Estimate the illuminance a bulb delivers in a room, from its wattage, efficacy, and the room's floor area.

How to Use the Watts to Lux Calculator

Enter the bulb's wattage, a luminous efficacy figure for its technology, and the room's floor area in square meters. Both the intermediate lumen output and the resulting illuminance in lux appear below.

Of everything in this batch of conversions, this is the one that answers the question most people actually have: if I put this bulb in this room, how bright will it really be? Getting there takes two steps chained together, and it's worth seeing both rather than one opaque number at the end.

Step 1 — light output: lm = W × efficacy (lm/W)
Step 2 — illuminance:  lux = lm ÷ area (m²)

Step one converts electrical power into total light output using the bulb's efficacy — the same relationship the Watts to Lumens Calculator uses on its own. Step two spreads that light output across the room's floor area to get illuminance, the actual brightness experienced in the space, which is what a lux reading or a lighting-code target really measures.

Run the defaults — an 8-watt LED at 100 lm/W in a 20 m² room — and the result lands at 800 lumens spread thin enough to give just 40 lux, well under the 300–500 lux most task-lighting standards call for. Halve the area to 10 m² instead and that same 800 lumens' worth of light doubles to 80 lux — for a fixed bulb, room size does as much work as bulb choice in determining how bright a space actually feels. That's the real value of running the numbers: it's easy to assume one modern LED bulb lights a whole room adequately, and the math is usually what reveals you need two or three fixtures, not one.