Bulb basics: kelvin, lumens and CRI explained

September 24, 2026

Bulb boxes are covered in numbers and almost none of them are explained. Three of those numbers decide how a room will feel. Here is what each one means, what the official guidance actually says, and which ones are worth checking before you buy.

Kelvin: the colour of the light

Kelvin, written as a number followed by K, describes the colour of the light rather than how much of it there is. Lower numbers are warmer and more orange. Higher numbers are cooler and more blue. The ENERGY STAR light bulb purchasing guide puts it plainly: lower K means warmer, yellowish light, while higher K means cooler, bluer light.

2200K to 3000K is warm white and soft white. ENERGY STAR describes this band as ideal for most indoor applications. 2700K is the standard for living rooms and bedrooms, and the closest common bulb to the old incandescent look.
3500K to 4100K is neutral white and cool white, the band ENERGY STAR suggests for kitchens and work spaces. It is also the temperature of most offices and shops.
5000K to 6500K is daylight, which the same guide compares to a blue sky at noon. Useful for a workbench or a garage, wrong for a sitting room.

Below 2200K you are into candlelight territory, very warm and very dim, more atmosphere than illumination.

The practical advice is simple. For anywhere you relax, look for 2700K. If you are mixing bulbs within one room, keep them within a few hundred kelvin of each other, because the eye is very good at spotting one bulb that is cooler than the rest.

Lumens: how much light there is

Lumens measure brightness. This is the number that replaced watts when LEDs arrived, because watts only ever measured how much electricity a bulb consumed, not how much light came out. In the United States the Lighting Facts label on the box states brightness in lumens, and the Federal Trade Commission's shopping guide gives these equivalents for the old incandescent wattages:

40W is about 450 lumens
60W is about 800 lumens
75W is about 1,100 lumens
100W is about 1,600 lumens

For a table lamp you are sitting next to, somewhere between 400 and 800 lumens is usually plenty. People routinely overbuy here. If you are building a room out of three or four small sources, each one can be far dimmer than you would expect.

CRI: how honest the light is

Colour rendering index, written as CRI or sometimes Ra, is a score out of 100 describing how accurately a light source shows colour compared with a reference source. The US Department of Energy's fact sheet on LED colour characteristics defines a score of 100 as rendering colours identically to the reference, and rates a CRI in the 70s as acceptable for interior use, the 80s as good and the 90s as excellent.

It is the number most people have never heard of and the one that quietly changes rooms. At the acceptable end, wood reads greyer, skin reads flatter and fabric loses its depth. At 90 and above the same room looks like it was decorated by someone with taste. Nothing physically changed, the light is just reporting the colours properly.

Look for CRI 90 or higher anywhere you care how things look, which is most of the house. It is often printed in small type and it is worth hunting for. One caveat from the same fact sheet: two bulbs with the same CRI can still render colours differently, so treat the number as a guide rather than a guarantee.

The three other things on the box

Dimmable. Not all LEDs are, and even the ones that are can misbehave on the wrong switch. The National Academies' Assessment of Advanced Solid-State Lighting notes that existing incandescent dimmers may not work with LED lamps even when the lamp is labelled dimmable, with flicker, lamps popping on at full brightness before settling, or cutting out before they reach the low end. Check the bulb is marked dimmable and, where you can, that it is listed as compatible with your dimmer.

Base type. E26 in the US and E27 in the UK are the common screw fittings, and in practice they are interchangeable. E14 is the small screw. B22 and B15 are the common UK bayonet fittings. GU10 is the little twist lock used in spotlights. Measuring or photographing the old bulb before you go shopping saves a return.

Beam angle. Mostly relevant to spotlights and directional fittings. A narrow angle throws a tight pool, a wide one spreads. For general room lighting, wider is usually what you want.

What to actually check

If you only read two numbers on the box, read the kelvin and the CRI. Brightness can be fixed with a dimmer or by moving the lamp. A bulb that is the wrong colour, or that renders colour badly, cannot be fixed at all.