Watt to Lux Calculator
Convert watts (W) to lux (lx): lx = (W × η) ÷ A. 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
- Illuminance from power, efficacy, and area (or sphere radius) — or power from lux, area, and efficacy.
- Scope
- Uniform illuminance over the stated area
- Verification
- Engine tested · Specification checked · v1.1.0
- Named expert review
- Optional · Not performed
- Specification basis
- E = (P × η) / A; 1 lx = 1 lm/m²; 1 m² = 10.76391 ft²
Formulas
Core equations used by this calculator.
How to use
Find watts and efficacy
Enter lamp power (W) and η in lm/W (package value or typical for the bulb type).
Enter area or sphere radius
Use the surface you want to light (m² / ft²) or an isotropic sphere radius.
Read lux (or reverse to watts)
lx = (W × η) ÷ A. Larger area → lower lux for the same lamp.
Example calculations
Common configurations with formula and result.
50 W · 100 lm/W · 10 m²
Default
1 W · 90 lm/W · 100 m²
How many lux per watt?
50 W · 100 lm/W · 100 ft²
Feet area
lx → W
300 lx · 10 m² · 20 lm/W
Sphere
10 W · 100 lm/W · r=1 m
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 | 80–140+ | Common modern lamps |
| Metal halide | 75–100 | HID |
| High-pressure sodium | 85–150 | Street / industrial |
Watt to Lux calculator specification
Version 1.1.0 · Engine tested
- Engine tested 4 published cases
- Named expert review Not performed
- Calculation version 1.1.0
- Definition
- Watts and lux are different quantities — you cannot convert them without luminous efficacy and area. Illuminance: lx = (W × η) ÷ A with A in m². With A in ft²: lx = 10.76391 × (W × η) ÷ A(ft²). Chain: first lm = W × η, then lx = lm ÷ A.
- What it calculates
- Illuminance from power, efficacy, and area (or sphere radius) — or power from lux, area, and efficacy.
- Inputs
- Watts (W) or lux (lx)
- Area (m² or ft²) or sphere radius (m)
- Luminous efficacy η (lm/W)
- Outputs
- Lux (W → lx) or watts (lx → W)
- Formula
E=(P·η)/A; P=(E·A)/η- Assumptions
- Uniform illuminance over the stated area
- All flux reaches that surface
- Constant luminous efficacy
- Units
- W, lm/W, m² → lx
- W, lm/W, ft² → lx
- Boundary conditions
- A = 0 or η = 0 → undefined (UI returns 0)
- Example
- 50 W, 100 lm/W, 10 m² → 500 lx
- Validation cases
4 published on this page
- 50 W, 100 lm/W, 10 m² → 500 lx
- 1 W, 90 lm/W, 100 m² → 0.9 lx
- 300 lx, 10 m², 20 lm/W → W → 150 W
- 10 W, 100 lm/W, r=1 m → ≈ 79.5775 lx
- Specification basis
- E = (P × η) / A; 1 lx = 1 lm/m²; 1 m² = 10.76391 ft²
- Calculation version
- 1.1.0
Background
Interpretation and common distinctions.
Convert watts (W) to lux (lx) using luminous efficacy eta and area.
E((lx)) = (P((W)) × eta((lm/W)))/(A((m^(2)))) E((lx)) = 10.76391 × (P × eta)/(A((ft^(2))))
Default: 50 W · 100 lm/W · 10 m² → 500 lx.
Calculation steps
- Watts (W) — lamp electrical power
- Efficacy η (lm/W) — e.g. LED often ~90–140
- Lumens — lm = W × eta
- Lux — lx = lm ÷ A(m²)
Watts vs lux
| Quantity | Unit | Meaning |
|---|---|---|
| Power | W | Electricity the lamp draws |
| Illuminance | lx | Light on the surface (lm/m²) |
| Efficacy | lm/W | Light produced per watt |
There is no fixed lux-per-watt number without area and η.
Related tools
Other calculators in this family: Lux to Watt, Watt to Lumen, Lumen to Lux, Watts to Amps .
Frequently asked questions
Key distinctions behind the calculation.
How do I convert watts to lux?
Multiply watts by luminous efficacy, then divide by area: lx = (W × η) ÷ A with A in m². Example: (50 × 100) ÷ 10 = 500 lx.
Can I convert watts to lux without area?
No. Lux is per square meter. You need the lit area (or sphere radius) and luminous efficacy.
How many lux is one watt?
It depends on η and area. Example: 1 W at 90 lm/W over 100 m² → 0.9 lx.
What efficacy should I use for LED?
Use the package value if available. Otherwise start near 90–100 lm/W (some LEDs reach 120–140+).
What are the calculation steps?
1) Note watts. 2) Get η (lm/W). 3) Compute lumens = W × η. 4) Divide by area in m² to get lux.
How do I convert 300 lux to watts?
W = (lx × A) ÷ η. Example: 300 lx × 10 m² ÷ 20 lm/W = 150 W.
Does this include fixture losses?
No — assumes all light reaches the area uniformly. Use CU and LLF for design layouts.
Can I go from lux back to watts?
Yes — use lx → W: W = (lx × A) ÷ η.