Lighting · Explainer · 4 min ·
That K on a bulb box is the colour of the light, not how hot it gets. Low numbers are golden, high numbers are blue-white, and the same room looks like a different place at each.
Slide the light from candle to daylight and watch a sofa, a rug and a wooden table change. Then see which number suits which room, why evenings like it warm, and what to check before you swap a bulb.
Look at the box of any LED bulb and you will find a number followed by a capital K, like 2700K. It is the bulb's colour temperature, measured in kelvin. The name comes from physics: a glowing piece of metal turns from red to orange to white to blue-white as it gets hotter, and kelvin is the scale for that. A bulb does not actually get hotter as the number goes up. The number only tells you which colour of white the light is.
The part that trips people up is that it runs the opposite way to what you might expect. Lower numbers are warmer and more golden, higher numbers are cooler and bluer. A candle flame sits at the very low end, around 1900K. The old filament bulbs most of us grew up with were close to 2700K, which is why that number still means "cosy" on a box. Open daylight on a bright day is far higher, roughly 5000K to 6500K.
Warm light brings out reds, oranges and yellows, so wood, brick, wool, terracotta and skin look rich. It makes whites look creamy. Cooler light does the opposite: it lifts blues and greens, makes whites look crisp, and can make the same wooden table look a bit grey. Neither is wrong. The question is what the room is for.
| Roughly | Looks like | Often used for |
|---|---|---|
| 2200–2700K | Candle to old filament bulb | Bedside lamps, dining tables, reading corners |
| 2700–3000K | Warm white | Living rooms and bedrooms |
| 3500–4000K | Neutral white | Kitchens, bathrooms, home desks |
| 5000–6500K | Daylight | Workshops, garages, task lights |
Makers draw the lines slightly differently, so you will see "warm white" used for anything from about 2700K to 3000K. Treat these as ranges, not rules.
Bright, blue-rich light late in the evening tends to keep people feeling more awake, which is part of why many of us prefer warm light for winding down. It is not a magic switch, and how bright the light is matters as well as its colour. A dim lamp at 2700K is a gentler bet than a bright ceiling light at 5000K when you are heading for bed. In the daytime, cooler light is fine and can feel more alert.
CRI or Ra. A higher number shows fabrics and wood more truthfully. Many bulbs sit around 80, and 90 or above is nicer for textiles and art.You can also try it for free before buying anything. Swap a bulb between two lamps, or move a lamp from the hallway to the sofa corner, and see how the evening feels for a few days. If a bulb still works, there is no need to replace it just to change a number.
If you do decide to change a bulb, the same few things matter at every budget:
E27, E14, GU10) so the new one screws or twists in, and check the maximum bulb size your lampshade allows.Replacing a bulb is a plug-in job. If you ever want to change a ceiling light or anything wired into the wall, that is work for a qualified electrician.
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A low-wattage warm-white bulb with a standard E27 screw cap, a gentle choice for a bedside or corner lamp. Check the kelvin number on the box, and that the shade has room for a 6 cm bulb.
Warm white · E27 · 3.4 W · Ø 6 cm
Two warm-white bulbs of the same type, handy for keeping every lamp in a room at one colour. Check the kelvin number on the box before you buy.
Warm white · E27 · 8 W · 2 bulbs
A brighter warm-white bulb from a well-known lighting brand, suited to a main living-room lamp. Confirm the box says 2700K and whether it is dimmable if you use a dimmer.
Warm white · E27 · 8 W · Ø 6.3 cm
