What Is an Illuminance Unit Calculator?
Illuminance measures how much light falls on a given surface area. This calculator converts between the two units used to express it worldwide: lux (the metric/SI unit) and foot-candles (the imperial/US unit), so you can compare readings from a light meter, a lighting spec sheet, or a photography app regardless of which unit they use.
Lighting designers, photographers, plant growers, and workplace safety inspectors all rely on illuminance targets — and those targets are sometimes published in lux and sometimes in foot-candles, which is where this converter comes in handy.
How to Read Your Results
Lux (lx)
Lux measures lumens per square meter. It's the standard unit almost everywhere outside the United States, and the one most light meters default to.
Foot-candles (fc)
A foot-candle measures lumens per square foot. It remains common in US building codes, horticultural lighting guides, and older photographic light meters.
Conversion Factor Used
This shows the fixed ratio applied: 1 foot-candle equals 10.764 lux, because a square meter contains 10.764 square feet.
Real-World Example
A workplace safety standard calls for a minimum of 50 foot-candles of task lighting on a factory floor. A facilities manager using a lux meter needs to know the equivalent target.
| Unit | Value |
|---|---|
| Foot-candles | 50 fc |
| Lux | 538.2 lx |
Multiplying 50 fc by 10.764 gives 538.2 lux — so the meter should read at least about 538 lux to meet the 50 foot-candle requirement.
Tips for Working With Illuminance
- Match the unit to your meter's display before comparing to a published spec, since mixing them up leads to readings that look 10x off.
- Typical reference points: overcast daylight ≈ 1,000 lux (93 fc), office lighting ≈ 300–500 lux (28–46 fc), moonlight ≈ 0.1–0.3 lux.
- Illuminance is not the same as luminance or brightness — it measures incoming light on a surface, not light emitted or reflected toward a viewer.
- Take readings at the working surface, not at the light source, for accurate task-lighting comparisons.
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Last updated: August 2026 · Reviewed by: Simple Calculator Tools Editorial Team