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Illuminance Converter (lux, foot-candles)

Type a value — the result and every other unit update as you type.

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Understanding the illuminance converter — concept and principle

Illuminance is the amount of visible light falling on a surface per unit area. This converter switches between lux (lx), the SI unit, foot-candles (fc), still used in US lighting design and photography, the CGS unit phot (ph) and kilolux. It helps when comparing lighting recommendations, light meter readings or grow-light specifications written in different units.

Foot-candles and luxA foot-candle is one lumen per square foot and a lux is one lumen per square metre. One square metre is 10.764 square feet, so 50 fc = 50 × 10.764 = 538.2 lx.A square metre holds 10.76 square feet1 ft²1 m²1 fc = 1 lumen on 1 ft²1 lx = 1 lumen on 1 m²so 1 fc = 10.764 lx50 fc × 10.764= 538.2 lxoffice desk: 300–500 lxsunlight: ~100,000 lx50 foot-candles = 538.2 luxsame light, smaller unit area
One foot-candle equals 10.764 lux, so 50 fc is 538.2 lx — a well-lit desk.

How it is calculated

Every unit is converted to lux and then to the target unit: result = value × (lx per source unit) ÷ (lx per target unit).

The principle

All three units measure the same thing, luminous flux in lumens spread over an area; they differ only in the area unit. One square foot is exactly 0.3048² = 0.09290304 m², so one lumen spread over a square foot is concentrated 1 ÷ 0.09290304 = 10.7639 times more than over a square metre. A square centimetre is 1/10,000 of a square metre, which gives the phot factor. The lumen itself is weighted by the sensitivity of the human eye, so illuminance describes brightness as people perceive it, not raw radiant power.

Worked example

  1. Inputs: 50 fc, convert to lux.
  2. 1 fc = 1 ÷ 0.09290304 = 10.76391042 lx.
  3. 50 × 10.76391042 = 538.1955208 lx.
  4. In phot: 538.1955208 ÷ 10,000 = 0.0538195521 ph; in kilolux: 0.5381955208 klx.

How to read the result

538 lx is a typical level for office desks. As benchmarks, the European workplace lighting standard recommends 500 lx for writing, reading and computer work; corridors need far less, while detailed inspection tasks need more. A bright overcast day outdoors gives several thousand lux and direct summer sun can reach around 100,000 lx, while a full moon gives well under 1 lx. Because the eye adapts, a room that feels bright can be tens of times dimmer than daylight.

Common mistakes

Keep in mind: Lighting levels in standards depend on the task, the users and the country. For workplace, school or safety lighting, follow the applicable local standard and consult a lighting professional.

Frequently asked questions

How do I convert lux to foot-candles?

Divide by 10.76391. For example, 500 lx ≈ 46.5 fc.

How many lux do plants need?

It varies widely; plant growers often use PPFD (µmol/m²/s) instead, because lux is weighted for human vision.

Can a phone app measure lux accurately?

Phone sensors give a rough indication, but they are not calibrated like a proper light meter.

Sources: BIPM, The International System of Units (SI Brochure), 9th edition · EN 12464-1, Light and lighting: Lighting of work places, Part 1: Indoor work places · NIST Special Publication 811 (2008), Appendix B

Checked 8 Nov 2026. Factors from the SI Brochure (BIPM, 9th ed.) and NIST Special Publication 811 (2008), Appendix B.

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