Lux to Watts Calculator
Calculate the watts needed to achieve a target lux level over a given area. Essential for lighting design, energy compliance, and estimating power consumption for any light level.
Typical rooms: bedroom 10–15 m², office 30–50 m², warehouse 500+ m²
Power (Watts)
50 W
Lumens needed
5,000 lm
Footcandles
46.451 fc
Formula
W = (Lux × Area) / Efficacy = (500 × 10) / 100
Typical Illuminance Levels
About the Lux to Watts Calculator
A lux to watts calculator determines how much electrical power is needed to achieve a target illuminance level over a given area, combining the lux-to-lumens conversion with the luminous efficacy of the chosen bulb type. This combined calculation answers the complete lighting design question: "How many watts of LED (or fluorescent, or halogen) lighting do I need to achieve 300 lux in my 20 m² office?" The answer depends on both the area to be illuminated and the efficiency of the light source chosen. Our calculator chains together two steps: first converting lux and area to required lumens (lm = lux × m²), then dividing by efficacy to find watts (W = lm / efficacy). The efficacy selector shows values for LED (100-140 lm/W), CFL (60 lm/W), fluorescent (90 lm/W), halogen (20 lm/W), and incandescent (15 lm/W), enabling direct comparison of technology options. Used by lighting designers, electrical engineers, architects, and building managers determining lighting power loads for energy compliance calculations under ASHRAE 90.1, Part L of UK Building Regulations, and equivalent standards.
Formula
W = (Lux × Area) / Efficacy | LPD (W/m²) = W / Area | ASHRAE limit: 8.8 W/m² (offices)
How It Works
Step 1: lm = Lux × Area (m²). Step 2: W = lm / Efficacy (lm/W). Combined: W = (Lux × Area) / Efficacy. Example: 300 lux target in a 25 m² office. Step 1: lm = 300 × 25 = 7,500 lumens. Step 2 options: LED (100 lm/W): W = 7,500/100 = 75W total LED power. CFL (60 lm/W): W = 7,500/60 = 125W. Halogen (20 lm/W): W = 7,500/20 = 375W. Incandescent (15 lm/W): W = 7,500/15 = 500W. The LED option uses 85% less power than incandescent for the same lux level. Practical design notes: divide theoretical watts by utilization factor (0.5-0.8) for real-world estimates. Lighting power density (LPD) = W / Area = 75 / 25 = 3 W/m² for LED. ASHRAE 90.1-2019 allows up to 8.8 W/m² for offices — LED easily complies at 3 W/m².
Tips & Best Practices
- ✓Lighting Power Density compliance: ASHRAE 90.1 specifies maximum LPD values for building spaces. With LED technology, modern designs typically achieve 2-4 W/m² for office spaces — well below the 8.8 W/m² limit. The lux-to-watts calculator confirms whether your design will comply.
- ✓UK Part L Building Regulations limit installed lighting efficacy to a minimum of 75 lm/W for new buildings. Using the calculator with efficacy below 75 lm/W identifies non-compliant designs before specification is finalised.
- ✓Australian NCC Section J energy requirements specify minimum average lamp efficacy by space type: offices require minimum 80 lm/W. The calculation confirms that LED lighting at 100+ lm/W far exceeds this requirement while fluorescent at 90 lm/W just meets it.
Who Uses This Calculator
Architects sizing electrical panels and specifying lighting loads for permit applications. Lighting designers selecting between technologies based on energy compliance requirements. Building managers comparing the wattage reduction achievable by switching from fluorescent to LED. Energy consultants calculating energy savings from LED retrofits based on target illuminance maintained unchanged.
Optimised for: USA · UK · Canada · Australia · Calculations run in your browser · No data stored
Frequently Asked Questions
How do I calculate watts from lux?
W = (Lux × Area) / Efficacy. First find lumens needed: lm = lux × m². Then divide by efficacy: W = lm / (lm/W). Example: 300 lux in 20 m² with LED (100 lm/W): W = (300 × 20) / 100 = 60W total LED power needed.
What is the lighting power density (LPD) limit for offices?
ASHRAE 90.1 (US energy code) limits office LPD to 0.82 W/ft² (8.8 W/m²). A 2,000 ft² office can use max 1,640W of lighting. At 300 lux requirement: 0.82 W/ft² × 100 lm/W = 82 lm/ft² ≈ 882 lux/100 ft — more than enough for 300 lux target.