Lux and Foot-Candles: How Bright Is Bright Enough?

Illuminance is the answer to "how bright is it where I'm standing?" — the light actually landing on a surface, measured in lux (lumens per square meter) or, in the US, foot-candles (lumens per square foot). The two differ by one exact factor, and the scale they span runs from a moonless night to a desert noon: a factor of a million.

One factor: the square of the foot

A foot-candle is one lumen spread over a square foot; a lux is one lumen spread over a square meter. Since the international foot is exactly 0.3048 meters, the ratio is just the square of that:

1 ft²   = 0.09290304 m²
1 fc    = 10.7639104167 lx   (exact)
1 lx    = 0.09290304 fc

For mental arithmetic, multiply foot-candles by 10 to approximate lux — a 46 fc desk is 500 lux. The older CGS unit, the phot, is one lumen per square centimeter, exactly 10,000 lux, and survives in old photography literature; a lumen per square inch is 1,550.0031 lux. The Illuminance Converter handles all six units, keeping small readings like moonlight meaningful rather than rounding them away.

The brightness scale from moonlight to sunlight

The illuminance numbers of everyday life are worth memorizing, because they calibrate everything else:

Direct sunlight      30,000 – 100,000 lx  (3,000 – 9,300 fc)
Overcast day          1,000 – 10,000 lx
Bright office             ~500 lx         (~46 fc)
Corridor / stairs         100 – 150 lx
Streetlight pool           10 – 30 lx
Full moon                   ~0.2 lx       (~0.02 fc)

Sun to moon is nearly a factor of a million, and human vision adapts across all of it — which is why a camera phone struggles at dusk while your eye does not, and why a room can feel fine by eye while measuring an order of magnitude off spec. The moon's own contribution to the bottom of that table depends on its phase, which the Moon Phase Calculator tracks: a quarter moon delivers about a tenth of the full moon's 0.2 lux.

Lux is not lumens

The most common mix-up in lighting retail is reading a lumen rating where a lux reading is needed. Lumens measure the total light a source emits; lux measures how densely it lands. The link is area:

lux = lumens ÷ area (m²)

A 1,000-lumen bulb filling 4 m² of wall gives about 250 lux there; the same bulb focused into 0.5 m² on a desk gives 2,000 lux at that desk — five times brighter work lighting from the identical bulb. Beam angle and distance decide the geometry, the lumens decide the budget, and only the lux number at the work surface tells you whether a task is lit. This division-by-area is also why lux instruments are just light meters with a cosine-corrected surface of known area.

How bright is bright enough?

Lighting standards translate tasks into target lux levels. EN 12464-1, the European workplace standard, and the IES levels it mirrors in the US put offices at about 500 lux (46 fc) on desks, corridors at 100–150 lux, warehouses at 150–300 lux, and precision work — drafting, inspection, jewelry — at 750 to 1,500 lux. Kitchens and bathrooms land around 300–500 lux at counter height. Indoor farming uses the same instrument with different targets: leafy greens do well at 5,000–10,000 lux for 12–16 hours a day, while fruiting plants such as tomatoes want sunlight-class light, 30,000 lux and up, which is exactly why grow lights are rated in lux or micromoles rather than lumens on the box.

Why two units at all

Lux and foot-candle split along the same seam as every metric-imperial pair: a light meter priced for one market reads the other's unit, and a fixture spec crosses the Atlantic untranslated. The mechanics of that seam — and why the foot itself is now exactly 0.3048 meters, making this one of the rare conversions that is truly exact rather than agreed — are covered in Why Two Measurement Systems. The area side of the division (square meters to square feet) is the Area Converter's job, and it pairs naturally with this one: specify lumens and area, get lux; specify lux and area, get lumens.

Try it yourself

Three checks with the Illuminance Converter. First, enter 500 lux and confirm 46.45 fc — the office standard in both dialects. Second, enter 100,000 lux and read 9,290 fc, full sun in imperial dress. Third, enter 0.2 lux and see 0.019 fc — then check tonight's moon phase and decide whether your stargazing site will even reach that number. With those anchors, a cheap lux meter and one converter, you can audit every room in the house against its standard.