💡 Watts to Lux Calculator
Convert electrical power to illuminance (lux)
Typical values: Incandescent 10-17, Halogen 15-25, Fluorescent 50-100, LED 80-200 lm/W
Distance from the light source to the illuminated surface
How to Use This Calculator
Select Calculation Method
Choose to calculate using distance from light source (for point sources) or illuminated area (for uniform illumination over a surface).
Enter Power and Light Source
Input electrical power in watts. Select light source type or enter custom luminous efficacy in lumens per watt (lm/W). Different light sources convert electrical power to light with different efficiencies.
Enter Distance or Area
For distance method, enter distance from light source in meters. For area method, enter total illuminated area in square meters.
Calculate
Click "Calculate Lux" to get illuminance in lux. Lux measures light intensity on a surface (1 lux = 1 lumen per square meter).
Formula
Φ = P × η
Luminous Flux
E = Φ / (4π × d²)
From Distance (Point Source)
E = Φ / A
From Area
Where:
- E = Illuminance (Lux, lx) = lumens per square meter
- Φ = Luminous Flux (Lumens, lm)
- P = Electrical Power (Watts, W)
- η = Luminous Efficacy (Lumens per Watt, lm/W)
- d = Distance from Source (meters, m)
- A = Illuminated Area (square meters, m²)
Example Calculation 1 (Distance Method):
For P = 100 W, η = 120 lm/W, d = 2 m:
Φ = 100 × 120 = 12,000 lm
E = 12,000 / (4π × 2²) = 12,000 / 50.27 = 238.7 lux
A 100W LED at 2m distance provides approximately 239 lux of illumination.
Example Calculation 2 (Area Method):
For P = 60 W, η = 60 lm/W, A = 10 m²:
Φ = 60 × 60 = 3,600 lm
E = 3,600 / 10 = 360 lux
A 60W fluorescent lamp illuminating 10 m² provides 360 lux.
Example Calculation 3 (Incandescent):
For P = 100 W, η = 15 lm/W, d = 1.5 m:
Φ = 100 × 15 = 1,500 lm
E = 1,500 / (4π × 1.5²) = 1,500 / 28.27 = 53.1 lux
A 100W incandescent bulb at 1.5m provides much less lux than LED due to lower efficacy.
About Watts to Lux Calculator
The Watts to Lux Calculator converts electrical power consumption to illuminance (lux), which measures light intensity on a surface. The conversion involves two steps: (1) Calculate luminous flux from power using luminous efficacy (Φ = P × η), and (2) Calculate illuminance from luminous flux using distance or area. Different light sources have vastly different luminous efficacies, significantly affecting the lux output for the same electrical power.
When to Use This Calculator
- Lighting Design: Determine illuminance from electrical power requirements
- Energy Efficiency: Compare lighting efficiency of different light sources
- Lighting Planning: Calculate required power for desired illuminance levels
- Retrofit Analysis: Compare old vs. new lighting systems
- Educational: Understand luminous efficacy and illuminance relationships
Why Use Our Calculator?
- ✅ Light Source Database: Pre-filled luminous efficacy for common sources
- ✅ Dual Methods: Calculate from distance or area
- ✅ Quick Conversion: Instantly convert watts to lux
- ✅ Free Tool: No registration or payment required
- ✅ Educational: Learn lighting efficiency and illuminance
Common Applications
LED Retrofit Planning: Calculate illuminance when replacing incandescent or fluorescent lights with LEDs. A 100W incandescent (15 lm/W) produces 1,500 lm, while a 15W LED (120 lm/W) produces 1,800 lm - same or better light with 85% less power! At 2m distance, LED provides ~36 lux vs. ~30 lux for incandescent, demonstrating LED efficiency.
Workspace Lighting Design: Design adequate lighting for workspaces. Office lighting typically requires 300-500 lux. For a 20 m² office with fluorescent lights (60 lm/W), achieving 400 lux requires: Φ = 400 × 20 = 8,000 lm. At 60 lm/W, this needs 133W of fluorescent lighting. This calculator helps size lighting systems.
Outdoor Lighting: Calculate illuminance for outdoor spaces. Street lighting may require 10-30 lux. For a 100W LED (120 lm/W) at 5m height: Φ = 12,000 lm, E = 12,000 / (4π × 5²) = 38 lux at ground level. This helps design efficient outdoor lighting installations.
Tips for Best Results
- Use actual luminous efficacy from manufacturer specifications
- Distance method assumes point source - use area method for extended sources
- Typical illuminance: Office 300-500 lux, Reading 500 lux, Workshop 750 lux
- LED efficacy ranges: Standard 80-120 lm/W, High-performance 150-200+ lm/W
- Account for light losses (fixtures, aging, dirt) - reduce by 20-30%
Frequently Asked Questions
What is luminous efficacy?
Luminous efficacy (η) measures how efficiently a light source converts electrical power to visible light, expressed in lumens per watt (lm/W). Higher efficacy means more light per watt. Incandescent: 10-17 lm/W, Halogen: 15-25, Fluorescent: 50-100, LED: 80-200+ lm/W. LEDs are most efficient, converting more electrical power to visible light.
What is the difference between lux and lumens?
Lumens measure total light output (luminous flux) from a source. Lux measures light intensity on a surface (illuminance). 1 lux = 1 lumen per square meter. A 1,000 lm source provides 1,000 lux on 1 m², 100 lux on 10 m², or 1 lux on 1,000 m². Lux depends on distance and surface area.
When should I use distance vs. area method?
Use distance method for point sources (small bulbs, spotlights) where light spreads in all directions. Use area method when you know the total illuminated area and light is distributed uniformly. For most indoor lighting with fixtures, area method is more practical. Distance method is theoretical and assumes isotropic (equal in all directions) light emission.
Why do LEDs use less power for the same light?
LEDs have much higher luminous efficacy than incandescent or fluorescent lights. A 100W incandescent (15 lm/W) produces 1,500 lm. A 12.5W LED (120 lm/W) produces the same 1,500 lm with 87.5% less power! Higher efficacy means less electrical power needed for the same luminous output, reducing energy costs and heat generation.
What is a typical illuminance level?
Typical illuminance levels: Outdoor (moonlight) 0.1-1 lux, Ambient indoor 100-200 lux, Office work 300-500 lux, Detailed work 500-1,000 lux, Surgical operating 10,000+ lux. Standards (IES, EN) specify minimum illuminance for different tasks. This calculator helps ensure adequate lighting levels are achieved.