W
Please enter a valid wattage.
Total lamp power (one or multiple fixtures)
lm/W
Please enter a valid efficacy in lm/W.
Lumens per watt — auto-filled from source
m²
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Area to calculate illuminance (lux)
Illuminance
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Sources & Methodology
Lux definition from BIPM SI units. Illuminance standards from EN 12464 and IESNA Lighting Handbook. Efficacy values from manufacturer datasheets and DOE SSL Program.
US DOE — Solid-State Lighting (SSL) Program
DOE reference for LED luminous efficacy values (lm/W), LED lifetime data, and comparisons to traditional light sources used in the source presets of this calculator.
BIPM — SI Definition of the Candela and Lumen
BIPM official definitions: 1 lux = 1 lm/m², and the candela as the SI base unit for luminous intensity, used to verify the lux conversion formula in this calculator.
Methodology: Lumens = Watts × Efficacy (lm/W). Lux = Lumens / Area (m²). For ft², convert to m² by dividing by 10.7639. Foot-candles = Lux / 10.764. This calculator assumes uniform illuminance over the area (average maintained illuminance). Real installations vary based on fixture distribution, mounting height, room reflectances, and maintenance factor. Use a utilization factor of 0.5–0.75 for directional lighting; this calculator assumes 1.0 for simplicity.
⏱ Last reviewed: April 2026
How to Convert Watts to Lux — Luminous Efficacy and Illuminance
Converting watts to lux requires two steps: first find lumens using the lamp’s luminous efficacy (lm/W), then find lux by dividing lumens by the illuminated area. Lux measures how much light actually falls on a surface, making it the key metric for lighting design and compliance with workplace standards.
The Two-Step Formula
Lumens = Watts × Efficacy (lm/W)
Lux = Lumens / Area (m²)
Example 1 — 10W LED at 100 lm/W over 10 m²:
Lumens = 10 × 100 = 1000 lm. Lux = 1000 / 10 = 100 lux
Example 2 — 60W incandescent at 13 lm/W over 10 m²:
Lumens = 60 × 13 = 780 lm. Lux = 780 / 10 = 78 lux
Conclusion: The 10W LED provides more illuminance than the 60W bulb, using 83% less electricity.
Lumens = 10 × 100 = 1000 lm. Lux = 1000 / 10 = 100 lux
Example 2 — 60W incandescent at 13 lm/W over 10 m²:
Lumens = 60 × 13 = 780 lm. Lux = 780 / 10 = 78 lux
Conclusion: The 10W LED provides more illuminance than the 60W bulb, using 83% less electricity.
Luminous Efficacy by Light Source Type
| Light Source | Efficacy (lm/W) | Typical Life (h) | Notes |
|---|---|---|---|
| Incandescent | 8–15 | 1,000 | Mostly phased out globally |
| Halogen | 15–25 | 2,000 | Better CRI than LED |
| CFL (compact fluorescent) | 45–75 | 6,000–15,000 | Slow warm-up |
| T8 fluorescent tube | 80–100 | 15,000–30,000 | Office and industrial |
| LED standard | 80–120 | 25,000–50,000 | Current mainstream |
| LED high-efficiency | 130–200 | 50,000+ | Best available 2024 |
| High-pressure sodium (HPS) | 85–150 | 16,000–24,000 | Street lighting |
| Metal halide | 65–115 | 10,000–20,000 | Sports, industrial |
Recommended Illuminance Levels (EN 12464 / IESNA)
| Application | Lux (lx) | Foot-candles (fc) |
|---|---|---|
| Residential corridor / hallway | 100 | 9.3 fc |
| Residential living room | 100–300 | 9.3–28 fc |
| Residential kitchen | 300–500 | 28–46 fc |
| Office general work | 300–500 | 28–46 fc |
| Office detailed / drawing | 500–750 | 46–70 fc |
| Retail general | 300–500 | 28–46 fc |
| Retail display / fashion | 750–1000 | 70–93 fc |
| Warehouse storage | 100–200 | 9.3–19 fc |
| Outdoor sports field | 200–500 | 19–46 fc |
| Surgical theatre | 10,000–100,000 | 929–9290 fc |
💡 Key insight: A 10W LED (1000 lm) over a 10 m² room gives 100 lux, which is sufficient for a corridor but not for reading or office work. For 500 lux in a 20 m² office at 100% utilization: 500 × 20 = 10,000 lumens needed → 10,000/100 lm/W = 100W of LED. With a realistic utilization factor of 0.65, you need 10,000/0.65 = 15,385 lm → 154W of LED. Multiple fixtures spread across the space.
Frequently Asked Questions
Two steps: (1) Lumens = Watts × Efficacy (lm/W). (2) Lux = Lumens / Area (m²). Example: 10W LED at 100 lm/W = 1000 lumens. Over 10 m²: 1000/10 = 100 lux. The conversion depends on the lamp’s efficacy — different light sources produce very different lumens per watt.
Lumens = total light output from the source. Lux = lumens falling on 1 square metre of surface. A 1000 lm bulb creates 1000 lux on a 1 m² surface, 100 lux on 10 m², or 10 lux on 100 m². Lumens is a property of the lamp; lux depends on both the lamp and the distance/area. Light levels on a desk or workplane are measured in lux.
Standard LEDs: 80–120 lm/W. High-efficiency LEDs: 130–200 lm/W. Compare to incandescent (8–15 lm/W), halogen (15–25 lm/W), CFL (45–75 lm/W), T8 fluorescent (80–100 lm/W). LEDs at 100 lm/W use 90% less electricity than incandescent for the same light output. A 10W LED replaces a 75–100W incandescent while providing equivalent or greater brightness.
EN 12464-1 and IESNA recommend 300–500 lux for general office work, 500–750 lux for detailed tasks and drawing offices, and 750–1000 lux for very fine work. For a typical 20 m² office at 500 lux: you need 10,000 lumens (before accounting for utilization factor). With LED at 100 lm/W and a utilization factor of 0.65: 10,000/0.65 = 15,385 lm → 154W of LED total.
1 foot-candle = 10.764 lux. Foot-candles = Lux / 10.764. For example, 500 lux = 46.5 foot-candles. US standards (IESNA) use foot-candles; European standards (EN 12464) use lux. An office requiring 50 fc (IESNA) = 538 lux. Lux is the SI unit; foot-candles are used in North America for architectural lighting.
Luminous efficacy = lumens / watts (lm/W). It measures how efficiently a light source converts electricity to visible light. Higher efficacy = more light per watt = lower running cost. The theoretical maximum for white light is approximately 260–300 lm/W. Current LEDs reach 150–220 lm/W. Incandescent lamps waste ~92% of input energy as heat, with only ~8% becoming visible light.
For a point source, lux follows the inverse square law: lux = candela / distance². Doubling the distance from a light source reduces lux to one-quarter. At 1m: 1000 lux. At 2m: 250 lux. At 4m: 62.5 lux. This is why task lighting positioned close to a work surface is far more effective than a high ceiling fixture. For area sources (LED panels), the falloff is less severe but still significant.
A 60W incandescent produces about 800 lumens (13 lm/W). An LED at 100 lm/W producing 800 lm needs 800/100 = 8W. A 9–10W LED is the standard replacement. It uses 83% less electricity and lasts 25–50 times longer. For 100W incandescent (1600 lm): 16W LED replacement. Always match lumens, not watts, when replacing traditional bulbs.
Direct sunlight: 100,000–130,000 lux. Overcast sky: 1,000–25,000 lux. Indoor near a window: 1,000–5,000 lux. Standard office: 300–500 lux. Full moon: 1 lux. Candle at 1m: 1 lux. The human eye can function at 1 lux but is most comfortable at 500–1000 lux. Daylight-equivalent lighting in offices (1000–10,000 lux) improves circadian rhythm and productivity.
Utilization factor (UF) = fraction of lamp lumens that actually reach the workplane. Typical values: direct LED panel 0.60–0.75, indirect uplighter 0.30–0.50, general recessed luminaire 0.55–0.70. Required lumens = target lux × area / UF. At UF 0.65 for 500 lux in 20 m²: 500 × 20 / 0.65 = 15,385 lm required, not 10,000. Always apply UF for realistic lighting designs.
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