You've picked the perfect chandelier, unboxed your new table lamp, and ordered a box of LED bulbs — only to discover they don't fit. The culprit is almost always the bulb base: the part of the bulb that connects to the socket. Base codes like E26, E27, E14, E12, G9 and GU10 tell you exactly how a bulb attaches and which sockets it fits. In Edison screw bases, the letter E stands for Edison and the number is the base diameter in millimetres — E27 is 27 mm wide, E14 is 14 mm. Match the code stamped on your fixture's socket to the code on the bulb box, and the bulb will fit.
How to Read a Bulb Base Code
Base codes come from the IEC 60061 international standard. For Edison screw bases (E-series), the number is the diameter of the threaded base in millimetres: E26 is 26 mm, E27 is 27 mm, E14 is 14 mm, E12 is 12 mm. For pin bases (G-series), the number is the distance between the pins: GU10 has pins 10 mm apart, G9 has pins 9 mm apart. For bayonet caps (B-series), the number is a diameter again — B22 is 22 mm across. If the code doesn't start with E, G or B, you may be reading a shape code, not a base code.
Edison Screw Bases: E26, E27, E14 and E12
Screw bases dominate home lighting, and four codes cover nearly every fixture. The E26 medium base is the North American standard — table lamps, floor lamps, pendants and ceiling lights — and the classic pear-shaped A19 bulb sits on it. The E27 is the European equivalent, 1 mm wider, and the standard across the EU and most of the world. The small bases live in decorative fixtures: E12, the US candelabra base, is the classic chandelier and ceiling-fan socket, and E14, the small Edison screw, is its European counterpart in chandeliers, wall sconces and compact lamps.
Pin Bases: GU10, G9 and G4
Pin bases push into the socket instead of screwing in. The GU10 is a twist-lock bi-pin with 10 mm pin spacing, running at mains voltage (120 V or 230 V): push in, quarter-turn, locked — the standard for spotlights and track heads. The G9 is a much smaller loop-pin base with 9 mm spacing, also mains voltage, made for compact decorative fittings like wall lights and small pendants. The G4 is smaller still at 4 mm spacing and runs at 12 V low voltage with a transformer, typical in under-cabinet and display lighting. Pin spacing is exact, so measure before you buy.
Bulb Base Comparison Table
| Base | Type | Size | Voltage | Typical fixtures |
|---|---|---|---|---|
| E26 | Edison screw | 26 mm diameter | 120 V | Table lamps, floor lamps, pendants (North America) |
| E27 | Edison screw | 27 mm diameter | 230 V | Standard lamps and pendants (Europe, most of world) |
| E12 | Edison screw, candelabra | 12 mm diameter | 120 V | Chandeliers, ceiling fans, decorative fixtures |
| E14 | Edison screw, small | 14 mm diameter | 230 V | Chandeliers, wall sconces, compact lamps |
| GU10 | Twist-lock bi-pin | 10 mm pin spacing | 120/230 V mains | Spotlights, track heads |
| G9 | Loop-pin bi-pin | 9 mm pin spacing | 120/230 V mains | Compact decorative fixtures, wall lights |
| G4 | Bi-pin | 4 mm pin spacing | 12 V low voltage | Under-cabinet, display lighting |
| B22 | Bayonet cap | 22 mm diameter | 230 V | Ceiling lights and lamps (UK, Commonwealth) |
The Traps That Catch Shoppers
First, E12 and E14 are not interchangeable. Only 2 mm apart, they look nearly identical, but an E12 bulb wobbles loosely in an E14 socket and risks arcing from poor contact, while an E14 bulb won't screw into an E12 socket at all. Second, shape codes are not base codes. A19 describes the glass shape — the same bulb carries an E26 base. B10 on a chandelier bulb is also a shape code, usually riding on an E12 base. Read the base code, not the shape code. Third, adapters change the shape, not the rules. E12-to-E14 adapters exist for imported fixtures, but an adapter does not convert voltage: never run a 120 V-only bulb on 230 V just because it now physically fits.
How to Find Your Fixture's Base Type
The surest method is the socket itself — the code is usually stamped on it, on the fixture's rating label, or in the manual. No label? Measure. Inside diameter of a screw socket in millimetres gives you the Edison number: about 26 mm means E26, about 14 mm means E14, about 12 mm means E12. Pin-hole spacing gives you the pin base: 10 mm means GU10, 9 mm means G9. After matching the base, check the socket's maximum wattage rating (never exceed it — the limit is about heat, not brightness) and the voltage, which must match your mains supply.
Products That Show Each Base in Action
The Corin Halo Glass Chandelier uses small E12/E14 candle-style sockets, the classic case where getting the base code exactly right matters. The Arden Silva Ceramic Table Lamp takes the standard E26/E27 medium base, so almost any household bulb fits. The Auria Ridge Resin Floor Lamp also uses the E26/E27 medium base, giving you the widest choice of brightness and colour temperature.
For more on choosing the bulb itself, see our light bulb buying guide and our explainer on lumens, kelvin, CRI and wattage. If you're pairing bulbs with a multi-light fixture, our chandelier buying guide covers sizing and placement.
Can I use an E12 bulb in an E14 socket?
No. The E14 socket is 2 mm wider, so an E12 bulb sits loosely, risks arcing from poor contact, and may work its way out. Buy the exact base your fixture specifies, or use a purpose-made E12-to-E14 adapter — and check the adapter's voltage and wattage ratings first.
Are E26 and E27 bulbs interchangeable?
Physically, yes — the 1 mm difference is negligible. Electrically, only if the bulb's voltage rating matches your supply: a 120 V E26 bulb will fail on a 230 V E27 circuit, and a 230 V E27 bulb will glow dimly on 120 V. Always match the voltage rating, not just the base.
Can I put a higher-wattage bulb in if the base fits?
No. The socket's maximum wattage rating is about heat: exceeding it can melt wiring insulation and is a fire risk. For more light, buy a brighter LED at the same or lower wattage — a 10–12 W LED delivers the roughly 1,100 lumens of an old 75 W incandescent.