Illuminance Converter

Frequently Asked Questions

What is illuminance?

Light falling on a surface, measured in lux (lumen per m²) or foot-candle (lumen per ft²). 1 fc = 10.7639 lux. The converter pivots through lux.

What are typical lux levels?

Moonlight ~0.25 lux, living room ~50, office ~300–500, overcast day ~1,000, direct sunlight ~32,000–100,000 lux.

Is illuminance the same as brightness?

No - illuminance is light arriving on a surface; luminance (cd/m²) is light leaving a surface toward the eye. They are not interconvertible without geometry.

Why does photography use foot-candles?

Cinematography and architectural lighting in the US still spec foot-candles; this tool maps them to the SI lux used elsewhere.

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Estimates for informational purposes only.

Important Disclaimer: Estimates for informational purposes only.

This calculator provides estimates for informational purposes only. Results are based on assumptions and may not reflect actual outcomes. Consult qualified professionals in relevant fields before making important decisions based on these results.

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How This Calculator Works

Illuminance measures how much luminous flux (light, weighted for human vision) lands on a surface per unit area. The SI base unit is the lux (lx), defined as one lumen per square metre - so lumen/m² and lux are numerically identical. Every other unit here is a fixed multiple of lux, and conversion pivots through that base: result = amount × (luxPerFrom ÷ luxPerTo). The factors: 1 foot-candle = 1 lumen per square foot = 10.7639104 lux (since one square metre is 10.7639104 ft²); 1 phot = 1 lumen per square centimetre = 10,000 lux; 1 kilolux = 1000 lux; and 1 nox = 0.001 lux, a unit historically used for very low (twilight) light levels. Because all are linear, the pivot is exact. Note illuminance differs from luminance (cd/m², brightness of a surface) and from luminous flux (lumens, total light output) - this tool converts surface illumination only.

Worked Example

A US lighting plan specifies 50 foot-candles on a desk and your meter reads lux. Step 1: 50 × 10.7639104 = 538.2 lux. Step 2 (already lux): you need about 538 lx, comfortably above the ~500 lx office recommendation. Reverse it: a workshop measuring 750 lux equals 750 ÷ 10.7639104 ≈ 69.7 fc. A photometry rig reading 2 phot is 2 × 10,000 = 20,000 lux - bright overcast daylight. Twilight at 5 nox is just 5 × 0.001 = 0.005 lux.

Where You Use This

Lighting designers and electricians convert code requirements between foot-candles (US) and lux (metric/IES & EN standards). Photographers and cinematographers reason about scene illuminance in lux or foot-candles. Architects verify daylighting against lux targets. Horticulturists and indoor-growers check canopy lux. Occupational-safety inspectors measure workplace illuminance against minimum lux thresholds.

Common Mistakes

Illuminance vs luminous flux: lux is light per area; lumens is total output. A bright bulb in a huge room can still give low lux.

Illuminance vs luminance: lux (incident light) is not cd/m² (surface brightness/nits); they are different photometric quantities.

fc factor rounding: 1 fc ≈ 10.76 lux, not 10; using 10 underestimates by ~7%.

phot scale: a phot is 10,000 lux - mistaking it for lux understates light by four orders of magnitude.

Distance ignored: illuminance falls off with the inverse square of distance; a single conversion does not account for moving the meter.

Frequently Asked Questions

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