Kelvin sounds like physics because it is, but the idea is simple. Heat a piece of metal and it glows dull red, then orange, then white, then bluish as it gets hotter. A bulb's Kelvin rating names where its color falls on that scale. Low numbers look like candlelight and old incandescent bulbs. High numbers look like noon daylight through a north window.
Two things follow from that. First, Kelvin is about color, not brightness — a 2700K bulb and a 5000K bulb can put out exactly the same amount of light. Second, Kelvin has nothing to do with how hot the bulb itself gets. With that straight, choosing bulbs gets a lot easier, and it's why the old warm look of an incandescent is a temperature you can still buy today rather than a technology you lost — the LED vs incandescent differences lie elsewhere.
2700K — warm white. The classic household glow. Flattering to skin, wood, and anything you'd describe as cozy. This is the default for rooms where people relax.
3000K — soft white, a touch crisper. Still warm, but whites look whiter and colors read a little truer. A good middle step for kitchens and bathrooms that feel dingy at 2700K.
4000K — cool white. Neutral and alert, the color of a decent office or a bright laundry room. Great for task areas, slightly clinical in a bedroom.
5000K and up — daylight. Blue-leaning and stark, close to outdoor light at midday. Superb in a workshop or garage where you need to see detail and judge colors honestly. In living spaces, most people find it harsh by evening.
| Room | Color Temperature | Why |
|---|---|---|
| Living room | 2700–3000K | Warm and relaxed, kind to faces and wood tones |
| Bedroom | 2700K | The warmest light supports winding down at night |
| Kitchen | 3000–4000K | Crisper light for counters, cooking, and cleanup |
| Bathroom / vanity | 3000–4000K | True colors for mirrors, shaving, and makeup |
| Home office | 3500–4500K | Alert without going full daylight |
| Garage / workshop | 4000–5000K | Detail and honest color for tools and finishes |
| Dining room | 2700–3000K | Food and faces both look better warm |
| Laundry / utility | 4000–5000K | Stains and sorting need honest light |
Watts measured how much power an old bulb burned, and people used them as a brightness shortcut for a century. LEDs broke the shortcut: a 9-watt LED outshines a 60-watt incandescent. So shop by lumens, which actually measure light output.
| If You Used To Buy | Look For |
|---|---|
| 40W incandescent | About 450 lumens |
| 60W incandescent | About 800 lumens |
| 75W incandescent | About 1,100 lumens |
| 100W incandescent | About 1,600 lumens |
As a rough rule of thumb, living spaces feel right around 10–20 lumens per square foot, with kitchens, bathrooms, and workbenches wanting more at the spots where tasks happen. When in doubt, go a little brighter and put it on a dimmer — you can always turn light down, but you can't turn a dim bulb up.
CRI — color rendering index — measures how truthfully a bulb shows colors compared to natural light, on a scale up to 100. Plenty of cheap bulbs sit at 80, which is fine for hallways and garages. Where color judgment matters — a bathroom mirror, a kitchen counter, artwork, a closet where navy and black have to be told apart — look for 90 or higher. Two bulbs with identical Kelvin ratings can render the same sweater noticeably differently, and CRI is the reason why.
One temperature per room, and per sightline. Mixed temperatures in the same view are what make a room feel subtly wrong — a warm lamp glowing next to a daylight ceiling bulb reads as a mistake even if you can't name it. Rooms that flow into each other, like a kitchen open to a living room, should share a temperature or stay one step apart.
Buy one bulb first. Boxes are cheap in multipacks and expensive to be wrong about six times. Test a single bulb in the actual fixture, at night, before committing.
Check the fixture before the bulb. Some LEDs aren't rated for fully enclosed fixtures or recessed cans, and trapped heat shortens their life even when the base fits. The box says "suitable for enclosed fixtures" when it is. Match the base, check the rating, and a bulb that should last a decade actually will.
Mind the dimmer. Not every LED dims, and older dimmers made for incandescents can make LEDs flicker, buzz, or cut out early. Check that the box says "dimmable," and if you're swapping hardware anyway, an LED-rated control solves most of it — here's how to install a dimmer switch yourself, and the same hour of work covers a plain light switch replacement if a switch is what's actually failing.
Daylight bulbs in the bedroom. 5000K at 10 p.m. feels like a waiting room. Bedrooms belong at 2700K, no debate.
Shopping by watts out of habit. Grab the "60W equivalent" line on the box or, better, the lumen number. Guessing from the LED's actual wattage gets you a bulb half as bright as you wanted.
Ignoring CRI at the bathroom mirror. Low-CRI light washes out skin tones and shifts makeup colors. It's the one room where the extra dollar for 90+ CRI earns its keep every morning.
Match the Kelvin to what the room is for: 2700K where you relax, 3000–4000K where you groom and cook, 4000–5000K where you work. Buy brightness in lumens — 800 is your standard bulb — keep one temperature per sightline, and spend the small CRI upgrade at mirrors and counters. Do that and the lighting stops being something you notice at all, which is the entire point.
Both work; it depends on the kitchen. 3000K keeps a warm, evening-friendly feel and suits kitchens that open into living space. 4000K gives crisper, more neutral light over counters and makes it easier to judge whether food is done. Pick one and use it for every bulb in the room, including under-cabinet lights.
Like daylight — bright, white leaning blue, similar to light near a window at noon. It's excellent in garages, workshops, and laundry rooms. In bedrooms and living rooms most people find it cold and stark, especially at night.
They're close enough that most people won't notice, especially in different fixtures like a lamp and a ceiling light. What you want to avoid is a wide gap in the same sightline — a 2700K lamp beside a 5000K ceiling bulb looks like two different rooms glued together.
As a rough guide, aim for about 10–20 lumens per square foot of total light in living spaces — a 200-square-foot room lands somewhere around 2,000 to 4,000 lumens spread across all its fixtures. Layered lighting (ceiling plus lamps) hits that total more comfortably than one bright fixture.
LEDs are commonly rated for around 15,000 to 25,000 hours, versus roughly 1,000 for an old incandescent. Real life varies with heat and fixture type — an LED in a fully enclosed fixture runs hotter and may not reach its rating — but even so, a bulb you install today is likely to be there for years of normal use.