Bulb size: how to identify the right base without getting it wrong

What size are light bulbs?

Measure the diameter of the base, not the bulb: an E27 is 27 mm, an E14 is 14 mm, a B22 is 22 mm. This measurement takes ten seconds with a tape measure or a simple ruler held against the socket. Shape and wattage are chosen afterwards.

A shelf organised by base saves more time than a filter by wattage: E27, E14, G9 and GU10 LED bulbs are sorted this way, which saves opening twenty product pages to find the right one.

The diameter can be read in the base name, but the pin-based family needs a fresh eye: G4, G9, GU10 and GU5.3 look alike in a product page photo.

Unsure between two similar references? The details of each family are covered in our article on light bulb base sizes.

E27 bulb size: 27 mm in diameter

The E comes from Edison, the 27 gives the width of the base in millimetres. It is the format used for pendants, ceiling lights and living room floor lamps. The E27 base bulbs collection has 96 references as of 5 October 2026, from amber filament to opal globe.

This depth of range has a concrete benefit: you can replace a burnt-out bulb without changing the tint or shape, ten years after buying the fixture.

To match wattage and tint within the format, our guide to the E27 bulb goes through the reference points one by one.

E14 bulb size: 14 mm in diameter

Twice as slim as the E27, the E14 base is used on bedside lamps, small wall lights and accent fixtures. It is a format that leaves little room for error: in a narrow lampshade, a bulb that is too long sticks out and shows. There is plenty of choice among E14 bulbs, from the small globe to the candle.

Small base bulb sizes: G9, GU10, G4

Small bases do not screw in, they push in or clip in: G9 for wall lights and compact fixtures, GU10 for spotlights, G4 for small 12 V bulbs. The reference is simple, the number gives the gap between the pins in millimetres.

These formats are narrower than an E27, but often just as long in filament versions. The G9 bulbs collection groups compatible models without any extra sorting.

What is the diameter of a socket?

The diameter of a socket can be read from the base that screws into it: 27 mm for an E27, 14 mm for an E14, 22 mm for a B22. On a screw socket, you measure the inside of the thread. On a pin socket, you measure the gap between the two holes, 10 mm for a GU10.

Measuring the base diameter in 30 seconds

No professional tool is needed, a ruler or a soft tape measure is enough.

1. Unscrew the bulb and leave the socket bare.
2. Lay the ruler across the opening, without forcing.
3. Compare the measurement with 14, 22 or 27 mm: the base names itself.
4. Check the thread: a bayonet base is pushed in and turned a quarter turn, it does not screw in.

The bulb footprint, the second figure that matters

The base gets the bulb in, the shape decides whether it stays. Three points to check before ordering:

• The total height, from the base to the top of the glass, to compare with the depth of the lampshade.
• The globe diameter, which must stay narrower than the opening of the fixture.
• The mounting direction, head up or head down, which changes the available space.

On a wall light or a closed ceiling light, three extra centimetres are enough to stop the diffuser from going back on.

The bedside lamp case

A bedside lamp combines a small base and a shallow depth. The E14 is the majority there, with short bulbs that light downwards for reading. Our article on the bedside lamp bulb details the formats that fit under a low lampshade.

Then there is the case of second-hand fixtures sold without their original bulb: the socket is often the only reliable reference.

If you are still unsure about the fixture in front of you, send us a photo from our contact page: we will confirm the base before you order the wrong bulb.

What are the dimensions of an LED bulb and a spotlight?

An LED bulb almost always takes the footprint of the classic bulb it replaces. It is the spotlight trim, not its pins, that decides the housing: a GU10 goes into a GU10 holder, whatever its wattage. Measure the holder, not the product page.

LED dimensions: what really changes

The LED chip heats up less than a filament, so the glass can stay compact. The space saving is mainly in length, on globes and candles. Some filament LEDs, however, reproduce the exact silhouette of an incandescent bulb, which settles the footprint question without any calculation.

The standard width of an LED spotlight

A spotlight is chosen by family, not to the nearest centimetre. Three families cover most suspended ceilings and tracks.

These three families do not mix in the same holder. For a like-for-like replacement, the GU10 bulbs collection gathers spotlights compatible with a pin holder.

GU10 spotlight size and drill hole diameter

The GU10 is the format to know when drilling. Note the trim diameter of the model already in place, then keep a little play to remove it from the ceiling without forcing. Our guide to changing the bulb in a recessed spotlight shows how to release the spring without damaging the plaster.

A19 and G40: what do these bulb shapes correspond to?

A19 and G40 come from American usage, where the number is read in eighths of an inch. An A19 is about 6 cm in diameter, it is the classic bulb, called A60 in Europe. A G40 is close to 12 cm and is called G125 here. Both exist with an E27 base.

A19 in cm: about 6 cm in diameter

The A19, or A60, is the bulb you do not notice: straight, regular, it lights without showing off. It is the format that disappears into a closed pendant and fills a fabric lampshade properly. The summary of bulb types helps place the other shapes on the same scale.

G40 and XXL decorative bulbs

The G40, or G125, changes status: at 12 cm in diameter, a globe is no longer hidden, it is looked at. It is the bulb for exposed-filament pendants and large dining room fixtures.

It needs a fixture that owns it: a pendant fixture with an exposed socket shows it off, a closed lampshade stifles it.

Tint matters as much as size on these large globes. Our article on XXL decorative bulbs details the choice of colour temperature by room.

Are 2000 lumens a lot?

2000 lumens is a lot for an accent lamp and is the scale of an entire living room. The more light a bulb delivers, the larger its globe grows. So the real question is not the number of lumens, but where the bulb will be seen.

What 2000 lumens changes in a room

A high output is justified in a living room, a long hallway or an open kitchen. In a bedroom or a bathroom, it crushes the atmosphere and tires the eyes in the evening. When the output rises, the globe size follows, because the most powerful modules are also the widest. Our guide to choosing your bulbs cross-checks output, tint and footprint before purchase.

Why an oversized bulb breaks the line of a fixture

On a fixture with a visible socket, the bulb is part of the design. A few millimetres too many and the silhouette loses its balance.

• The curve sticks out of the socket and breaks the line intended by the manufacturer.
• The height prevents a diffuser or a cup from going back on.
• The weight tilts a wall light or a loosely tightened articulated arm.
• The diameter touches the lampshade and creates an ugly shadow zone.

The selection is commented on by buyers: their feedback is visible on the shop's Google listing, updated continuously.

Then there is the question of volume: the silhouette of a pendant above a table is judged with the bulb in place, never on a screen.

For a project with several light points, the order is prepared differently, one base per project avoids a second trip. The trade area centralises orders and account tracking.