How to use the illuminance converter
- Enter the reading from your light meter or lighting plan in Value.
- Choose the unit it is in under From.
- Choose the unit you need under To. The answer appears at once.
- Check the table for all six units, or press the swap button to go the other way.
What it does and when to use it
Illuminance is the amount of light falling on a surface. Most of the world measures it in lux. North American lighting design, photography and grow-room guides often use foot-candles. Light meters and phone apps may show either, and older books use the phot.
Common uses: checking a lighting plan written in foot-candles against a standard written in lux, setting up a desk lamp or workshop light, comparing photography light readings, or turning a horticulture guide’s foot-candle target into lux for your meter. Light colour is a separate question, covered by the mired and colour temperature converter, and the brightness of a screen or lit sign belongs in the luminance converter.
How it works
Every unit is stored as lux (lumens per square metre). The result is value × from-factor ÷ to-factor.
| Unit | Lux | Why |
|---|---|---|
| kilolux | 1,000 | SI prefix |
| foot-candle | 10.7639104167 | 1 lm ÷ 0.09290304 m² |
| phot | 10,000 | 1 lm ÷ 0.0001 m² |
| nox | 0.001 | millilux |
| lm/in² | 1,550.0031 | 1 lm ÷ 0.00064516 m² |
Because the foot is exactly 0.3048 m, the foot-candle factor is exact. All the “per area” units are just one lumen spread over a different area, so you can check them with the area converter.
Worked examples
Grow-room or shop target. 50 fc = 538.195520835 lux (lm/m²).
Office desk. 500 lux, a level often used as a target for desk work in lighting guidelines, is 46.45152 foot-candles (lm/ft²).
Outdoors. 100 kilolux, a figure often quoted for direct sunlight, equals 9,290.304 foot-candles (lm/ft²).
Limits and tips
- Meter position matters more than units. Lux changes with distance and angle, so measure on the actual work surface, facing the light.
- Phone light meters are handy but rough. Treat their readings as estimates, not certified values.
- Lumens cannot be converted to lux without knowing the area the light covers.
- Results are shown to 12 significant digits; lighting work rarely needs more than three.
Frequently asked questions
How many lux is one foot-candle?
What is the difference between lux and lumens?
Can I convert lux to PPFD for plants?
What is a phot?
Is lux the same as brightness on my screen?
Sources
- NIST Guide to the SI, Appendix B.8: Factors for units listed alphabetically — NIST, accessed 2026-10-08
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