How to Use This Tool
Enter the floor area and what the room is for. The room use is the input that matters most, and it is the one a wattage-based answer cannot represent at all.
Watts have not meant brightness for years
A 60 W incandescent gives about 800 lumens. A 9 W LED gives the same 800 lumens. The wattage tells you what the bulb costs to run; the lumen figure tells you how much light comes out. Anyone still sizing a room by "a 100 watt bulb" is using a unit that stopped tracking brightness when the technology changed.
150 sq ft room, 8 ft ceiling, mid-tone walls bedroom 150 x 15 = 2,250 lm 3 bulbs at 800 living room 150 x 20 = 3,000 lm 4 bulbs kitchen general 150 x 35 = 5,250 lm 7 bulbs workshop 150 x 80 = 12,000 lm 15 bulbs same room at a 10 ft ceiling: x 1.25 same room with dark walls: x 1.20
Ceiling height, because light spreads out
Illuminance at the working plane falls as the fitting gets further away. A 10 ft ceiling needs roughly 25 per cent more output than an 8 ft one to put the same light on a worktop, and a 12 ft ceiling about 50 per cent. This is why a converted barn or a room with a vaulted ceiling always feels dim when it is lit to a normal specification.
Dark walls eat light
A white ceiling and pale walls bounce a large share of the light back down. Dark paint, exposed brick and timber absorb it. The gap between a pale room and a very dark one is comfortably 30 per cent, which is larger than the difference between most fitting choices — so the paint decision is a lighting decision.
Layers beat one big fitting
Reaching the total from a single ceiling fitting produces glare directly under it and shadow at the edges, and every measurement in this tool assumes light reasonably distributed. Splitting the same lumens across several sources, with task lighting where work happens, gives a better result at the same total output — which is also why kitchen worktop lighting is listed separately from kitchen general lighting rather than replacing it.
Colour temperature is not brightness
A cooler light at 4000 K often reads as brighter than a warm 2700 K at the same lumen output, because of how the eye responds. It is a comfort choice rather than a quantity one; do not use it to make up a shortfall in lumens.
