Illuminance Converter
Convert lux, foot-candles, phot, nox and more — instantly
Instantly convert between all major illuminance units including lux, foot-candle, phot, nox, kilolux and millilux. Used by lighting designers, photographers, and engineers worldwide.
What Is Illuminance?
Illuminance is the measure of how much luminous flux (light energy visible to the human eye) falls on a given surface area. It describes how well-lit a surface is, independent of the intrinsic brightness of the light source itself. The SI unit of illuminance is the lux (lx), defined as one lumen per square metre (lm/m²).
Unlike luminance — which measures the brightness perceived when looking at a surface — illuminance measures the light arriving at the surface. This makes it the key metric in architectural lighting design, workplace safety regulations, photography exposure calculations, and plant growth lighting.
Illuminance follows the inverse-square law: doubling the distance between a point source and a surface reduces illuminance to one quarter of its original value. This is why illuminance matters so much in practical lighting: small changes in fixture height or distance produce large changes in the light level experienced at work-surface height.
How to Use the Illuminance Converter
- Enter your value — Type the illuminance number you want to convert into the input field on the left (or top on mobile).
- Select the source unit — Use the "From" dropdown to choose your current unit: lux, foot-candle, phot, nox, kilolux, or millilux.
- Select the target unit — Use the "To" dropdown to pick the unit you need the result in.
- Read the result instantly — The converted value appears immediately in the result field. The formula multiplier is also shown below the result.
- Copy or Share — Use the Copy button to copy the result to your clipboard, or Share to send the current page link.
Illuminance Conversion Formulas
All illuminance units convert through their relationship to lux (the SI base unit). Each unit has a fixed factor relative to lux:
| Unit | Symbol | Factor (= X lux) |
|---|---|---|
| Lux | lx | 1 |
| Foot-candle | fc | 10.7639 |
| Phot | ph | 10,000 |
| Nox | nx | 0.001 |
| Kilolux | klx | 1,000 |
| Millilux | mlx | 0.001 |
To convert from unit A to unit B: result = value × factor(A) / factor(B). For example, to convert 500 lux to foot-candles: 500 × 1 / 10.7639 ≈ 46.45 fc.
Common Illuminance Reference Values
| Light Source / Setting | Approx. Lux | Foot-candles |
|---|---|---|
| Direct midday sunlight | 100,000 lx | 9,290 fc |
| Full daylight (overcast) | 10,000–25,000 lx | 930–2,320 fc |
| TV studio lighting | 1,000 lx | 93 fc |
| Office (detailed work) | 500–750 lx | 46–70 fc |
| Office (general) | 300–500 lx | 28–46 fc |
| Corridor / hallway | 50–100 lx | 4.6–9.3 fc |
| Full moon | ~1 lx | 0.093 fc |
| Starlit night | 0.001 lx | 0.0001 fc |
Worked Examples
Example 1 — Lux to foot-candles: A photography studio measures 800 lux at the subject. To convert to foot-candles (used by US exposure guides): 800 ÷ 10.7639 ≈ 74.3 fc. This is a well-lit professional studio environment.
Example 2 — Foot-candles to lux: An American building code requires 50 fc in a reading room. To convert for an international specification: 50 × 10.7639 = 538.2 lux. This aligns with the ISO standard of 500 lux for reading tasks.
Example 3 — Kilolux to lux: A grow-light system outputs 45 klx at canopy level. In lux that is 45 × 1,000 = 45,000 lux — comparable to bright outdoor conditions and ideal for high-light-demand crops like tomatoes or cannabis.
Practical Tips for Illuminance
- Use lux for international work. Lux is the SI standard and is used in ISO, EN, and most national standards outside the US.
- Use foot-candles for US projects. American architects and electrical engineers typically specify illuminance in foot-candles (fc).
- Measure at the task surface. Standards specify lux at the working plane (desk height ~0.75 m), not at floor level or ceiling level.
- Account for uniformity. A single average lux reading is not enough — standards also require a minimum uniformity ratio (min/avg) to avoid harsh shadows.
- Lux meters are inexpensive. A good smartphone light-meter app or a dedicated lux meter (under $30) can verify whether a space meets lighting standards.
- LED vs fluorescent. Comparing LED and fluorescent fixtures should be done by lux at the work surface, not by wattage — LEDs deliver more lux per watt.
- Plant lighting. Most horticultural references use PPFD (µmol/m²/s) rather than lux, but for broad estimates, 1,000 lux ≈ 15–20 µmol/m²/s for white LED.
Frequently Asked Questions
Illuminance measures the total luminous flux incident on a surface per unit area. The SI unit is lux (lx), equal to one lumen per square metre. It quantifies how brightly a surface is lit, not the intrinsic brightness of a light source.
One lux equals one lumen of light spread over one square metre of surface. It is the SI standard for measuring illuminance and is widely used in lighting design, photography, and safety regulations.
Divide the lux value by 10.764 to get foot-candles. For example, 500 lux ÷ 10.764 ≈ 46.4 foot-candles. To go the other way, multiply foot-candles by 10.764 to get lux.
Recommended office illuminance is 300–500 lux for general work areas, 500–750 lux for tasks requiring visual precision, and 750–1,000 lux for drafting or inspection work.
Direct sunlight measures approximately 100,000 lux at noon. Overcast daylight is roughly 10,000–25,000 lux. Indoor lighting is typically 100–1,000 lux, and a full moon provides about 1 lux.
Illuminance (lux) measures light falling onto a surface, while luminance (cd/m²) measures light leaving or reflected from a surface toward an observer. Illuminance is about the incoming light; luminance is about what you actually see.
A phot (ph) is a CGS unit equal to 10,000 lux, used historically in scientific contexts. A nox (nx) is an old unit equal to 0.001 lux, originally used to measure very low nighttime illuminance levels. Both are largely obsolete but still appear in older technical literature.