Watts to Lux Calculator

Created by Mateusz Szpila
Reviewed by Hanna Pamuła, PhD and Steven Wooding
Last updated: Nov 03, 2022

Use our watts to lux calculator to calculate the amount of light falling on the chosen surface. In photometry, this measure is known as illuminance – it is total luminous flux based on electric power, luminous efficacy, and the illuminated object's surface area or spherical radius.

With this tool, you can compare how the power can affect the efficiency of the light source and, for example, save money to change the power to have efficient light for a given area. For example, you can find how many lux is a watt for a specific space and light source. On the other hand, you can also use the calculator to convert lux to watt, and then you will know how much power you need to get a well-lit area.

If the surface area is unimportant to you, and you know the bulb's brightness, you may want to use our other tool: lumens to watts calculator. And if you want to calculate the optimal illumination level for different types of rooms, make sure to check Omni's lighting calculator.

Definition of lux

Lux is a unit of illuminance. One lux is equal to the illumination of an area of one square meter that is one meter away from a single candle. Also, one lux is equivalent to one lumen per square meter.

Check out our lumen to lux to candela converter to learn more about conversion between these three most popular units of measuring light.

Lux vs. watts – what's the difference?

Watt and lux units represent different physical quantities, so you can't convert them directly. Our application calculates and converts watts to lux to show how power affects the light efficiency on a given surface.

A higher number of lux makes the source more efficient in spreading (the surface is brighter), and a lower one means that your surface is darker, dimmer.

How to calculate watts to lux

Below you'll find the formulas for watts to lux calculations, separately for metric and imperial surface area units:

  • Watts to lux calculation with area in square meters:

    The illuminance EvE_v in lux (lx) is equal to the power PP in watts (W), times the luminous efficacy ηη in lumens per watt (lm/W) divided by the surface area AA in square meters (m2m^2):

Ev(lx)=Pwη(lm/W)/A(m2)\small \qquad E _{v(\text{lx})} = P_w\, \eta _{(\text{lm/W})} / A_{({\text m^2})}
  • Watts to lux calculation with area in square feet:

    The illuminance EvE_v in lux (lx) is equal to 10.76391 times the power PP in watts (W), times the luminous efficacy ηη in lumens per watt (lm/W) divided by the surface area A in square feet (ft2\text{ft}^2):

Ev(lx)=10.76391×Pwη(lm/W)A(ft2)\small \qquad E _{v(\text lx)} = 10.76391 \times \frac{P_w \, \eta _{(\text{lm/W})} }{ A_{({\text{ft}^2})}}

Luminous efficacy

The luminous efficacy describes how well a light source produces visible light - in other words, it's the amount of light emitted for power used. The table below contains the most popular light sources and their efficacy.

Light bulb type

Luminous efficacy (lumens/watt)

Compact fluorescent bulbs


Fluorescent tube lights


Halogen light bulbs




LED bulbs


Neon Lamp


Metal halide lights


Mercury vapor lights


High-pressure Sodium


Low-pressure Sodium


Sulfur plasma lights


Tungsten Incandescent bulbs


There's also another similar metric connected to luminous efficacy – luminous efficiency. The luminous efficiency of a light source is expressed in percentages, and it's defined as the actual luminous efficacy divided by the theoretical maximum value of the efficacy.

How many lux is a watt?

There's no simple answer to such a question, as lux and watts are not the same physical quantities: the formula depends on the surface area lit and the light source type (to be precise, on its luminous efficacy)

As an example, let's find the illuminance (measured in lux) of the 1 W LED bulb illuminating 100 m2100\ \text{m}^2 room:

  1. First, get the information about the luminous efficacy of your light source – or choose the exemplary one from the drop-down list of the calculator. Let's choose the preset implemented for a LED lamp: its luminous efficacy is 90 lm/W.

  2. Enter the values into the formula to convert watts to lux:

Ev=1 [W]×90 [lm/W]100 [m2]\small \qquad E _{v} = 1\ [\text{W}] \times \frac{90\ [\text{lm/W}]}{ 100\ [\text{m}^2]}
  1. Thus, your 1 W LED lamp gives us 0.9 lux for 1 watt. Now, you can compare the result with another light source using our watts to lux calculator.

In the same way that you can find out how many lux is a 1000 watt bulb, after setting up the values for luminous efficacy and the surface area of your illuminated object.

How do I convert 300 lux to watts?

To convert 300 lux to watts in our lux to watts calculator, take the following steps:

  1. Enter 300 lux300\ \text{lux} into the "Illuminance" field.
  2. Input the luminous efficacy of your light source (remember that you can also use our built-in presets). Let's assume the luminous efficacy of the source is 20 lm/W20 \ \text{lm/W}.
  3. Input the value of the illuminated area, e.g. 10 m210\ \text{m}^2.
  4. The calculator shows the result: the power of your source is 150150 watt.
Mateusz Szpila
Luminous efficacy
lm /
Surface area
Spherical radius
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