Convert electrical watts to illuminance (lux) or calculate watts needed for target lux. Supports 10 light sources, utilization/maintenance factors, lux scale visual, and source comparison table.
Converting watts to lux requires knowing the luminous efficacy of the light source (lumens per watt), the room area, and how efficiently light reaches the work surface. The lumen method — the standard approach for indoor lighting design — calculates illuminance as: Lux = (Total Lumens × Utilization Factor × Maintenance Factor) / Area.
Different light sources produce vastly different amounts of visible light per watt. An incandescent bulb converts only 5% of electrical energy to light (~15 lm/W). Modern LEDs achieve 90-130 lm/W — nearly 10× more efficient. High-pressure sodium lamps reach 120 lm/W but with poor color rendering (CRI 25). The right source for an application balances efficacy, color quality, and cost.
This calculator works in both directions: compute lux from a given wattage, or calculate the watts needed to achieve a target illuminance level. It accounts for utilization factor (how much light actually reaches the work plane) and maintenance factor (light loss from aging and dirt accumulation). An illuminance scale puts your result in context from moonlight to direct sunlight, and a comparison table shows how different sources compare for the same lux target.
Lighting design affects productivity, comfort, safety, and energy cost. Under-lit spaces strain eyes; over-lit spaces waste electricity. The lumen method is the standard calculation but involves multiple steps and lookup tables. This calculator automates the process and compares light sources so you can make informed decisions. Keep these notes focused on your operational context.
Lux = (W × η × CU × MF) / A Watts = (Lux × A) / (η × CU × MF) Where: η = luminous efficacy (lumens/watt) CU = coefficient of utilization (0–1) MF = maintenance factor (0–1) A = area (m²) 1 lux = 1 lumen/m² 1 foot-candle = 10.764 lux
Result: 264 lux (24.5 fc)
A 100 W LED neutral fixture produces 100 × 110 = 11,000 lumens. With CU = 0.6 and MF = 0.8, effective lumens = 11,000 × 0.6 × 0.8 = 5,280. Over 20 m²: 5,280/20 = 264 lux. This is adequate for a living room but below the 500 lux recommended for offices.
The lumen method (also called zonal cavity method) divides a room into three cavities: ceiling, room, and floor. Effective reflectances are calculated for each cavity, then used with CU tables published by fixture manufacturers. The total lumens needed = (Lux × Area) / (CU × MF). This method is accurate enough for uniform general lighting but not for task or accent lighting, which requires point-by-point calculation software.
Modern energy codes (ASHRAE 90.1, IECC, Title 24) limit lighting power density (LPD) in W/m² by building type. Offices: 8.5-10 W/m². Retail: 10-14 W/m². Warehouses: 5-7 W/m². Compliance typically requires LED lighting. Some codes also require daylight-responsive dimming, occupancy sensors, and automatic shutoff — measures that reduce actual energy use below the installed LPD.
Research shows that light affects circadian rhythms, alertness, and mood beyond just visibility. Tunable white LED systems adjust color temperature from warm (2700K) in the evening to cool (5000K) during the day, mimicking natural daylight. Illuminance recommendations are evolving: standards like EN 12464-1 now include recommendations for melanopic illuminance (the component that affects circadian biology), not just photopic lux.
Lumens measure total light output. Lux measures illuminance (lumens per square meter) at a surface. A 1,000 lm lamp produces 1,000 lux on 1 m², 500 lux on 2 m², 100 lux on 10 m². Lux tells you how bright the surface is; lumens tell you how bright the source is.
It depends on the source. For LED lighting at 500 lux: roughly 5-8 W per m² of floor area. For incandescent: 25-40 W/m². Always calculate with the lumen method for accurate results.
Foot-candles (fc) are the imperial illuminance unit: 1 fc = 1 lumen per square foot = 10.764 lux. US lighting standards often specify foot-candles. Multiply fc by 10.764 to get lux.
Not all light from a fixture reaches the work surface. Light hits walls, ceiling, and furniture; dark surfaces absorb more. In a worst case (dark room, indirect lighting), only 30% of lumens reach the desk — the CU is 0.3. Direct downlights in a light room might achieve 0.7.
LED: 90-130 lm/W. CFL: 60 lm/W. Halogen: 20 lm/W. Incandescent: 15 lm/W. HPS: 120 lm/W (but CRI 25). LEDs offer the best combination of efficacy and color quality for most applications.
Color Rendering Index (0-100) measures how accurately a light source renders colors compared to sunlight (CRI 100). CRI > 80 is acceptable for offices. CRI > 90 is preferred for retail, art, and medical. High-pressure sodium (CRI 25) makes everything look orange — fine for parking lots but terrible for color discrimination.