Lighting Calculator (Lumens)

Multiply the floor area by the light level (in foot-candles) the room’s use calls for, then allow for light lost to walls and fittings. Typical targets: bedrooms about 10 fc (100 lux), living rooms 15 fc (150 lux), kitchens and bathrooms 20–30 fc (200–300 lux), home offices and workshops 50 fc (500 lux). 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.

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, and one lux is one lumen per square metre. Fittings are sold in lumens, rooms are designed in foot-candles, and the floor area is what connects them.

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 (12 m²) kitchen needing about 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 spacing from the walls.

Not every lumen a fitting emits reaches the work surface — dark walls, high ceilings, shades and dust all absorb light. Dividing by a factor of about 0.7–0.8 for a normal room (lower for dark or tall rooms) converts delivered lumens into the installed lumens you must buy. The calculator exposes this factor so you can tune it.

Loosely — efficiency varies by product. Decent LEDs deliver roughly 80–120 lumens per watt, so an 800 lm downlight draws around 7–10 W. Always buy on lumens (and color temperature), not watts; “60 W equivalent” labels are marketing shorthand for about 800 lumens.

No — it sizes the general (ambient) layer. A proper design layers task lighting where work happens and accent lighting for depth, checks glare and color rendering, and uses photometric data for the exact fittings. For homes and small commercial spaces this gets the fitting count right; for offices and retail, run a photometric layout too.