50 lx ambient
Room size, lux, kelvin
A bulb's wattage tells you almost nothing about how bright a room will feel. Multiply your floor area by a target lux level, pick a colour temperature on the kelvin scale, and check the CRI and rated hours printed on the package — those three numbers decide the result.
50 lx ambient
80 lx hallway
320–500 lx work
500 lx worktop
2700 K warm white
6500 K daylight
Multiply the floor area in square metres by a target lux level and you have the lumens the room needs.
One lux equals one lumen per square metre. The same 1000 lumens spread over 1 m² give 1000 lx; spread over 10 m² they give 100 lx. The room's area therefore sets how much flux you must supply, not the wattage of the bulb.
Published illuminance examples give the target lux directly: 50 lx for family living-room lighting (Australia, 1998), 80 lx for office building hallways, 320–500 lx for office lighting, and 500 lx for a kitchen worktop. Pick the row that matches how the surface is used, multiply, and you have the figure to shop for.
Read the kelvin value on the package: 2700 K for warm relaxing light, 6500 K for daylight.
Warm white is 2700 K — the colour of a tungsten lamp at about 2700 K, yellow. Daylight sits between 5500 K and 6500 K, and D65 / sRGB use 6500 K. Phosphor white LEDs span 2200 K to 7000 K, so every value in that range is available off the shelf.
In interiors, warmer (lower colour temperature) light is used in public areas to promote relaxation, while cooler (higher) light is used to enhance concentration, for example in schools and offices. The choice is a use decision, not a brightness decision: the same lumens at 2700 K and at 6500 K light the room equally.
CRI runs 0 to 100 with 100 as the ideal; rated hours tell you how long the stated light output lasts.
The colour rendering index runs from 0 to 100 with 100 as the ideal. Many LED bulbs sat near 80 in 2019, commercial blue-pump phosphor LEDs cluster around 82, high-CRI products exceed 95, and LEDs with CRI below 75 are not recommended for indoor lighting. Treat 80 as the floor for general rooms and 90+ where colour matters.
A typical domestic LED lamp is stated at 15000 hours average life — about 15 years at 3 hours a day — and 50000 switch cycles. The best LED lamps offer 30000 hours or more against about 1000 hours for incandescent and about 8000 hours for compact fluorescents. Energy Star requires less than 10% lumen drop after 6000 hours.
Each card pairs a room use with the lux figure and the kelvin band that the sources attach to it.
Two geometry errors explain it: the cosine loss on tilted surfaces and the same flux spread over a larger area.
Illuminance falls with the cosine of the angle between the light direction and the surface normal, and it drops when the same flux is spread over a larger area. A fixture aimed at a wall or a sloping ceiling delivers less lux to the working surface than the package lumens suggest.
A third cause is sizing by watts instead of lumens. An incandescent lamp runs at 10–17 lm/W (about 16 lm/W at 120 V, 13 lm/W at 230 V), while a typical white LED runs near 100 lm/W — so an 8 W LED replaces a 60 W tungsten bulb at the same 806-lumen output. Swapping watt for watt leaves the room underlit.
| Use of the room | Target lux | Lumens needed (12 m²) | Typical bulb count |
|---|---|---|---|
| Family living room, ambient | 50 lx | 600 lm | 1 × 600 lm or 2 × 350 lm |
| Home corridor / hallway | 80 lx | 960 lm | 2 × 500 lm |
| Office building hallway | 80 lx | 960 lm | 2 × 500 lm |
| Office lighting, low end | 320 lx | 3840 lm | 4 × 1000 lm |
| Office lighting, high end | 500 lx | 6000 lm | 5 × 1200 lm |
| Kitchen worktop | 500 lx | 6000 lm | 1 × 12000 lm fixture (area-dependent) |
| 60 W tungsten equivalent (EU) | — | 806 lm minimum | 1 × 806 lm at about 8 W |
Figures are taken from published illuminance recommendations, European Union lamp standards, and measured LED efficacy and life data.