Watts to Lux Calculator

Convert electrical power to illuminance using luminous efficacy, distance, and area calculations.

Calculate illuminance in lux from electrical power in watts. Consider luminous efficacy, distance, angle, and area for accurate lighting measurements.

Examples

Click on any example to load it into the calculator.

LED Light Bulb

LED Light Bulb

A 15W LED bulb with 120 lm/W efficacy at 2m distance.

Power: 15 W

Luminous Efficacy: 120 lm/W

Distance: 2 m

Angle: 0 °

Area: 5 m²

Fluorescent Tube

Fluorescent Tube

A 40W fluorescent tube with 80 lm/W efficacy at 1.5m distance.

Power: 40 W

Luminous Efficacy: 80 lm/W

Distance: 1.5 m

Angle: 0 °

Area: 8 m²

Incandescent Bulb

Incandescent Bulb

A 100W incandescent bulb with 15 lm/W efficacy at 3m distance.

Power: 100 W

Luminous Efficacy: 15 lm/W

Distance: 3 m

Angle: 0 °

Area: 12 m²

LED Spotlight

LED Spotlight

A 50W LED spotlight with 130 lm/W efficacy at 45° angle.

Power: 50 W

Luminous Efficacy: 130 lm/W

Distance: 4 m

Angle: 45 °

Area: 6 m²

Other Titles
Understanding Watts to Lux Calculator: A Comprehensive Guide
Master the conversion from electrical power to illuminance with our comprehensive guide. Learn about luminous flux, illuminance, and the factors affecting lighting calculations.

What is the Watts to Lux Calculator?

  • Core Concepts
  • Why Lighting Calculations Matter
  • Fundamental Relationships
The Watts to Lux Calculator is an essential tool for lighting designers, engineers, and anyone working with illumination systems. It converts electrical power consumption (watts) to illuminance (lux) by considering the luminous efficacy of the light source, distance from the source, angle of incidence, and illuminated area. This conversion is crucial for designing efficient lighting systems and ensuring proper illumination levels.
The Fundamental Relationship: Power to Light
Electrical power (watts) represents the energy consumed by a light source, but not all of this energy is converted to visible light. The luminous efficacy (measured in lumens per watt) determines how efficiently the electrical power is converted to luminous flux. This efficiency varies greatly between different lighting technologies, from about 10-17 lm/W for incandescent bulbs to 100-150 lm/W for modern LEDs.
Luminous Flux: The Total Light Output
Luminous flux (measured in lumens) represents the total amount of visible light emitted by a source in all directions. It's calculated by multiplying the electrical power by the luminous efficacy: Φ = P × η, where Φ is luminous flux (lm), P is power (W), and η is luminous efficacy (lm/W). This gives us the total light output regardless of direction.
Illuminance: Light on a Surface
Illuminance (measured in lux) represents the amount of light falling on a surface per unit area. It's calculated by dividing the luminous flux by the area: E = Φ / A, where E is illuminance (lux), Φ is luminous flux (lm), and A is area (m²). However, distance and angle also affect the illuminance through the inverse square law and cosine law.

Key Lighting Units Explained:

  • Watt (W): Unit of electrical power - energy consumed by the light source
  • Lumen (lm): Unit of luminous flux - total visible light output
  • Lux (lx): Unit of illuminance - light intensity on a surface
  • Luminous Efficacy (lm/W): Efficiency of converting power to light
  • Candela (cd): Unit of luminous intensity - light in a specific direction

Step-by-Step Guide to Using the Calculator

  • Gathering Your Data
  • Inputting Values
  • Interpreting Results
Using the Watts to Lux Calculator requires understanding the characteristics of your light source and the geometry of your lighting setup. Follow these steps to get accurate illuminance calculations.
1. Determine Light Source Characteristics
Start by identifying the electrical power consumption of your light source in watts. This is typically marked on the device or can be measured with a power meter. Next, determine the luminous efficacy of your light source. This varies by technology: LED lights typically have 100-150 lm/W, fluorescent lights 60-100 lm/W, and incandescent lights only 10-17 lm/W.
2. Measure Distance and Geometry
Measure the distance from the light source to the illuminated surface in meters. If the light is not perpendicular to the surface, measure the angle between the light beam and the surface normal. For area-based calculations, measure the area of the illuminated surface in square meters.
3. Input Values and Calculate
Enter all your measured values into the calculator. The minimum required inputs are power and luminous efficacy. The calculator will use default values for distance (1m), angle (0°), and area (1m²) if not specified. Click calculate to get your illuminance results.
4. Analyze and Apply the Results
Review the calculated luminous flux and illuminance values. Compare these to recommended lighting levels for your application. Typical illuminance requirements range from 50-100 lux for general ambient lighting to 500-1000 lux for detailed work areas.

Common Calculation Scenarios:

  • Power + Efficacy → Luminous Flux
  • Power + Efficacy + Distance → Illuminance at Distance
  • Power + Efficacy + Distance + Angle → Angular Illuminance
  • Power + Efficacy + Area → Area-Based Illuminance
  • Complete Setup → Comprehensive Lighting Analysis

Real-World Applications and Practical Uses

  • Lighting Design
  • Energy Efficiency
  • Safety and Compliance
The Watts to Lux Calculator has numerous practical applications in lighting design, energy management, and regulatory compliance. Understanding the relationship between power consumption and illuminance helps optimize lighting systems for both performance and efficiency.
Architectural and Interior Lighting Design
Lighting designers use these calculations to determine the number and placement of light fixtures needed to achieve desired illuminance levels. For example, an office space might require 500 lux for general work, while a surgical suite might need 1000+ lux. The calculator helps determine if a 100W LED fixture with 120 lm/W efficacy at 3m distance will provide adequate illumination.
Energy Efficiency and Cost Analysis
Compare the efficiency of different lighting technologies by calculating the illuminance per watt consumed. A 15W LED bulb with 120 lm/W efficacy produces 1800 lumens, while a 100W incandescent bulb with 15 lm/W efficacy produces only 1500 lumens. This efficiency difference directly impacts energy costs and environmental impact.
Safety and Regulatory Compliance
Many workplaces and public spaces have minimum illuminance requirements for safety and productivity. The calculator helps ensure compliance with these standards. For example, emergency exit lighting typically requires 1 lux minimum, while detailed inspection work might require 1000+ lux.

Typical Illuminance Requirements:

  • Emergency Lighting: 1-5 lux
  • General Ambient: 50-100 lux
  • Office Work: 300-500 lux
  • Detailed Work: 500-1000 lux
  • Precision Work: 1000-2000 lux
  • Surgical Lighting: 2000-5000 lux

Common Misconceptions and Lighting Myths

  • Power vs. Brightness
  • Distance Effects
  • Efficiency Myths
Several misconceptions exist about lighting calculations and the relationship between electrical power and illuminance. Understanding these myths helps make better lighting decisions and avoid costly mistakes.
Myth: Higher Wattage Always Means Brighter Light
While higher wattage generally means more light output, the luminous efficacy is crucial. A 20W LED bulb with 120 lm/W efficacy produces 2400 lumens, while a 100W incandescent bulb with 15 lm/W efficacy produces only 1500 lumens. The LED is both brighter and more energy-efficient.
Myth: Distance Doesn't Matter Much
Distance has a dramatic effect on illuminance due to the inverse square law. Doubling the distance reduces illuminance to one-fourth. A light source providing 1000 lux at 1m will provide only 250 lux at 2m and 111 lux at 3m. This relationship is crucial for proper fixture placement.
Myth: All LED Lights Are Equally Efficient
LED efficiency varies significantly between products. High-quality LEDs can achieve 150+ lm/W, while cheap LEDs might only achieve 80-100 lm/W. The calculator helps identify truly efficient lighting solutions by considering both power consumption and luminous efficacy.

Efficiency Comparison Examples:

  • High-Efficiency LED: 150 lm/W (15W = 2250 lumens)
  • Standard LED: 120 lm/W (15W = 1800 lumens)
  • Fluorescent: 80 lm/W (40W = 3200 lumens)
  • Incandescent: 15 lm/W (100W = 1500 lumens)
  • Halogen: 25 lm/W (50W = 1250 lumens)

Mathematical Derivation and Advanced Calculations

  • Inverse Square Law
  • Cosine Law
  • Area Integration
The mathematical foundation of illuminance calculations involves several physical laws and geometric relationships. Understanding these principles helps explain why the calculator works and enables more advanced lighting calculations.
The Inverse Square Law
The inverse square law states that illuminance decreases with the square of the distance from a point source: E = I / d², where E is illuminance, I is luminous intensity, and d is distance. This law explains why moving a light source twice as far away reduces illuminance to one-fourth. For extended sources, the relationship is more complex but still follows a similar pattern.
The Cosine Law
When light strikes a surface at an angle, the illuminance is reduced by the cosine of the angle of incidence: E = E₀ × cos(θ), where E₀ is illuminance at normal incidence and θ is the angle from normal. This is why tilted surfaces receive less light than surfaces perpendicular to the light beam.
Area Integration and Average Illuminance
For practical applications, we often need to calculate average illuminance over an area. This involves integrating the illuminance over the surface area: E_avg = (1/A) × ∫∫ E(x,y) dA. The calculator simplifies this by assuming uniform illuminance over the specified area, which is a good approximation for many practical situations.

Mathematical Examples:

  • Point Source: E = Φ / (4πd²) for isotropic emission
  • Directional Source: E = I × cos(θ) / d²
  • Area Source: E = L × Ω × cos(θ) where L is luminance
  • Multiple Sources: E_total = Σ E_i (superposition principle)
  • Reflected Light: E_reflected = E_incident × ρ where ρ is reflectance