Which Smart Bulb Cap Do You Need? E27, B22, E26 and the Rest

Smart lighting has one failure mode that has nothing to do with the software, the app, or the hub. You order the bulbs, they arrive, and they do not fit the lamp.

It happens constantly, because bulb fittings are regional and the internet is not. A review written for one country recommends a kit that is sold with a different cap in yours. This guide is the boring, useful part that most smart lighting articles skip.

First, the word

The "cap" (or "base") is the bit that connects the bulb to the fitting. It is the only part that decides whether the bulb physically goes in. Everything else about a smart bulb, the colours, the brightness, the protocol, is irrelevant if the cap is wrong.

Caps are named for their shape and size. E27 is an Edison screw, 27 mm across. B22 is a bayonet, 22 mm across, with two pins that you push and twist.

The caps you will actually meet

If you have a standard table lamp or ceiling pendant, you almost certainly need E27, E26 or B22, and which one depends on where you live. If you have spotlights sunk into the ceiling, you are looking for GU10.

The one mismatch that matters

E26 and E27 differ by a single millimetre of thread. In practice they thread into each other, and people discover this and assume the two are simply the same thing with different names.

They are not, and the difference is electrical rather than mechanical.

E26 is the North American standard, built around mains supply of roughly 120 V. E27 is the European standard, built around roughly 230 V. The cap tells you whether the bulb will go in. It tells you nothing about whether the bulb is designed for the voltage that is about to arrive down the wire.

The direction that should worry you is a bulb rated only for 120 V going into a 230 V fitting, because the bulb then receives roughly double what it was built for. Many modern LED bulbs are rated for a wide input range and print something like "100-240 V" on the base, in which case they are happy either side of the Atlantic. Plenty of others are not.

So the rule is simple. The cap tells you whether the bulb fits. The voltage printed on the bulb tells you whether it is safe. Read both. If the stated range does not cover your country's mains supply, do not use it, however neatly it screws in.

The easiest way to avoid the question entirely is to buy from your own region's store rather than importing a listing that looked cheaper. Which brings us to the thing that catches people out most often.

The same product is sold with different caps

This is not a trap manufacturers set deliberately. It is just how a global product gets sold into local markets.

A Philips Hue starter kit may contain E27 bulbs in one country, E26 in another, and B22 in a third. The box art looks identical. The product name is often identical. The bulbs are not.

It is one of the reasons we mark the Hue kit down slightly despite scoring it 8.6, which is the highest score on this site. The lighting is excellent. The buying experience assumes you already know which fitting you are looking for.

Kit contents vary too, so confirm the bulb count as well as the cap before you pay.

How to tell what you have, in thirty seconds

Turn the light off and let the bulb cool. Take it out.

If it unscrewed, it is an Edison screw, and you need to know whether it is E27 or E26. Where you live answers that: North America means E26, most of Europe and Oceania means E27.

If you had to push it in and twist it a quarter turn, it is a bayonet, so B22 in almost every home that has them.

If it is a small squat cylinder with two stubby pins that twisted out of a ceiling recess, that is GU10.

If it is a much smaller screw bulb from a chandelier or a lamp with a little shade, it is E14 in Europe and E12 in North America.

Then take a photograph of the old bulb before you throw it out. It is the cheapest insurance there is against ordering the wrong thing twice.

A note on smart bulbs specifically

Smart bulbs are usually physically larger than the dumb bulbs they replace, because there is a radio and a driver inside. In an enclosed fitting, or a lampshade with a tight clip, one may simply not fit even when the cap is correct.

Check the bulb's length as well as its cap if the fitting is at all snug. This is the second most common way a perfectly good smart bulb ends up back in its box.

If you are just starting out and want the bigger picture first, our smart home beginner's guide covers which ecosystem to commit to before you spend anything on bulbs.

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