The five common fittings
In the UK, Ireland, Australia, New Zealand and much of the world, five bulb fittings cover almost everything. Here's how to recognise each:
- B22 (bayonet) — a smooth base with two small pins on the sides. Push in and twist a quarter-turn to lock.
- E27 (large screw) — an Edison screw about 27mm across, screwed in like a jar lid. The most common fitting worldwide.
- E14 (small screw) — a smaller screw, about 14mm across, used in candle bulbs, lamps and chandeliers.
- GU10 — a mains-voltage twist-lock spotlight, with two chunky pins with rounded tips.
- MR16 — a low-voltage spotlight (usually 12V through a transformer), with two thin, straight pins.
The quickest way to tell them apart is how they connect: push-and-twist means bayonet; a spiral thread means a screw fitting; two pins mean a spotlight. From there, size (large vs small screw) or pin style (chunky twist-lock vs thin straight) narrows it to one.
Still not sure?
Fittings can be genuinely hard to place from memory, which is why the tool above walks you through it with a question or two. But whatever it suggests, the foolproof check is to match your old bulb exactly — take the old bulb, or a clear photo of its base, when you shop. For a closer look at the two most common fittings, see B22 vs E27; for the two spotlights, see GU10 vs MR16.