How Much Light a Room Actually Needs

Lighting is usually decided by where the ceiling rose happens to be, which is why so many rooms are simultaneously too dim to work in and too bright to relax in. The arithmetic is straightforward and it starts from what the room is for rather than how big it is.

The targets

Illuminance is measured in lux — lumens per square metre — and the recommended level varies by task. For a 20 m² room with 800-lumen fixtures, from the lighting calculator:

Room	Target	Total lumens	Fixtures
living	150 lux	3,000 lm	4
kitchen	300 lux	6,000 lm	8
bedroom	100 lux	2,000 lm	3
bathroom	200 lux	4,000 lm	5
dining	200 lux	4,000 lm	5
office	400 lux	8,000 lm	10

The same floor area wants 10 fixtures as an office and 3 as a bedroom — 4 times the light for identical square metres. A room lit for one and used as the other will always feel wrong, and most people blame the bulb colour rather than the quantity.

Why a single ceiling light fails

A 20 m² living room needs 3000 lumens at the 150 lux target. One 800-lumen fitting supplies 26.7% of that, and it supplies all of it from one point on the ceiling.

The quantity problem and the distribution problem are separate. Even at the right total, light from a single source casts hard shadows, leaves corners dark and puts glare in exactly one place. Multiple smaller sources reaching the same total is a materially different room.

Layering, in the order that matters

Ambient is the general level the table above is describing — the background the room sits at.

Task is local and much brighter: a desk, a worktop, a reading chair. A kitchen at 300 lux ambient still wants dedicated light on the counter, because the person standing at it casts a shadow onto exactly the surface being used.

Accent is the small proportion that makes a room look considered rather than illuminated, and it contributes almost nothing to the total.

A room that hits its lux target entirely through ambient light is technically correct and usually unpleasant.

What the calculation does not include

Daylight. The targets describe artificial light, so a room with large windows needs the full figure only after dark.

Reflectance. Dark walls, floors and ceilings absorb a great deal of the light you put in. The same fixtures in a dark room deliver noticeably less usable illuminance than in a pale one, and the difference can be large enough to matter.

Ceiling height. Lux is measured at the working plane, and light spreads as it falls. In a tall loft the same fixture delivers less to the floor than it would in a standard room.

Fixture type. 800 lumens from a bare bulb, a diffused shade and a narrow downlight illuminate very different areas.

Buying by lumens, not watts

Watts measure consumption, not output, and the relationship between the two changed completely with LEDs. The number on the box that matters is lumens, and it is the only one the calculator asks for.

As a rough orientation, a traditional 60 W incandescent produced around 800 lumens — which is why 800 lm is the common LED equivalent and the figure used in the table above.

Working out your own

Measure the room and get its area — the square footage calculator handles irregular shapes by summing rectangles. Decide what the room is genuinely for, which for a multi-use room means the most demanding task in it. Then run it through the lighting calculator with the lumen figure from the fittings you are actually considering.

Add dimming to anything at or above 200 lux. A room lit correctly for a task is too bright for an evening in it, and the fix is a dimmer rather than a compromise on the installation.

The commonest mistake

Lighting a whole flat to the living-room target. Bedrooms end up glaring, offices end up at under half what they need, and the kitchen worktop stays in shadow.

Room by room is barely more work and it is the difference between a flat that is lit and one that merely has lights in it.

Colour temperature, which is a separate axis

Everything above is about quantity. Colour temperature — measured in kelvin — is about what the light looks like, and the two get conflated constantly.

Warm (around 2700K) suits living rooms and bedrooms, where the point is to feel relaxed.

Neutral (around 3500–4000K) suits kitchens, bathrooms and workspaces, where seeing accurately matters.

Cool (5000K and up) reads as clinical in a home and is rarely the right answer outside a garage or a workshop.

Mixing temperatures within a room is the fault that makes a space feel subtly wrong without an obvious cause — one warm lamp among neutral downlights reads as a fault rather than a choice.

The loft problem specifically

Converted and open-plan spaces break the room-by-room assumption, because one volume contains several functions. A kitchen corner at 300 lux and a sofa at 150 lux are in the same room and want different levels.

The answer is zones rather than an average. Calculate each functional area as though it were its own room, light it accordingly, and put them on separate switches — an averaged single circuit gives you a kitchen that is too dim and a sofa that is too bright, which is the commonest lighting outcome in an open-plan flat.

A worked example

A 35 m² open-plan space with a 10 m² kitchen zone and a 25 m² living zone needs 3,000 lumens over the kitchen and 3,750 over the living area — 4 and 5 fixtures respectively at 800 lm each.

Lit as one 35 m² living room it would take 7 fixtures, and the kitchen would be at barely half what it needs.

That gap is the whole argument for zoning, and it costs nothing at the planning stage and a great deal to retrofit.