Lux to Watts Calculator: Convert Illuminance to Power

A lux-to-watts calculator estimates the electrical power needed to produce a specified illuminance over a known area, using a light source’s luminous efficacy. Enter illuminance, area, and efficacy to get estimated watts; the result is an estimate because lux alone does not determine a lamp’s power.

Formula

The estimate follows from the definition of illuminance and the relationship between luminous flux and electrical power:

• Required luminous flux: Φ = E × A
• Estimated electrical power: P = Φ ÷ η = E × A ÷ η

E is illuminance in lux (lumens per square metre); A is illuminated area in square metres; Φ is luminous flux in lumens; η is luminous efficacy in lumens per watt (lm/W); and P is electrical input power in watts. The calculation assumes the specified illuminance is uniform across the stated area and that the efficacy value applies to the light source in use.

Worked Example

Using the defaults above—500 lux over 10 m² and a luminous efficacy of 100 lm/W:

1. Required luminous flux: Φ = 500 lux × 10 m² = 5,000 lm.
2. Estimated power: P = 5,000 lm ÷ 100 lm/W = 50 W.

The calculator therefore gives an estimated electrical power of 50.00 W and a required luminous flux of 5,000.00 lm.

Why Lux Does Not Convert Directly to Watts

Lux and watts measure different things. Lux measures how much visible light arrives on a surface: one lux is one lumen per square metre. Watts measure the rate at which a device uses or transfers energy. A lux reading does not say how much electrical power produced the light, because sources vary in how efficiently they turn electricity into visible light.

For the same target illuminance and area, a more efficacious source needs fewer electrical watts. For example, the calculator’s 5,000-lumen requirement needs 50 W at 100 lm/W, but 25 W at 200 lm/W. Those figures are mathematical estimates, not a promise that a particular lamp will deliver exactly the target lighting in a real room.

Also distinguish electrical watts from optical watts. Optical watts describe radiant energy in the light output; this calculator estimates electrical input power from lumens and efficacy. Converting between optical watts and lumens requires information about the light’s spectrum, because human vision responds differently to different wavelengths.

How to Choose Inputs

• Illuminance: Enter the target or measured lux at the surface of interest, not automatically the value printed on a lamp package. Lighting needs vary with the task and location.
• Area: Use the surface area receiving the target light. For a rectangular surface, multiply length by width. Convert measurements before multiplying: square metres are not the same as square feet.
• Luminous efficacy: Use a credible lm/W figure for the actual light source if available. Efficacy depends on the source and operating conditions; a generic value is only a rough planning assumption.

The calculator accepts area in square metres. To convert square feet, multiply by 0.092903. For instance, 100 ft² is about 9.29 m². Lux is metric; the related US customary illuminance unit is the foot-candle, with 1 foot-candle equal to about 10.764 lux. Convert the illuminance to lux before using this form.

When to Use the Estimate—and Its Limits

This calculation is useful for a first-pass comparison of power requirements when you know an area’s target illuminance and a source’s efficacy. It can also help explain why two lighting setups with similar brightness may have different wattage. It does not calculate how many fixtures to buy, the precise illuminance at every point, or a guaranteed energy bill.

Real installations lose or redistribute light. Fixture optics, mounting height, beam shape, room dimensions, wall and ceiling reflectance, obstructions, dirt, dimming, and ageing can all affect the light reaching a work surface. A lamp’s rated efficacy may describe the source under specified conditions rather than the complete fixture. For a practical design, use fixture photometric data and consider the layout and uniformity, not just total lumens.

Illuminance also varies across a space. The formula treats the chosen lux value as an average over the entire area. A single lux-meter reading at one point may not represent that average. If the goal is to verify lighting, take measurements at relevant positions and heights, with the source in its normal operating state.

Common Mistakes

• Trying to convert lux straight to watts: Area and luminous efficacy are needed. The same lux value can correspond to different electrical power.
• Using lamp wattage as efficacy: Watts are power; lm/W is efficacy. If a package lists lumens and watts, divide lumens by watts to get an approximate source efficacy.
• Mixing units: Enter lux, square metres, and lm/W together. Mixing square feet with square metres creates a substantial error.
• Confusing lumens with lux: Lumens describe total visible luminous flux; lux describes flux per area. A lumen figure cannot be treated as a lux figure without considering the illuminated area and distribution.
• Reading decimals as certainty: Two decimal places are only display rounding. Input uncertainty and real-world lighting conditions usually matter more than the last digit.

Frequently Asked Questions

Can you convert lux to watts directly?
No. Lux alone is insufficient. You also need the illuminated area and the light source’s luminous efficacy in lm/W to estimate electrical power.

How many watts is 500 lux?
There is no single answer without an area and efficacy. At 500 lux over 10 m² and 100 lm/W, the estimate is 50 W. Changing either area or efficacy changes the result.

What is the difference between lux and lumens?
Lumens measure luminous flux emitted or delivered; lux measures illuminance on a surface. One lux is one lumen per square metre, assuming the flux is distributed over that area as described.

How do I find luminous efficacy?
If you have a suitable lumens and electrical-watts rating for the same source, divide lumens by watts. The result is lm/W. Make sure the figures describe comparable operating conditions and whether they refer to a lamp or a complete fixture.

Does this tell me how many bulbs I need?
Not by itself. It estimates total power for an assumed average illuminance, area, and efficacy. Fixture output, beam distribution, spacing, mounting, and room conditions are needed to plan a reliable fixture count.

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in E | Illuminance | lux | 500 | 0
in A | Area | m^2 | 10 | 0
in eta | Luminous efficacy | lm/W | 100 | 0
out P | Estimated electrical power | W | E * A / eta | 2
out F | Required luminous flux | lm | E * A | 2