Light bulb base types: a guide to fittings

The short version

In Australia, nearly every standard globe is one of two bases: E27, the large screw fitting, or B22, the large bayonet. If you're replacing a globe, the quickest check is the base of the old one, where the size code is usually printed. This guide covers the bases you'll actually meet in an Australian home, how to tell them apart, and what to look for when you buy.

How to tell which base you have

1. Read the old globe. The code (E27, B22, E14, GU10) is printed on the metal base or the packaging.
2. Look at how it connects. A screw base has a spiral thread. A bayonet base has two side pins and pushes in with a quarter-turn. A spotlight has two straight pins out the back.
3. Measure across the base. E27 and B22 are about 22 to 27mm wide; E14 and B15 are the smaller 14 to 15mm versions.

Screw or bayonet?

Both do the same job; they just hold the globe differently.

• Screw (Edison screw, ES): thread it in clockwise, like a jar lid. Codes start with E, then the width in millimetres: E27 (large), E14 (small).
• Bayonet (BC): line up the two pins, push in, and twist a quarter-turn to lock. Codes start with B: B22 (large), B15 (small).

Australia has long used both. Older homes lean bayonet; newer fittings and most imported lamps lean screw. Neither is better, you just need to match what's in the ceiling.

The common bases, at a glance

Spotlights and downlights

Downlights don't use screw or bayonet bases. They use pins. GU10 runs straight off mains power and locks with a push-and-twist, the standard in modern downlights. GU5.3 is the two push-in pins on an MR16 spotlight, which runs at low voltage through a transformer and is common in older downlights. Deciding between the two, or upgrading old halogen downlights? That's covered in GU10 vs MR16.

Does the base change your smart bulb choice?

Only in that you need to buy the right base. A smart bulb is the same bulb whichever base it's on, so once you know your fitting, choose on brightness, colour and ecosystem instead. Most smart globes come in both E27 and B22, so bayonet homes aren't left out.

For picking an actual globe, see Best smart light bulbs, or browse smart bulbs.