What This Calculator Does
This tool estimates how many light bulbs you need to properly illuminate a room. It combines the room's floor area, the desired light level (in foot-candles), and the brightness of each bulb (in lumens) to recommend a bulb count. It's useful when planning lighting for living rooms, kitchens, offices, garages, or any space where you want enough — but not excessive — light.
How to Use It
Enter three values: the room area in square meters, the required light level in foot-candles, and the lumens per bulb printed on the packaging of the bulb you plan to use. The calculator returns the number of bulbs, always rounded up so the room is never under-lit.
The Formula Explained
Foot-candles and lux measure illuminance. One foot-candle equals about 10.764 lux. To find the total light output (lumens) a room needs, multiply its area in square meters by the target foot-candles and by 10.764. Dividing that by the lumens each bulb produces gives the number of bulbs, which we round up:
$$\text{Bulbs} = \left\lceil \frac{\text{Area (m}^2\text{)} \times \text{Light Level (fc)} \times 10.764}{\text{Lumens per Bulb}} \right\rceil$$
Worked Example
Suppose you have a 20 m² living room that needs 30 foot-candles, using 800-lumen bulbs. Total lumens = $$20 \times 30 \times 10.764 = 6{,}458.4 \text{ lm}.$$ Dividing by 800 gives \(8.07\), which rounds up to 9 bulbs.
FAQ
What foot-candle level should I use? Living rooms and bedrooms typically want 10–20 fc, kitchens and offices 30–50 fc, and detailed task areas 50–75 fc.
Why round up? Partial bulbs don't exist, and rounding up ensures the space meets or exceeds the target light level.
Do fixtures matter? Yes — shades, reflectors, and ceiling height affect delivered light. This is a planning estimate; adjust for tall ceilings or heavy fixtures.
