How many lumens for a room

How it works

Each space needs a certain lighting level (lux). Total lumens are the lux times the square meters, then split across the light points.

Tip

On bulb packaging look for lumens, not watts: lumens tell you how much light they really produce.

Watts no longer mean anything

With incandescent bulbs the watt had become a habit: 60 W meant a certain amount of light. With LEDs that link snapped, because the same light comes from a tenth of the power, and two 9 W LEDs from different makers can differ by thirty per cent in light output. The number to look for on the box is therefore the lumen, which measures light, not electricity.

As a quick reference: an old 40 W bulb gave about 470 lumens, a 60 W about 810, a 100 W about 1,520. Those are the three numbers that let you translate a habit into a purchase.

Lux and lumens are not the same thing

The lumen is how much light leaves the bulb, the lux is how much lands on a surface: one lux is one lumen per square metre. The same bulb gives plenty of lux over a side table and very few in a large lounge, which is why the calculation goes through square metres.

The levels usually recommended run from about 100 to 150 lux for a hallway or a bedroom, to 300 for a living room or an office, up to 500 and beyond on a kitchen worktop or a desk used for reading. One figure doubles with age: a sixty-year-old needs a great deal more light than a twenty-year-old to read the same text, and it is one of the most useful corrections when lighting an elderly parent's home.

Why a single ceiling light is never enough

Concentrating every lumen at one point in the middle of the ceiling produces hard shadows, dark corners and unpleasant glare. The same quantity of light spread over three or four sources makes the room brighter and more restful. The classic way to think about it is in layers: a diffuse general light, a targeted light where things get done (the hob, the desk, the side of the bed), and a low light for the evening.

On a work surface, where it comes from matters too: if the light is behind you, your shadow falls exactly where you are working, and no quantity of lumens removes it.

The number always forgotten: colour temperature

Lumens say how much light, kelvins say what colour. Below 3,000 K the light is warm and yellowish, suited to bedrooms and living rooms; around 4,000 K it is neutral, good for kitchens and bathrooms; above 5,000 K it is cold and bluish, and it is the one that disturbs sleep in the evening because it is closest to daylight.

Then there is the colour rendering index, CRI: below 80 colours go flat and skin looks ill, above 90 everything looks right. In the kitchen, in the bathroom and in front of a mirror it is the difference you notice more than quantity.

Nearby tools

The room dimensions come from Room area and volume, the consumption of the bulbs from Electricity cost. For the rest of the wiring there are Resistor colour codes and Ohm's law.