Room Lighting Power Calculator
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/8/2026
The luminous flux needed is calculated with surface × recommended illuminance (in lux). For a 20 m² living room (200 lux recommended), you need about 4,000 lumens, or 40 W with a 100 lm/W LED.
Explanation
The illuminance level suited to a room depends on its use: a living room, a relaxation space, generally needs only about 200 lux, a bedroom even less (about 150 lux), while a kitchen (a food preparation area, where visual precision matters) needs more, around 300 lux, and an office or a work surface even more, up to 500 lux according to professional lighting standards (AFE, the French Lighting Association). Multiplying the room's surface by this target illuminance gives the total luminous flux needed, expressed in lumens — the unit that measures the amount of light emitted, unlike watts, which measure the electrical power consumed to produce that light. It is precisely this confusion between lumens and watts that has made choosing bulbs more complex since LEDs became widespread: a 60 W incandescent bulb produced about 800 lumens, while a modern LED can produce that same flux with only 6 to 10 W, thanks to much higher luminous efficacy (lumens produced per watt consumed). This calculator therefore converts the luminous flux needed into equivalent LED power, based on the luminous efficacy indicated on the packaging of the chosen product (often between 80 and 120 lm/W for a common household LED) — this value remains freely editable rather than fixed, since it varies noticeably between models. These benchmarks remain general averages: localized task lighting (a desk lamp, a reading spotlight) may need more lumens over a specific area, in addition to the room's general lighting.
Example: a 20 m² living room, 100 lm/W LED
Inputs
Surface: 20 m². Room type: living room (200 lux). Bulb efficacy: 100 lm/W.
Calculation
Flux needed = 20 × 200 = 4,000 lm. Power = 4,000 ÷ 100 = 40 W.
Result
This living room needs about 4,000 lumens, or 40 W of LED at this luminous efficacy.
Frequently asked questions
Why look at lumens rather than watts when choosing a bulb?
Because watts measure electrical consumption, not the amount of light produced: an LED consumes far fewer watts than an incandescent bulb to produce the same luminous flux. Comparing bulbs by wattage alone, a habit inherited from the incandescent era, no longer gives a reliable indication of their actual brightness since LEDs became widespread.
Does this calculation assume a single ceiling light source?
No, it gives the total luminous flux needed for the room, whether it comes from a single ceiling fixture or is spread across several sources (spotlights, task lamps, wall lights). If you plan several light points, simply add up the lumens of each to reach the total calculated here.
Do rooms with dark walls need more lumens?
Yes, this benchmark assumes light-colored surfaces, which reflect light better. Dark-colored walls and floors absorb more light and may require a higher luminous flux than this calculation to achieve an equivalent perceived brightness.