Watts to Lux Calculator
Convert watts to lux using luminous efficacy and surface area. Free online illuminance calculator with presets for LED, CFL, halogen, and incandescent bulbs with comparison charts.
About This Calculator
The Watts to Lux Calculator converts electrical power (watts) into illuminance (lux) based on the light source's luminous efficacy and the illuminated surface area. This tool helps electricians, lighting designers, architects, and homeowners determine the brightness a specific bulb will provide in a given space, or conversely, how many watts are needed to achieve a target illuminance level.
How the Conversion Works
Illuminance in lux is calculated using the formula: Ev = P × η / A, where Ev is illuminance in lux, P is power in watts, η is luminous efficacy in lumens per watt, and A is surface area in square meters. First, we multiply wattage by luminous efficacy to find total luminous flux in lumens. Then dividing by area gives the illuminance — the light intensity falling on each square meter of the surface.
Light Source Luminous Efficacy
Different light sources convert electricity to visible light with varying efficiency. LED bulbs achieve 80–120 lm/W, making them the most efficient. CFL bulbs range from 45–75 lm/W, halogen from 16–30 lm/W, and traditional incandescent bulbs from 12–18 lm/W. The calculator includes presets for all major bulb types and allows custom efficacy values for specialized sources.
Recommended Illuminance Levels
Living spaces: Living rooms and bedrooms typically need 100–300 lux. Task areas: Kitchens and offices require 300–750 lux for comfortable food preparation and desk work. Reading: Detailed tasks benefit from 500–750 lux. Exterior: Pathways need 10–50 lux, while security lighting ranges from 100–200 lux.
Frequently Asked Questions
What is the difference between watts and lux?
Watts measure electrical power consumed by a light source, while lux measures illuminance — the amount of light falling on a surface. One lux equals one lumen per square meter. The conversion depends on the light source's luminous efficacy (lumens per watt) and the illuminated surface area.
How do you convert watts to lux?
To convert watts to lux, first multiply power in watts by luminous efficacy (lm/W) to get total lumens, then divide by the surface area in square meters: lux = watts × efficacy / area. For example, a 50 W LED bulb (90 lm/W) illuminating a 10 m² room produces 50 × 90 / 10 = 450 lux.
What is a good luminous efficacy for a light bulb?
LED bulbs offer the highest efficacy at 80–120 lm/W, followed by CFLs at 45–75 lm/W, halogen at 16–30 lm/W, and traditional incandescent bulbs at 12–18 lm/W. Higher efficacy means more light output per watt of electricity consumed, making LEDs the most energy-efficient choice.
How many lux do I need for different rooms?
Recommended illuminance levels vary by space: living rooms 100–300 lux, kitchens 300–750 lux, offices 300–500 lux, bedrooms 100–300 lux, bathrooms 300–500 lux, and reading areas 500–750 lux. Use this calculator to determine the wattage needed to achieve your target lux level.
What is the difference between lumens and lux?
Lumens measure the total light output from a source, while lux measures the light intensity on a surface area. One lux equals one lumen spread over one square meter. A 1000-lumen bulb produces 1000 lux on a 1 m² surface but only 100 lux on a 10 m² surface.
Can I use this calculator for exterior lighting?
Yes, this calculator works for any lighting application. For exterior lighting, typical ranges are 10–50 lux for pathways, 50–100 lux for gardens, and 100–200 lux for security lighting. Just enter the appropriate surface area and choose your light source type.
Does the calculator support square feet instead of square meters?
The calculator uses square meters by default. To use square feet, multiply square feet by 0.0929 to convert to square meters before entering the value. Alternatively, you can convert the result: 1 lux equals approximately 0.0929 foot-candles.
What light source should I select for best energy efficiency?
LED bulbs provide the best energy efficiency with 90–120 lm/W, consuming up to 85% less power than incandescent bulbs for the same light output. For example, a 10 W LED produces similar light to a 60 W incandescent bulb. The calculator compares illuminance across all bulb types to help you choose.