Watts to Lux Calculator
Use our watts to lux calculator to calculate the amount of light falling on a chosen surface. In photometry, this measure is known as illuminance — the 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 affects the efficiency of the light source and, for example, save money by changing the power to have efficient light for a given area. 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 watts, 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 already know the bulb's brightness, you may want to use our Lumens to Watts calculator instead. And if you want to calculate the optimal illumination level for different types of rooms, check out the 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 (lm/m²).
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 at spreading light (the surface is brighter), while a lower number means that your surface is darker and 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 with area in square meters
The illuminance Ev in lux (lx) is equal to the power P in watts (W), times the luminous efficacy η in lumens per watt (lm/W), divided by the surface area A in square meters (m²):
Ev (lx) = P (W) × η (lm/W) ÷ A (m²)
Watts to lux with area in square feet
The illuminance Ev in lux (lx) is equal to 10.76391 times the power P in watts (W), times the luminous efficacy η in lumens per watt (lm/W), divided by the surface area A in square feet (ft²):
Ev (lx) = 10.76391 × P (W) × η (lm/W) ÷ A (ft²)
The factor 10.76391 comes from the fact that 1 square meter equals 10.76391 square feet, and 1 foot-candle (lm/ft²) = 10.76391 lux.
Spherical radius option
If you know the spherical radius r instead of a flat surface area, the calculator computes the surface area of a sphere as A = 4 × π × r², then applies the formulas above.
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 the power used. The table below contains the most popular light sources and their efficacy.
| Light bulb type | Luminous efficacy (lumens/watt) |
|---|---|
| Compact fluorescent bulbs | 45–75 |
| Fluorescent tube lights | 40–105 |
| Halogen light bulbs | 16–30 |
| LED | 30–40 |
| LED bulbs | 80–120 |
| Neon Lamp | 15–40 |
| Metal halide lights | 75–100 |
| Mercury vapor lights | 35–65 |
| High-pressure Sodium | 85–150 |
| Low-pressure Sodium | 100–190 |
| Sulfur plasma lights | 50–100 |
| Tungsten Incandescent bulbs | 12–18 |
There's also another similar metric connected to luminous efficacy — luminous efficiency. The luminous efficiency of a light source is expressed as a percentage, 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 is no single answer — it depends on the bulb's luminous efficacy and the area being lit. For example, a 1 W LED with an efficacy of 100 lm/W spread over a 1 m² surface produces 100 lux. The same watt from an incandescent bulb (15 lm/W) over 1 m² produces only 15 lux. Increase the area and the lux value drops proportionally.
How Do I Convert 300 Lux to Watts?
Switch the calculator to Lux → Watts mode and enter your target illuminance (300 lux), the luminous efficacy of your light source, and the surface area. The calculator rearranges the formula:
P (W) = Ev (lx) × A (m²) ÷ η (lm/W)
Example: To reach 300 lux over a 10 m² room with an LED (100 lm/W): P = 300 × 10 ÷ 100 = 30 W of LED power.
American vs. Metric Measurement Systems
The calculator supports both measurement systems:
- Metric: illuminance in lux (lx), surface area in square meters (m²), radius in meters.
- American (imperial): surface area in square feet (ft²), radius in feet. The result is still shown in lux, with a foot-candle (fc) equivalent (1 fc = 10.76391 lux).
Electricity Cost Estimation
Enable the Electricity Cost option to estimate how much it costs to run your light. Enter the hours per day, days per year, your electricity rate per kWh, and the currency. The calculator supports the US dollar, the Russian ruble, the euro, the British pound, and 9 other world currencies, and shows your daily, monthly, and annual running cost.
Frequently Asked Questions
What is the difference between lumens and lux?
Lumens measure the total light output of a source in all directions, while lux measures how much of that light falls on a given surface area (1 lux = 1 lumen per square meter). The same lumen output spread over a larger area gives fewer lux.
How many lux do I need for a room?
Typical recommendations are 100–300 lux for living rooms, 300–500 lux for kitchens and offices, and 500–1000 lux for detailed task work. Use the Lux → Watts mode to find the power needed to reach those levels.
Can I convert watts directly to lux?
Not directly — watts measure electrical power and lux measure illuminance. You also need the luminous efficacy (lm/W) of the bulb and the illuminated surface area to make the conversion, which is exactly what this calculator does.