E27, E14, GU10: Which Bulb Fitting Do You Actually Need?

You found the light you want, the listing says E27, and you have no idea whether that matters. It does, but only a little, and once you can recognise three shapes you will never have to think about it again.

A bulb fitting, also called a base or a cap, is simply the mechanical connection between the bulb and the lamp. It says nothing about brightness, colour or quality. It is a shape, nothing more.

The three you will actually meet

E27. The large screw base, about 27 mm across. This is the default for pendants, ceiling lights, floor lamps and most table lamps. If a listing does not say, E27 is the safe guess. The E stands for Edison, the number is the diameter in millimetres.

E14. The small screw base, 14 mm. Same thread, smaller. It turns up in small table lamps, wall sconces, chandelier arms and anywhere the shade is too narrow for a full size bulb. An E27 bulb will not fit an E14 socket and forcing it will damage both.

GU10. A twist-lock base with two stubby pins, no thread. Push in, twist a quarter turn, done. Used for spotlights, adjustable downlights, picture lights and directional wall fittings. GU10 bulbs are always mains voltage, so they need no transformer.

Those three cover the overwhelming majority of decorative lighting sold in Europe. A handful of others exist, and it is worth knowing they do:

• G9. Two thin loop pins, very small, common in compact wall lights and small chandeliers.
• B22 and B15. Bayonet bases, push and twist, standard in the UK and Ireland rather than mainland Europe.
• MR16 or GU5.3. Looks like GU10 but with thin pins, and runs on 12 volts through a transformer. Not interchangeable with GU10 even though the bulbs look similar.
• Integrated LED. No base at all, because there is no removable bulb. We cover what that means for you in integrated LED or replaceable bulb.

How to tell what a fitting takes when nobody told you

Turn the light off at the wall and let the old bulb cool. Then look at the socket, not the bulb:

• Visible spiral thread, wide enough that your thumb fits comfortably inside: E27.
• Visible spiral thread, narrow, roughly the width of a pen: E14.
• Two small slots on either side with a twist channel, no thread: GU10 or bayonet.
• Two small round holes and nothing else: G9 or a pin type.

If the old bulb is still in one piece, the base is usually printed on it along with the wattage.

The part that actually changes how a room feels

The fitting is a plug. Everything you notice about the light comes from what you screw into it.

Colour temperature. Measured in kelvin. 2700K is warm and domestic, 4000K is neutral, 5000K and above is cold and belongs in a workshop. This single choice does more to the feel of a room than the fixture does, and we go room by room in warm white or cool white.

Brightness. Measured in lumens, not watts. Watts describe consumption, and with LED they describe almost nothing useful. See lumens not watts.

Colour rendering. CRI, on a scale to 100. Below 80 and skin looks grey and wood looks flat. Aim for 90 or above anywhere people eat, dress or look at each other.

Dimmable or not. A non-dimmable LED in a dimmer circuit flickers, buzzes, or simply refuses to go low. If the fitting is on a dimmer, the bulb must say dimmable on the box.

Shape matters when the bulb is visible

In an open shade the bulb is part of the design. A clear globe behind clear glass looks considered. A frosted golf ball in the same fitting looks like a spare.

The common shapes, all available in E27 and most in E14: A60, the classic pear; G80, G95 and G125, globes in ascending diameter; ST64, the elongated tube with visible filament; C35, the candle shape for chandelier arms.

In an enclosed shade none of this is visible and you can buy whatever is cheapest, as long as the colour temperature is right.

Watts, heat and the limit on the label

Every fitting has a maximum wattage printed near the socket or in the manual. With LED you will almost never approach it: a bulb that replaces a 60 watt incandescent draws about 8 watts.

The number still matters for one reason. Enclosed fittings trap heat, and heat is what kills LED drivers. If the shade is fully closed, look for a bulb marked suitable for enclosed fixtures. It is a real distinction and an ordinary LED in a sealed globe will fail early.

A simple stocking rule

Keep two spares of each base you own, in the colour temperature you have standardised on, and keep them together in one drawer. The most common lighting problem in a home is not a broken fixture. It is a dark room and no bulb that fits.

Frequently asked questions

What is the difference between E27 and E14?

Only the diameter of the screw thread: 27 mm against 14 mm. E27 is the standard large base for pendants and floor lamps, E14 the small one for compact lamps and chandelier arms. They are not interchangeable.

Can I put an LED bulb in any fitting?

Yes, provided the base matches and the bulb is rated for the fitting. Check dimmable if the circuit has a dimmer, and enclosed rated if the shade is sealed.

Is GU10 the same as MR16?

No. They look alike, but GU10 has thick twist-lock pins and runs on mains voltage, while MR16 has thin pins and needs a 12 volt transformer. Fitting the wrong one will not work.

Does the bulb base affect brightness?

Not at all. Brightness comes from the bulb's lumen output. The base is only the mechanical connection.

How do I know what bulb my lamp takes?

Look at the socket with the power off, or read the base of the old bulb. A wide thread is E27, a narrow thread E14, twist-lock pins are GU10.

Once you know the base, the fixture is the fun part. Browse ceiling and pendant lights, wall lights and table lamps.