A 12 × 14 ft living room lit to 150 lux (14 foot-candles) needs 4,682 lumens of bulbs: 168 ft² × 14 fc ÷ 0.5. With 9 W LED bulbs of 900 lumens that is 6 bulbs, 54 W in all, laid out 3 × 2. The calculator above works this out for any room and bulb.
How the calculator works
Enter the room size, pick its use and the bulb type and wattage. The result is the light level in foot-candles and lux, the lumens needed, the number of bulbs, their total power and a grid layout on the plan.
Light level by room
| Room | Foot-candles | 12 × 12 ft room |
|---|---|---|
| Bedroom, hallway | 9 | 2,676 lm — 3 bulbs |
| Living room | 14 | 4,014 lm — 5 bulbs |
| Kitchen, bathroom, kids' room | 19 | 5,351 lm — 6 bulbs |
| Home office | 28 | 8,027 lm — 9 bulbs |
| Workshop, garage | 37 | 10,703 lm — 12 bulbs |
Bulbs of 900 lm (9 W LED). 1 foot-candle = 1 lumen per square foot = 10.76 lux.
Why divide by 0.5
Not all of a bulb's light reaches the floor and the counters: shades, diffusers, walls and the ceiling absorb part of it. For a typical room with light walls about half arrives, so the bulbs must give twice the lumens the surface needs. Dark walls or ceilings over 9 ft need more.
Watts and lumens
Bulbs are sold by lumens: an LED gives about 100 lm per watt, a CFL 60, an incandescent 12. A 60 W incandescent and an 8–10 W LED both give about 800 lm. Task lights over a desk or kitchen counter come on top of the general lighting.
Planning the circuit for the lights: Wire Size Calculator. Room area for other trades: Square Footage Calculator, ceiling grid: Drop Ceiling Calculator.