Lux to Watts Calculator
Convert illuminance in lux (lx) to electric power in watts (W). Enter the illuminance, select a light source or type the luminous efficacy in lumens per watt, then give the surface area in ft² / m² or the spherical radius in ft / m and press Calculate — the power comes from P(W) = Ev(lx) × A(m²) / η(lm/W). Everything runs locally in your browser.
Enter illuminance in lux
Type the illuminance Ev in lux (lx) falling on the surface — a lux meter reading or a lighting-design target. Zero or any positive value is accepted; zero lux gives zero watts.
Pick a light source or type the efficacy
Selecting a light source fills the luminous efficacy field with its typical lm/W value (tungsten 15, halogen 20, fluorescent 60, LED 90, metal halide 87, high-pressure sodium 117, low-pressure sodium 150, mercury vapor 50). You can then edit the value, or choose the placeholder entry and type your own.
Give the area or radius, then Calculate
Enter the surface area in ft² or m², or a spherical radius in ft or m — the two fields stay consistent through A = 4πr², and the unit dropdowns mirror each other. Press Calculate to read the power in watts, or Clear to start over.
Lux to watts calculation formula
- Lux to watts with area in square meters
- The power P in watts (W) is equal to the illuminance Ev in lux (lx) times the surface area A in square meters (m²), divided by the luminous efficacy η in lumens per watt (lm/W): P(W) = Ev(lx) × A(m²) / η(lm/W). For example, 500 lx over 10 m² at 15 lm/W needs 500 × 10 / 15 ≈ 333.33 W.
- Lux to watts with area in square feet
- The power P in watts (W) is equal to 0.09290304 times the illuminance Ev in lux (lx) times the surface area A in square feet (ft²), divided by the luminous efficacy η: P(W) = 0.09290304 × Ev(lx) × A(ft²) / η(lm/W). The factor converts square feet to square meters (1 ft² = 0.09290304 m²).
- Using a spherical radius instead of an area
- If the light spreads over a sphere of radius r, the surface area is A = 4πr². Enter the radius in ft or m and the area field fills itself (they sync both ways); the result then uses the formula for the matching unit. A radius of 3 m means 4π × 3² ≈ 113.1 m².
- Are the light-source efficacy values exact?
- No — the predefined values (15 to 150 lm/W across the eight lamp types) are typical / average values, exactly as on the reference calculator. Real lamps vary by model, age, and drive current, so edit the lm/W field whenever you know the actual efficacy of your lamp.
- How accurate is this calculator?
- Results use the ideal formulas above and are displayed with up to four decimals (trailing zeros trimmed). Lux-to-watts is inherently lossy: the same lux can come from very different lamps and geometries, so treat the watts figure as an engineering estimate and check real installations with a power meter.
- Can I convert watts back to lux?
- Yes, algebraically: Ev(lx) = P(W) × η(lm/W) / A(m²) for square meters, or divide by A(ft²) and multiply by 0.09290304 for square feet. This page computes only the lux → watts direction.
Privacy: every calculation runs in your browser tab — no value you type is sent anywhere and nothing is stored after the tab closes. The light-source efficacy presets are typical / average values; verify real installations with a lux meter and a power meter.