Watts to Lux Calculator

Convert a light's electrical power to illuminance using its efficiency.

How the Math Works

The Watts to Lux Calculator uses the fundamental relationship between electrical power and illuminance: lux = watts · efficacy ÷ area. This formula combines three key quantities: watts (electrical power in joules per second), efficacy (luminous efficacy in lumens per watt, which measures how efficiently a light source converts power to visible light), and area (the surface over which light is distributed in square meters). When you multiply watts by efficacy, you get total luminous flux in lumens - the total amount of visible light emitted. Dividing by area distributes this light across a surface, giving illuminance in lux, which tells you how much light hits each square meter. For example, a 10-watt LED bulb with 80 lumens per watt efficacy shining on a 2-square-meter surface produces 400 lux of illuminance.

Practical Applications

To use this calculation practically, first determine your light source's power consumption in watts and find its luminous efficacy rating (typically provided by the manufacturer or found in lighting specifications). Measure or define the area you're illuminating - for a desk lamp, this might be the surface area of your workspace; for room lighting, it could be the floor area of the room. Apply the formula by multiplying the wattage by the efficacy to get total lumens, then divide by the area to find the illuminance level. This calculation is essential for lighting design in homes, offices, studios, and retail spaces where specific brightness levels are required for visual tasks or aesthetic purposes.

Day-to-Day Use

Understanding this conversion helps you make informed decisions about lighting in everyday situations. When shopping for light bulbs, you can estimate whether a particular wattage will provide sufficient brightness for reading, working, or cooking by calculating the expected lux level. If you're renovating a room, you can determine how many fixtures you need to achieve recommended illuminance levels for different activities - brighter light for detailed work, softer light for relaxation. This knowledge also helps with energy efficiency: you can find lower-wattage bulbs with higher efficacy that deliver the same lux level, saving electricity and money while maintaining proper lighting for your daily activities.

Worked example

10 W LED at 80 lm/W over 5 m² → 160 lux.

FAQ