Illuminance converter

Lux and lumens are not the same thing

This is the commonest confusion when buying a bulb, and the one that leads people to get a room's lighting wrong. Lumens say how much light leaves the bulb in every direction; lux says how much light actually reaches a surface, that is lumens per square metre. The same 1000-lumen bulb makes plenty of lux on a table close by and very little on a floor three metres away, because light spreads out: double the distance and the illuminance drops to a quarter.

How much lux you actually need

The practical references, the ones used in workplace lighting standards too: a corridor or a staircase gets by on 100-150 lux; a living room or a passing-through area is fine at about 200; a desk, a kitchen worktop or a workbench wants 300-500; technical drawing, sewing and precision work go up to 750-1000.

To see how small those numbers are, compare them with outdoors: an overcast day is already around 1000-2000 lux and full sun reaches a hundred thousand. It's why a room that looks bright by day turns out dark in the evening with the same lamps on.

The foot-candle, and how to convert it in your head

In English-language documents and on American equipment illuminance is measured in foot-candles, that is lumens per square foot: 1 fc ≈ 10.76 lux. For mental arithmetic just multiply by ten and add a touch: 50 fc is a little over 500 lux. The phot and the nox, which turn up in some old manuals, are units of the centimetre-gram-second system: 1 phot is 10,000 lux and 1 nox a thousandth of a lux.

Nearby tools

To know how many lumens a whole room needs, starting from the square metres, the page is Lumens for a room. For what the lamps cost to run there is Electricity cost, and for colour temperature and colours in general Colour converter.