Every bulb has two things you need to know before you buy: the cap, which is how it attaches, and the shape, which is what it looks like. Get the cap wrong and it will not go in. Get the shape wrong and it goes in but looks silly.
Here is the whole subject in one page, for British fittings, with the measurements to check.
On this page
- The five caps you will actually meet
- How to identify yours in a minute
- Bulb shapes, and when each one matters
- Watts, lumens and the conversion everyone needs
- Dimmable, and why yours might flicker
- Colour, CRI and the numbers on the box
- Which bulb for which fitting
- Five mistakes worth avoiding
- In short
- Common questions
The five caps you will actually meet
The letter tells you the type, the number is the size in millimetres. E27 is an Edison screw measuring 27mm across. That is the entire code.
| Cap | Also called | How it fits | Typically found in |
|---|---|---|---|
| E27 | ES, Edison screw | Screws in, 27mm thread | Pendants, table lamps, most of our fittings |
| E14 | SES, small Edison screw | Screws in, 14mm thread | Chandeliers, wall lights, candle bulbs |
| B22 | BC, bayonet cap | Push and twist, two pins | Older British ceiling roses and lamps |
| GU10 | Twist-lock spotlight | Push and twist, two stubby pins 10mm apart | Downlights, spotlight bars, track |
| G9 | Capsule | Pushes straight in, two looped wires | Small chandeliers, compact wall lights |
Two more you may see and can usually ignore: GU5.3 / MR16, the 12V spotlight that needs a transformer, and G4, a tiny capsule used in under-cabinet fittings.
How to identify yours in a minute
Switch it off at the wall, let it cool, and look at the base of the old bulb.
- A spiral thread ? It is E14 or E27. Measure the thread across its widest point: 14mm or 27mm. If you have no ruler, an E14 is about the width of a pound coin's thickness against your finger — in practice, an E14 looks obviously slim and an E27 obviously chunky.
- Two pins on the side of a smooth metal cap ? B22 bayonet.
- Two short fat pins underneath a mirrored cup ? GU10.
- Two small wire loops and no metal cap at all ? G9.
If the old bulb has gone missing, look at the fitting itself: a screw socket for E-caps, a socket with two L-shaped slots for bayonet. And if you bought the light from us, the cap is written on the product page.
Bulb shapes, and when each one matters
Shape only matters when the bulb is visible. Inside an opaque shade, buy whatever is cheapest and warmest; behind clear glass, the bulb is part of the design.
| Shape | What it is | Best in |
|---|---|---|
| A60 / GLS | The standard pear bulb | Anything with a shade |
| ST64 | Elongated Edison tube | Open pendants, industrial fittings |
| G80 – G125 | Large globes, 8 to 12.5cm across | Bare-bulb pendants, statement fittings |
| C35 candle | Slim, tapered | Chandeliers and multi-arm wall lights |
| Capsule | Tiny, no envelope | Where the bulb must disappear |
We stock four, and they cover most of what our fittings need: the E27 Filament Bulb in thirteen filament shapes, the Gold Edison ST64 for an amber glow behind clear glass, the E14 Candle Bulb in packs of four or ten for chandeliers, and the G9 Ceramic Capsule from 3W to 18W. Shapes, glass and glow are compared in our filament bulb guide.
Watts, lumens and the conversion everyone needs
Watts measure what a bulb consumes. Lumens measure what it produces. With LEDs the two have come apart completely, so buying by watts now tells you almost nothing.
| Old incandescent | Lumens | LED equivalent |
|---|---|---|
| 25W | ~250 lm | 3 – 4W |
| 40W | ~470 lm | 5 – 6W |
| 60W | ~800 lm | 8 – 9W |
| 75W | ~1,050 lm | 10 – 12W |
| 100W | ~1,500 lm | 14 – 16W |
Most rooms need less than people expect. A bedside lamp is comfortable at 400 lumens; a single pendant over a dining table rarely needs more than 800. Room totals are in our guide to choosing colour temperature. For every room, see how many lumens do I need.
One warning the box will not give you. Every fitting has a maximum wattage, and it is about heat, not brightness. With LEDs you will almost never reach it, but never fit a bulb above the stated maximum in an enclosed fitting.
Dimmable, and why yours might flicker
LED bulbs are not dimmable unless the box says so, and a non-dimmable LED on a dimmer will buzz, flicker or simply refuse to go low.
The other half of the problem is the switch. Dimmers made for filament bulbs expect a large load, and a 6W LED does not provide one. If your lights flicker at the bottom of the range, the switch is usually the culprit rather than the bulb — the full diagnosis is in our guide to why LED lights flicker.
If you plan to dim, buy dimmable bulbs from the start. The E27 Filament Bulb is dimmable, and it is worth the small premium in any room used in the evening.
Colour, CRI and the numbers on the box
Three numbers matter, and only one of them is on the front of the box.
- Kelvin. 2700K for everywhere you relax, 4000K for a desk, 6500K for a garage. Nothing else at home.
- Lumens. The brightness, as above.
- CRI. Colour rendering, out of 100. Buy 90 or above anywhere you look at people or food. It is the number that separates a good LED from a cheap one, and it is usually printed small on the side.
Which bulb for which fitting
- Open pendants and bare-bulb fittings. The bulb is the design. An ST64 or a large globe at 2700K, and keep the output modest — 400 to 600 lumens, or it glares at eye level. Suits the Industrial Glass Globe and the Vintage Loft Pendant.
- Chandeliers and multi-arm fittings. E14 candles, bought as a set so they match. Six bulbs from two different batches never look the same. Suits the French Chandelier and the Industrial Chandelier.
- Shaded table and floor lamps. A plain A60 at 2700K, CRI 90+. Nobody sees it. Suits most of our table lamps and floor lamps.
- Spotlight bars and downlights. GU10, and choose the beam angle: narrow for art, wide for a worktop. Suits the 4-Head and 6-Head Foldable Bars.
- Small wall lights. G9 capsules, where the fitting is too shallow for anything else. Suits much of our wall lights range.
Five mistakes worth avoiding
- Buying by watts. An 8W LED and a 60W incandescent give the same light.
- Assuming screw fits everything. Britain still has plenty of B22 bayonet sockets, especially in older houses.
- Non-dimmable bulbs on a dimmer. Flicker, buzz, and a shorter life.
- Mismatched bulbs in one fitting. In a chandelier, one bulb of a different temperature ruins five good ones.
- Ignoring CRI. A warm bulb at CRI 80 still makes a room look flat.
In short
- E27 screw, 27mm — the default for pendants and lamps.
- E14 screw, 14mm — chandeliers and candle bulbs.
- B22 bayonet — push and twist, common in older UK homes.
- GU10 — twist-lock spotlights and downlights.
- G9 — push-fit capsule for small fittings.
- Buy on lumens, not watts. 60W ≈ 800 lumens ≈ 8W of LED.
- 2700K, CRI 90+, dimmable if the room is used in the evening.
Common questions
What is the difference between E27 and E14 ?
Only the size of the screw thread: E27 measures 27mm across, E14 measures 14mm. E27 is the standard cap for pendants and table lamps; E14 is used for chandeliers, candle bulbs and small wall lights.
How do I know what bulb fitting I need ?
Look at the base of the old bulb. A spiral thread means E14 or E27 — measure it. Two pins on a smooth metal cap means B22 bayonet, two stubby pins under a mirrored cup means GU10, and two small wire loops means G9.
What does B22 mean on a light bulb ?
B22 is a bayonet cap 22mm across: you push it into the socket and twist a quarter turn to lock the two pins. It is the traditional British fitting and still common in older houses.
What is a GU10 bulb used for ?
GU10 is the mains-voltage twist-lock spotlight cap, used in downlights, spotlight bars and track lighting. It needs no transformer, unlike the older 12V MR16.
How many lumens replace a 60W bulb ?
About 800 lumens, which an LED produces at roughly 8 to 9 watts. Buy on lumens rather than watts: with LEDs, wattage only tells you the running cost.
Are all LED bulbs dimmable ?
No. A bulb is only dimmable if the packaging says so, and a non-dimmable LED on a dimmer switch will flicker or buzz. Old dimmers designed for filament bulbs can also misbehave with low-wattage LEDs.
Can I put an LED bulb in any old fitting ?
In almost any, provided the cap matches and the fitting is not fully enclosed with poor ventilation. LEDs run far below the stated maximum wattage, so heat is rarely the issue it was with halogen.
Check the cap, then the shape
Cap first, because it decides whether the bulb goes in at all. Shape second, and only if you can see it.
If you are unsure what a particular fitting takes, send us the model or a photo of the socket at contact@thelightheritage.com. We answer every message ourselves.
Browse the bulbs that fit our range: E27 filament, E14 candle, gold ST64 and G9 capsule, or see the fittings themselves in pendant lights and chandeliers.
