Estimate the lighting power needed to reach a target illuminance over a given room area.

How to Use the Lux to Watts Calculator

Enter the target illuminance in lux, the floor area in square meters, and a luminous efficacy in lumens per watt — the default of 100 approximates LED lighting, but adjust it for whatever technology you're actually specifying.

This one runs in two steps rather than a single formula, and it's worth walking through both instead of treating the result as a black box.

Step 1 — lumens needed:  lm = lux × area (m²)
Step 2 — power required: W = lm ÷ efficacy (lm/W)

Step one is the same lux-to-lumens conversion as the Lux to Lumens Calculator: illuminance times area gives the total luminous flux a room needs. Step two is where the light source's technology actually matters, since a lumen requirement alone says nothing about wattage — a fluorescent tube, a halogen floodlight, and an LED panel each convert electrical power into light at very different rates. Luminous efficacy bridges the two, and because it varies by product rather than following a physical constant, this calculator asks for it directly instead of assuming one.

With the defaults — 500 lux over 20 m², at 100 lm/W — that works out to 10,000 lumens and 100 watts of LED lighting. Swap the efficacy to something closer to an old halogen fixture, around 20 lm/W, and the same 10,000-lumen target suddenly costs 500 watts: five times the power for identical brightness. That gap is the entire reason "how many watts do I need for this room" is really a two-part question — how much light, and how efficiently is it produced. Whichever number the calculator lands on, sanity-check it against the wattage printed on an actual fixture you're considering; the arithmetic is exact, but it's only as good as the efficacy figure feeding it.