Watts to Lux Calculator — Lamp Power to Illuminance
Estimate the illuminance (lux) a lamp delivers over a surface. Enter the wattage, lamp type (which sets lumens per watt), room area, and coefficient of utilization. Useful for basic lighting design and comparing lamp technologies.
W
Light source type
m²
Lux (lm/m²) on the work plane
- 1
Luminous flux
10 × 100 = 1,000Watts × efficacy (lm/W) gives total lumens. - 2
Useful flux (× CU)
1,000 × 0.75 = 750 - 3
Illuminance
750 ÷ 10 = 75
How does this calculator work?
Lux = (Watts × lm/W × CU) ÷ Area. Enter lamp wattage, choose the light source type for efficacy, set room area (m²), and adjust the coefficient of utilization. Get illuminance in lux and foot-candles for quick lighting design checks.
Formula
How this is calculated
Lux (lx) measures illuminance — the amount of luminous flux landing on one square metre of surface. To convert watts to lux you need two more pieces of information: how efficiently the lamp turns electricity into visible light (luminous efficacy, in lm/W), and what fraction of that light actually reaches the work plane (coefficient of utilization, CU).
First, lumens = watts × efficacy. A 10 W LED at 100 lm/W produces 1000 lm. Then lux = (lumens × CU) / area. If the CU is 0.75 (lights mounted overhead with some light absorbed by walls/ceiling) and the area is 10 m², the result is (1000 × 0.75) / 10 = 75 lux.
Luminous efficacy varies widely: incandescent bulbs produce roughly 10–15 lm/W, halogen 20–35 lm/W, CFLs 45–75 lm/W, and LEDs 80–160 lm/W depending on quality and colour temperature. The values used here are representative midpoints as of 2024–2025; check the manufacturer data sheet for exact figures. The coefficient of utilization (0.5–0.9 for most rooms) accounts for room geometry, reflectances, and luminaire distribution — use a full lighting design tool for precise results.
Frequently asked questions
A 10 W LED at 100 lm/W gives 1000 lm. With CU = 0.75 and 10 m², lux ≈ 75 lux — adequate for ambient background lighting but below the 300–500 lux recommended for detailed tasks.
ISO 8995 / EN 12464-1 recommends 300–500 lux for general office work and 500–750 lux for detailed drawing or inspection tasks. For corridors and common areas 100–200 lux is usually sufficient.
Lumens (lm) measure total luminous flux emitted by a lamp — the "amount" of light. Lux (lx) measures luminous flux per unit area — the "concentration" of light on a surface. The same 1000 lm lamp gives 1000 lux over 1 m² but only 100 lux over 10 m².
Also known as
TG we-Calculate Editorial Team. (2026). Watts to Lux Calculator — Lamp Power to Illuminance [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/watts-to-lux-calculator
TG we-Calculate Editorial Team. "Watts to Lux Calculator — Lamp Power to Illuminance." TG we-Calculate. 2026. https://we-calculate.com/calculator/watts-to-lux-calculator.
TG we-Calculate Editorial Team, "Watts to Lux Calculator — Lamp Power to Illuminance," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/watts-to-lux-calculator
@misc{wecalculate_watts_to_lux_calculator, title = {Watts to Lux Calculator — Lamp Power to Illuminance}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/watts-to-lux-calculator}}, year = {2026}, note = {TG we-Calculate} }
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