Watts to Lux Calculator
Convert electric power in watts (W) to illuminance in lux (lx). Enter the power, select a light source (or type its luminous efficacy in lumens per watt), then give the illuminated surface area in square feet / meters — or a spherical radius in feet / meters — and press Calculate. The illuminance comes from Ev(lx) = 10.76391 × P × η / A(ft²) or Ev(lx) = P × η / A(m²). Everything runs locally in your browser.
Enter the power in watts
Type the electric power P consumed by the lamp, in watts (W). Zero or any positive value is accepted; zero watts gives zero lux.
Pick a light source or type the efficacy
Selecting a source (tungsten incandescent, halogen, fluorescent, LED, metal halide, high/low pressure sodium, mercury vapor) fills the luminous efficacy field with a typical value in lumens per watt (lm/W) — or type your own. Typing later wins over the dropdown.
Add the area (or radius) and press Calculate
Enter the surface area A in ft² or m² — or a spherical radius r in ft or m, which auto-fills A = 4πr². The area and radius units stay in sync. Press Calculate to read the illuminance in lux, rounded to three decimals; Clear resets the whole form.
Watts to lux calculation
- Watts to lux formula 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). For example, a 100 W LED lamp rated at 90 lm/W spread over 10 m² gives 100 × 90 / 10 = 900 lx.
- Watts to lux formula with area in square feet
- The illuminance Ev in lux (lx) is equal to 10.76391 times the power P in watts, times the luminous efficacy η in lumens per watt, 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 converts square feet to square meters (1 ft² = 0.09290304 m²).
- Why does luminous efficacy matter?
- A watt is power, a lux is light landing on a surface — the bridge is luminous flux. Flux Φ (lumens) = P (watts) × η (lumens per watt), and 1 lux = 1 lumen per square meter. Two 10 W lamps can differ by more than 5× in lux: 10 W of tungsten at 15 lm/W emits 150 lm, while 10 W of LED at 90 lm/W emits 900 lm.
- What typical efficacy values does the dropdown use?
- The presets are typical / average values in lm/W: tungsten incandescent 15, halogen 20, fluorescent 60, LED 90, metal halide 87, high pressure sodium 117, low pressure sodium 150, mercury vapor 50. Real lamps vary by model and age — overwrite the field with the datasheet value when you have one.
- When should I use the spherical radius instead of the area?
- When light spreads uniformly in all directions it forms a sphere whose area grows with the square of the radius: A = 4πr². Enter the radius r (ft or m) and the area fills in automatically — 2 m gives about 50.265 m². At 2 m from a 100 W LED (90 lm/W) that is about 179 lx.
- Can I convert lux back to watts?
- Algebraically yes: P = Ev × A / η for square meters, or P = Ev × A / (10.76391 × η) for square feet. This page computes only the watts → lux direction.
Privacy: every calculation runs in your browser tab — no value you type is sent anywhere and nothing is stored after the tab closes. Results assume the luminous flux is spread evenly over the given surface; verify real installations with a lux meter.