Find the total lumens needed to light a room to a target lux level.
Lighting design starts from illuminance, not from bulbs. A lux is one lumen falling on one square metre, so the bare requirement is simply the target lux multiplied by the room area in square metres — which is why the area must be converted from square feet first, a factor of 10.76 that catches people out. Recommended levels vary by task: around 100-150 lux for a bedroom or hallway, 200-300 for a living room, 300-500 for a kitchen worktop or home office, and 500 or more for detailed work. Then comes the maintenance factor, the step amateurs skip: shades, diffusers, dust, wall colour and lamp ageing typically absorb a quarter of the light, so the installed output must exceed the theoretical figure by about a third.
Lumens required for a target illuminance
Area in sq m = area in sq ft x 0.0929; bare lumens = target lux x area in sq m; installed lumens = bare lumens / (1 - loss / 100); lamps = ceiling(installed lumens / lamp output)
Roughly 100-150 lux for bedrooms and circulation, 200-300 for living rooms, 300-500 for kitchens and home offices, and 500-750 where fine detail matters. Layering ambient and task lighting beats one bright ceiling fitting.
Because not all emitted light reaches the working plane. Shades, recessed trims, dark walls, dust and gradual lamp depreciation account for twenty to thirty percent, which the maintenance factor restores.
Not any more. Efficacy varies from about 12 lumens per watt for incandescent to over 100 for good LED, so wattage says what a lamp consumes, not what it delivers. Buy on lumens and colour temperature.