What 4000K Means on a Light Bulb

A bulb marked 4000K is not 4,000 degrees of anything. It describes the colour of the light it emits, and if you convert 4000 K to Celsius you get 3,727 °C — a figure that is arithmetically correct and tells you nothing at all.

This is one of the few places where a unit converter will happily give you a right answer to the wrong question. So here is the right one.

Where the number comes from

Heat a theoretical perfect radiator — a black body — and it glows. At low temperatures the glow is deep red. Heat it further and it runs orange, then yellow, then white, then faintly blue.

Lighting borrows that scale. A bulb described as 2700 K emits light the colour a black body would emit at 2,700 K. The bulb itself is nowhere near that temperature; a modern LED runs at perhaps 60 °C.

The scale has one thoroughly confusing feature: higher numbers look "cooler". 2700 K is warm and yellow; 6500 K is cold and blue. Physically it is the other way round — the higher figure is the hotter radiator. The words we use for colour and the physics run in opposite directions.

What to buy for each room

Rating	Looks like	Where it works
2700 K	Warm, yellowish — old incandescent	Living rooms, bedrooms, restaurants
3000 K	Soft white, slightly less yellow	Living spaces, hallways, hotel bathrooms
4000 K	Neutral white	Kitchens, bathrooms, offices, garages
5000 K	Cool, faintly blue — overcast daylight	Workshops, task lighting, security
6500 K	Distinctly blue, clinical	Display cases, some studios

The single most common mistake is mixing ratings in one room. Two bulbs in the same ceiling at 2700 K and 4000 K look obviously, unpleasantly wrong in a way neither does alone — the eye is very good at comparing two whites side by side and hopeless at judging one in isolation.

If you are replacing a blown bulb, match the rating of the ones that remain rather than picking what sounds right.

The number that matters more, and nobody prints

Colour temperature tells you the tint of the light. It says nothing about how accurately that light renders colour, which is measured separately as CRI — the colour rendering index, out of 100.

A cheap 4000 K bulb with a CRI of 70 and a good 4000 K bulb with a CRI of 95 are the same colour and produce visibly different rooms. Skin looks grey under the first; food looks unappetising; paint colours you chose in daylight come out wrong.

Sunlight is defined as 100. Anything above 90 is good, 80 is acceptable for a hallway, and below 80 is worth avoiding anywhere you look at people or food. It is usually printed in the specifications rather than on the front of the box, which tells you something about how manufacturers expect it to be used.

When the conversion is real

The Kelvin scale itself is a genuine temperature scale, and one of the most useful in science — it starts at absolute zero, so it has no negative numbers and ratios actually mean something. For real temperatures the conversion is exact and never approximate:

°C = K − 273.15

The Sun's surface is about 5,778 K, which is 5,505 °C. That one is a real temperature, and it is where the lighting scale borrowed its numbers from in the first place — "daylight" bulbs are labelled 5000–6500 K because that is roughly the colour of the sky.

You can run either kind of number through the Kelvin to Celsius converter or Celsius to Kelvin. Just know which one you have.