By James Batten, 299 Lighting · Published · Updated
In short
The cap is the base of the bulb: E27 and E14 screw in, GU10 twists and locks, G9 and G4 push in on two pins, GX53 twists into slim ceiling fittings, and R7S is a double-ended linear lamp. Always match the cap printed on the fitting page.
Many decorative fittings take a bulb rather than having built-in LEDs. Ordering the wrong cap is the most common mistake we see, and it is easy to avoid once you know what each code means.
The common caps
| Cap | Fitting | Typical use |
|---|---|---|
| E27 | Large Edison screw, 27 mm | Pendants, floor and table lamps, most decorative fittings |
| E14 | Small Edison screw, 14 mm | Chandeliers, small wall lights, candle bulbs |
| GU10 | Twist and lock, 10 mm pins | Spotlights and downlights |
| G9 | Push-in looped pins, 9 mm apart | Small glass and crystal fittings |
| G4 | Push-in pins, 4 mm apart, low voltage | Tiny capsule fittings |
| GX53 | Twist-in flat disc | Slim surface and recessed fittings |
| R7S | Double-ended linear lamp | Uplighters and floodlights |
Reading a bulb code
A code like "E27 4W 1800K" tells you three things: the cap (E27), the power (4 W) and the colour temperature (1800K, a very warm amber glow). The power is not brightness; check the lumens instead.
Brightness: lumens not watts
- Around 400–470 lm replaces an old 40 W bulb.
- Around 700–800 lm replaces a 60 W bulb.
- Around 1,000–1,500 lm replaces a 75–100 W bulb.
Always check the maximum wattage on the fitting, especially in enclosed glass shades.
Colour temperature and shape
1800–2200K amber filament bulbs suit clear glass and industrial fittings, where the bulb is on show. 2700K is the standard warm white for homes. Use globe or frosted bulbs in clear shades to avoid glare. Browse our full range of Ideal Lux bulbs with the right cap for every fitting.
Dimming
Not every LED bulb dims. Check for "dimm" in the product name and use an LED-compatible trailing-edge dimmer. On larger schemes, ask the 299 Lighting team to check dimmer compatibility.
Frequently asked questions
What is the difference between E27 and E14?
Both screw in. E27 is 27 mm wide and used in most pendants and lamps; E14 is 14 mm and used in chandeliers and small wall lights.
Can I use a GU10 bulb in a G9 fitting?
No. Caps are not interchangeable; use the cap listed on the fitting page.
Which bulb looks best in a clear glass pendant?
A warm 1800–2200K filament or globe bulb, because the bulb is visible.
