Lighting a room is two steps: work out the total light it needs (in lumens), then divide by the output of one fitting to get the count. The link between them is the floor area and the light level the room’s use calls for. Here is the method, with the levels to design to.
How many lumens?
Multiply the floor area by the light level (in foot-candles) for the room’s use, then allow for light lost to walls and fittings. A 130 sq ft (12 m²) kitchen at 30 fc needs roughly 4,800 lumens delivered — more like 6,400 lumens installed once losses are counted.
| Room | Light level |
|---|---|
| Bedroom | ~10 fc (100 lux) |
| Living room | ~15 fc (150 lux) |
| Kitchen / bathroom | 20–30 fc (200–300 lux) |
| Office / workshop | ~50 fc (500 lux) |
Lighting Calculator (Lumens)
Pick the room type and size for the foot-candles, total lumens and number of lights to install.
Lumens vs lux (or foot-candles)
Lumens measure how much light a fitting emits; foot-candles (or lux) measure how much lands on a surface — one foot-candle is one lumen per square foot, one lux is one lumen per square metre. Fittings are sold in lumens, rooms are designed in foot-candles, and the floor area connects the two.
How many downlights?
Divide the total installed lumens by the output of one fitting and round up. A typical LED recessed light gives 600–900 lumens, so that 130 sq ft kitchen needing ~6,400 installed lumens wants 8–9 lights at 800 lm each. Space them evenly — roughly 3–4 ft (1–1.2 m) apart for standard ceilings, half that from the walls.
The light-loss factor
Not every lumen reaches the surface — dark walls, high ceilings, shades and dust absorb light. Dividing by a factor of about 0.7–0.8 for a normal room (lower for dark or tall rooms) turns delivered lumens into the installed lumens you must buy. Buy on lumens and color temperature, not watts: “60 W equivalent” is just marketing shorthand for about 800 lumens.