One Lux Figure for a Whole Room Is a Starting Point, Not a Plan

A higher ceiling costs light

The same room with a 3-metre ceiling instead of the standard 2.4 asks for 5,000 lumens rather than 4,000, and seven fittings instead of five. The tool scales the requirement by the ratio of the heights, which is a deliberate simplification: light spreading from a point falls off faster than that, and a proper calculation would also account for how much the walls and ceiling reflect back. As a rule of thumb for a domestic room it is close enough, and it points the right way — high ceilings and dark walls both cost light, and a room that felt dim after decorating usually got darker paint rather than fewer lumens.

Lux is measured on a surface, not in the air

One lux is one lumen falling on one square metre, so the figure only means anything once you say where. The 500 lux target for food preparation is measured at the worktop, roughly 90 centimetres up, and a ceiling fitting three metres above it delivers far less there than the arithmetic on floor area suggests. This is why the professional approach specifies a working plane and a maintenance factor for dust and ageing, and why the household version works better as a ranking than as a specification: use it to see that a kitchen worktop needs roughly two and a half times a living room, then put the light where the work happens.

Frequently asked questions

Five bulbs in one fitting or five separate points?

The total lumens is the same either way, and the room will not be. One central fitting with five lamps puts everything in the middle and leaves the corners dark with hard shadows under anything solid; five spread points give a flatter, more even light that most people read as brighter at the same total. The count from this calculation is a budget, not a layout — spend it across at least three positions in any room bigger than a bathroom, and prefer more, dimmer sources to fewer, brighter ones.

Do these lux targets apply to a workplace?

No, and it matters. Workplace lighting is governed by standards that set minimum illuminance by activity, along with requirements on glare, uniformity and colour rendering that a floor-area calculation says nothing about — and in many places those standards are legally enforceable rather than advisory. The figures here are domestic rules of thumb for deciding what to buy. If the room is an office, a workshop or anywhere an inspector might visit, take the numbers from the applicable standard.