Lumens measure brightness, watts measure energy use, and Kelvin measures colour. To choose a bulb, pick the lumens for how bright you want it, the Kelvin for how warm or cool it looks, and check the CRI for colour accuracy — then match the base and confirm it's dimmable if you need it to be. Watts no longer tell you brightness.
Key takeaways
- Lumens (lm) = brightness. More lumens = brighter light.
- Watts (W) = energy consumed, not brightness. LEDs give the same lumens at far fewer watts.
- Kelvin (K) = colour: 2700K warm, 4000K neutral, 5000K+ daylight.
- CRI = colour accuracy (aim for 90+).
- Also check: base type (E27/E14/GU10), dimmable, beam angle and rated hours.
On this page
- What do lumens, watts and Kelvin mean?
- Why you should stop shopping by watts
- Watt-to-lumen conversion chart
- How to read a bulb label, line by line
- Five more specs worth checking
- Frequently asked questions
What do lumens, watts and Kelvin mean?
These three numbers describe three completely different things. Confusing them is the most common reason people end up with a bulb that's too dim, too cold, or too expensive to run.
| Spec | Measures | What to look for |
|---|---|---|
| Lumens (lm) | Brightness / light output | Higher = brighter |
| Watts (W) | Energy the bulb consumes | Lower = cheaper to run |
| Kelvin (K) | Colour of the light | 2700K warm – 5000K+ cool |
| CRI | Colour accuracy (0–100) | 90+ for living spaces |
Why you should stop shopping by watts
For a century, we chose bulbs by wattage — a "60-watt bulb" was a familiar brightness. That shorthand only worked because incandescent bulbs were all similarly inefficient. LEDs changed everything: an LED can produce the same 800 lumens as an old 60W incandescent using just 8–10 watts. So watts now tell you how much electricity you'll pay for, not how bright the light will be. Shop by lumens for brightness; check watts only to compare running costs.
Watt-to-lumen conversion chart
Use this chart to translate the old wattage you remember into the lumens you should now look for — and see how few watts an equivalent LED needs.
| Old incandescent | Brightness (lumens) | Equivalent LED |
|---|---|---|
| 25 W | ~250 lm | 3–4 W |
| 40 W | ~450 lm | 5–6 W |
| 60 W | ~800 lm | 8–10 W |
| 75 W | ~1100 lm | 11–13 W |
| 100 W | ~1600 lm | 14–18 W |
How to read a bulb label, line by line
Most bulb packaging lists the same details in the same order. Read them in this sequence and you'll never buy the wrong bulb:
- Lumens — set your brightness first (see our room-by-room lumen guide).
- Colour temperature (K) — choose warm or cool for the room (see warm vs cool light).
- Wattage — confirm running cost and that it fits the fixture's max.
- Base / cap — E27, E14, GU10, B22 etc. must match your fixture.
- Dimmable — only dimmable bulbs work on a dimmer.
- Rated life — shown in hours (e.g. 15,000–25,000 h).
Five more specs worth checking
- CRI (Colour Rendering Index): aim for 90+ so colours look true. Cheap bulbs sit at 80.
- Base type: the fitting. Screw bases are E-numbers (E27 large, E14 small); bayonet is B22; spotlights are usually GU10.
- Dimmable: check compatibility with LED-rated dimmers to avoid flicker or buzz.
- Beam angle: narrow (25–40°) for spotlights and accents; wide (120°+) for general room light.
- Rated hours: quality LEDs last 15,000–50,000 hours — years of everyday use.
Frequently asked questions
How many lumens is a 60-watt bulb?
About 800 lumens. A modern LED produces the same 800 lumens using only 8–10 watts.
Is higher lumens always better?
No — more lumens simply means brighter. Match lumens to the room's purpose: cosy spaces need fewer, task areas need more. A dimmer gives you flexibility.
What does CRI mean on a light bulb?
CRI (Colour Rendering Index) rates how accurately a bulb reveals colour, from 0 to 100. For homes, choose CRI 90 or above so food, art and skin tones look natural.
Do watts still matter with LED bulbs?
Only for energy cost. Watts tell you how much electricity the bulb draws; lumens tell you how bright it is. Compare lumens for brightness and watts for efficiency.
What's the difference between lumens and lux?
Lumens measure the total light a bulb emits; lux measures how much of that light lands on a surface (lumens per square metre). Lux is what matters when planning how bright a room will feel.
Last updated: July 2026 · Written by Artevaris Editorial
Related reading: Warm vs Cool Light: Colour Temperature Guide · How Many Lumens Do You Need? · The Art of Light
Once you know the numbers, choose fixtures worthy of a good bulb — browse our designer lighting, ceiling lights and table lamps.
