Bulb Base Guide: E26 vs E27, E12 vs E14, GU10, G9 and What Fits What
Share
Most "the bulb doesn't fit" returns come down to two letters and a number on the product page. Here is what each base code means, which ones are interchangeable, and which ones are not.
Screw bases (the "E" codes)
E stands for Edison screw. The number is the thread diameter in millimeters.
| Code | Diameter | Common name | Where you see it |
|---|---|---|---|
| E26 | 26 mm | Standard / medium base | Almost every US and Canadian lamp and fixture |
| E27 | 27 mm | Standard (Europe, Asia) | Most fixtures made outside North America |
| E12 | 12 mm | Candelabra | Chandeliers, sconces, night lights in North America |
| E14 | 14 mm | Small Edison (SES) | Chandeliers and sconces made in Europe and Asia |
| E17 | 17 mm | Intermediate | Appliance bulbs, some ceiling fans |
E26 and E27: interchangeable, with one caveat
The 1 mm difference is inside the tolerance of both standards. An E26 bulb screws into an E27 socket and vice versa. Many of our fixtures are specified as E27 because that is what the manufacturer stamps; any E26 LED bulb from a North American store fits. The caveat is voltage: the socket doesn't care, but a bulb rated only for 220–240 V will run dim or not at all on 120 V. Buy bulbs rated 120 V (or 100–240 V), which is what every bulb sold in the US and Canada is.
E12 and E14: not interchangeable
A 2 mm gap is too much. An E12 bulb wobbles in an E14 socket and makes poor contact; an E14 bulb won't thread into an E12 socket. If a fixture lists E14, you need E14 bulbs, which are less common in North America but sold online. Check the base code on chandelier and sconce listings before ordering; it is in the Specifications section where the supplier has supplied it, and we will confirm it on request.
Pin and bayonet bases
| Code | Type | Notes |
|---|---|---|
| GU10 | Twist-lock, two pins 10 mm apart | Line voltage (120 V) spotlights and track heads. Push in and twist. |
| GU5.3 / MR16 | Two straight pins 5.3 mm apart | Low voltage (12 V). Needs a transformer, usually built into the fixture. Not interchangeable with GU10. |
| G9 | Two looped pins 9 mm apart | Small halogen or LED capsules in chandeliers and sconces. Line voltage. |
| G4 | Two straight pins 4 mm apart | Tiny 12 V capsules in under-cabinet and accent lights. |
| B22 | Bayonet, two side pins | Standard in the UK, Australia and India. Push and twist. Not found in North American fixtures; no adapter needed if the fixture you ordered is E26/E27. |
"Bulb included" vs "bulb not included"
Fixtures with a built-in LED module have no replaceable bulb; the product page says "integrated LED" and lists the wattage and lumens directly. Everything else lists a base code and a maximum wattage. The maximum is the heat limit of the socket for an incandescent bulb; an LED bulb draws a fraction of that, so a 9 W LED (equivalent to 60 W) is fine in a socket marked "max 40 W".
Quick decision list
- Listing says E27 → use any E26 bulb, 120 V.
- Listing says E14 → buy E14 bulbs specifically; E12 will not seat.
- Listing says E12 → standard North American candelabra bulb.
- Listing says GU10 → 120 V spot bulb, twist-lock.
- Listing says G9 / G4 → capsule bulb; check whether the fixture is 12 V or 120 V before buying.
- Listing says integrated LED → nothing to buy.
Base type is listed under Specifications when the supplier gives it to us. If it isn't on the page, ask us at admin@brightideaslighting.store before ordering and we'll confirm with the supplier.