GU24 also comes on regular-shaped bulbs such as an A19.
On the package: GU10, or GU24
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Match the code on the two-pin bulb
Small two-pin bulbs come with different bases, such as G4, GU5.3 and G9, and the bulbs don’t swap.
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Before you change any bulb, turn the power off and let the old bulb cool.
Where the code is written
A bulb’s specs list its base. Feit’s microwave bulb shows “bulb base E17”, for example. If you have the old bulb’s box or its maker’s page, read the base there. If not, answer the questions above.
Screw, pins, or push and twist
- Screw bases start with E, from the small E12 up to the large E39, found on some torchiere-style 3-way lamps.
- Two pins that go straight in are small bi-pin bases, such as G4, or GU5.3 on 12 volt MR16 spots. A G9 has two looped pins.
- Push and twist bases, GU10 and GU24, have two pins that lock with a turn.
E26 or E27? E12 or E14?
E27 is the international equivalent of the US E26, used in Europe and other regions, and E14 is the international version of E12. The pairs are close in size, but E26 is used mainly on 120 volt systems, as in the US and Canada, and E27 mainly on 220-240 volt systems, as in Europe. In the US, the base to match is E26 or E12, on a 120 volt bulb.
GU10 or GU24?
GU10 bases are used almost only in spotlights. GU24 also comes on ordinary bulb shapes: Feit makes an A19 LED with a GU24 base.
Need the shape code too?
Bulb codes