Light Bulb Base Types: An FAQ for Choosing the Right Bulb

Quick answer

Start by checking the fixture label or the old bulb's base: E27 and E26 are both screw-in bases that are physically interchangeable in most fixtures, while B22 is the push-and-twist bayonet type. If you are buying new, match the base code exactly and then choose the bulb shape and light output to suit the shade.

Buying a bulb should take thirty seconds, but base codes turn it into a puzzle. The good news is that almost every domestic fitting falls into one of two families: screw or bayonet. Once you know which family your fixture uses, the rest is about shape, brightness, and whether you want dimming.

This FAQ answers the questions that come up most often when people are standing in a shop or scrolling a product page, trying to work out whether an E27 bulb will fit a lamp that came with an E26. Short answers first, then the practical detail.

What are the main light bulb base types?

In UK and European homes you will mostly meet three: E27 (a 27mm screw base), E26 (a 26mm screw base used widely in North America), and B22 (a 22mm bayonet base that pushes in and twists). Smaller fittings exist for decorative or specialist lamps, but for table lamps, pendants, and ceiling lights these three cover the vast majority.

The letter tells you the family, the number tells you the diameter in millimetres. So E27 and E26 are both Edison screw bases, just one millimetre apart. B22 is a bayonet base, held in by two small pins rather than threads. If you remember only one thing, remember that the number is the width.

• E27 and E26 are screw bases; B22 is bayonet.
• The number is the base diameter in millimetres.
• Check the fixture label first, then the old bulb.

E27 vs E26: are they interchangeable?

In practice, yes, in most fixtures. The one-millimetre difference is small enough that an E26 bulb usually threads into an E27 holder and vice versa. The risk is tolerance: a stiff holder or a slightly oversized bulb can feel tight, and a loose fit can leave the bulb not quite making contact.

If you are buying a bulb for a fixture sold in the UK, choose E27 where possible. It is the standard here and will seat correctly. If you already own an E26 bulb from a trip abroad, it will usually work, but check that it sits firmly and lights without flickering before you commit to a full set.

• E27 is the UK and EU standard; E26 is the North American equivalent.
• They usually fit each other's holders, but fit can vary by fixture.
• If a bulb feels loose or flickers, the base is the first thing to check.

Bayonet vs screw bulb: which do I have?

Look at the base of the old bulb. If it has a threaded metal collar, it is a screw base. If it is smooth with two small pins sticking out sideways, it is a bayonet base. Bayonet bulbs push in and twist about a quarter turn to lock; screw bulbs simply turn until snug.

Bayonet fittings are common in older UK homes and in some lamps designed for the British market. They are not interchangeable with screw bases, so this is the one distinction you cannot work around. If your fixture is bayonet, you need a B22 bulb, full stop.

• Threads mean screw; pins mean bayonet.
• Bayonet bulbs lock with a quarter turn.
• B22 and E27 are not interchangeable.

Which light bulb base should I buy if I am replacing a bulb?

Take the old bulb with you, or photograph the base and the fixture label. If neither is possible, check the product page for your light. Most listings state the base type in the specifications, and if they do not, that is a question worth asking the retailer before you order.

Once you know the base, the remaining choices are shape, brightness, and colour temperature. A bulb that fits but is too dim or too cool will still disappoint, so treat the base as the first filter rather than the whole decision.

• Photograph the old bulb's base before shopping.
• Check the fixture label or the product specification.
• Base first, then shape, brightness, and colour temperature.

Does the base type affect brightness or dimming?

No. The base is a mechanical and electrical connector; brightness comes from the bulb's lumen output and dimming depends on the bulb's driver and your dimmer switch. A dimmable E27 and a dimmable B22 can both work perfectly well, provided the dimmer is compatible.

Where base type does matter is physical space. A bulky bulb in a small shade can crowd the fitting or cast shadows. If you are choosing a statement bulb, check the diameter and length against the shade, not just the base code.

• Brightness is about lumens, not the base.
• Dimming needs a compatible bulb and dimmer.
• Check bulb dimensions against the shade.

Can I use an adapter to change the base type?

Adapters exist, but they add a layer of connection and can affect how securely the bulb sits. For a lamp you use daily, a correctly based bulb is the better choice. Adapters are more useful for a one-off or a decorative bulb you cannot otherwise fit.

If you do use one, check that the bulb still sits within the shade and that the adapter is rated for the bulb's wattage. Never force a base that does not want to go in.

• Adapters work but add a connection point.
• Prefer the correct base for everyday fixtures.
• Check wattage rating and fit before relying on an adapter.

Base-check FAQ: quick answers

Is E27 the same as E26?

They are both screw bases one millimetre apart and usually fit each other's holders, but E27 is the UK standard. Choose E27 for UK fixtures; treat E26 as a close substitute, not an exact match.

How do I tell bayonet from screw?

Bayonet bases have two small pins and lock with a quarter turn; screw bases have visible threads. If you see pins, you need B22, not E27.

Will any E27 bulb fit my pendant?

The base will fit, but the bulb's diameter and length may not suit a small shade or a tight fitting. Check bulb dimensions as well as the base code.

Can I dim any bulb?

No. The bulb must be dimmable and your dimmer must be compatible with it. Check both the bulb and the dimmer before buying.

Bulbs that show why base and shape matter

• Tala Oblo E27 Bulb - 3x: The Oblo is a bold and compact bulb, best suspended in a pendant, with a robust shape that breaks up space and colour with soft white light. A good example of why base alone is not enough: this E27 bulb is designed to be seen in a pendant, so its size and shape are part of the decision.
• Tala Elva E27 Bulb - 5x: The Elva is a compact, perfectly rounded E27 bulb with a clear glass finish, dimmable LED design, and a 30,000-hour lifespan. If you want a dimmable E27 bulb that suits most standard fixtures, this shows the kind of specification to look for beyond the base code.
• Tala Lurra E27 Bulb - 5x: Lurra is a compact, cylindrical E27 bulb with a traditional filament arrangement and an iron-tinted glass finish that helps emit a warm light output. Useful when you want a warm, filament-style look in an E27 fixture without changing the fitting.
• Tala Sphere II E27 Bulb - 5x: Sphere II is designed to complement most standard light fixtures, with Dim to Warm characteristics, flicker-free behaviour, and a matte white finish. A practical choice when you want a bulb that works across several E27 fixtures and dims warmly rather than just dimming down.

Match the base first, then choose the bulb that suits the shade and the room. If you are unsure, check the fixture label or ask before you order, and you will avoid the most common bulb-buying mistake.