Lux to Watt Calculator
Convert lux (lx) to watts (W): W = (lx × A) ÷ η. Free online tool — no sign-up. Clear formulas and worked examples. Runs locally in your browser.
Trust summary Engine tested · Specification checked · v1.1.0
- Input interpretation
- Enter values to calculate.
- Result
- —
- Model
- Electrical power from illuminance, area (or sphere radius), and efficacy — or lux from power, efficacy, and area.
- Scope
- Uniform illuminance over the stated area
- Verification
- Engine tested · Specification checked · v1.1.0
- Named expert review
- Optional · Not performed
- Specification basis
- P = (E × A) / η; 1 lx = 1 lm/m²; 1 m² = 10.76391 ft²
Formulas
Core equations used by this calculator.
How to use
Choose direction and geometry
lx → W or W → lx, with area in m² / ft² or sphere radius.
Enter lux (or watts), area/radius, and efficacy
Use package lm/W when available, or a typical value for the lamp type.
Read watts or lux
Higher lux or larger area → more watts (at fixed η).
Example calculations
Common configurations with formula and result.
500 lx · 10 m² · 100 lm/W
Default
300 lx · 10 m² · 20 lm/W
Lower efficacy
500 lx · 100 ft² · 100 lm/W
Feet area
W → lx
50 W · 100 lm/W · 10 m²
Sphere
80 lx · r=1 m · 100 lm/W
Typical luminous efficacy (guide)
Common values at a glance.
| Light source | Approx. η (lm/W) | Notes |
|---|---|---|
| Incandescent (tungsten) | 12–18 | Legacy bulbs |
| Halogen | 16–30 | — |
| CFL / fluorescent | 40–75 | Varies |
| LED | 80–120+ | Common modern lamps |
| Metal halide | 75–100 | HID |
| High-pressure sodium | 85–150 | Street / industrial |
Lux to Watt calculator specification
Version 1.1.0 · Engine tested
- Engine tested 4 published cases
- Named expert review Not performed
- Calculation version 1.1.0
- Definition
- Lux and watts are different quantities — you cannot convert them without area and luminous efficacy. Required power: W = (lx × A) ÷ η with A in m². With A in ft²: W = 0.09290304 × lx × A(ft²) ÷ η. Reverse: lx = (W × η) ÷ A. Chain: lm = lx × A, then W = lm ÷ η.
- What it calculates
- Electrical power from illuminance, area (or sphere radius), and efficacy — or lux from power, efficacy, and area.
- Inputs
- Lux (lx) or watts (W)
- Area (m² or ft²) or sphere radius (m)
- Luminous efficacy η (lm/W)
- Outputs
- Watts (lx → W) or lux (W → lx)
- Formula
P=(E·A)/η; E=(P·η)/A- Assumptions
- Uniform illuminance over the stated area
- All flux reaches that surface
- Constant luminous efficacy
- Units
- lx, m², lm/W → W
- lx, ft², lm/W → W
- Boundary conditions
- η = 0 or A = 0 → undefined (UI returns 0)
- Example
- 500 lx, 10 m², 100 lm/W → 50 W
- Validation cases
4 published on this page
- 500 lx, 10 m², 100 lm/W → 50 W
- 300 lx, 10 m², 20 lm/W → 150 W
- 50 W, 100 lm/W, 10 m² → lx → 500 lx
- 80 lx, r=1 m, 100 lm/W → ≈ 10.0531 W
- Specification basis
- P = (E × A) / η; 1 lx = 1 lm/m²; 1 m² = 10.76391 ft²
- Calculation version
- 1.1.0
Background
Interpretation and common distinctions.
Convert lux (lx) to watts (W) using area and luminous efficacy eta.
P((W)) = (E((lx)) × A((m^(2))))/(eta((lm/W))) P((W)) = (0.09290304 × E((lx)) × A((ft^(2))))/(eta)
Default: 500 lx · 10 m² · 100 lm/W → 50 W.
Lux vs watts
| Quantity | Unit | Meaning |
|---|---|---|
| Illuminance | lx | Light on the surface (lm/m²) |
| Power | W | Electricity the lamp draws |
| Efficacy | lm/W | Light produced per watt |
There is no fixed “lux per watt” — always include area (or sphere radius) and η.
Related tools
Other calculators in this family: Watt to Lux, Lux to Lumen, Lumen to Watt, Watts to Amps .
Frequently asked questions
Key distinctions behind the calculation.
How do I convert lux to watts?
W = (lx × A) ÷ η with A in m² and η in lm/W. Example: 500 lx × 10 m² ÷ 100 lm/W = 50 W.
Can I convert lux to watts without area?
No. Lux is per square meter; watts are total power. You need the lit area (or sphere radius) and luminous efficacy.
What is luminous efficacy?
How many lumens you get per watt (lm/W). Higher η means fewer watts for the same lux and area.
How do I convert 300 lux to watts?
Pick area and η. Example: 300 lx over 10 m² at 20 lm/W → (300 × 10) ÷ 20 = 150 W.
How many lux is one watt?
It depends on η and area. Example: 1 W LED at 90 lm/W over 100 m² → (1 × 90) ÷ 100 = 0.9 lx.
Why use feet modes?
When area is in ft², convert via 1 m² = 10.76391 ft²: W = 0.09290304 × lx × A(ft²) ÷ η.
Does this include fixture losses?
No — assumes all light reaches the area uniformly. Real designs apply utilization and light-loss factors.
Can I go from watts back to lux?
Yes — use W → lx: lx = (W × η) ÷ A.