Lumen Calculator — Illuminance & Luminous Efficacy
Enter the lumen output of a light source and the area it illuminates to get the illuminance in lux. Add the wattage to find luminous efficacy (lm/W) and compare your result to standard lighting recommendations.
lm
m²
W
Luminous flux per unit area (lm/m²) — the brightness landing on the surface
- 1
Luminous flux Φ
800 lm - 2
Surface area A
10 m² - 3
Illuminance E = Φ ÷ A
800 ÷ 10 = 80
How does this calculator work?
Illuminance (lux) = lumens ÷ area (m²). Luminous efficacy (lm/W) = lumens ÷ watts. Use 150–200 lux for living rooms and 300–500 lux for workspaces. A standard 800 lm LED bulb over a 10 m² room gives 80 lux — bright enough for a hallway, but add more bulbs for desk work.
Formula
How this is calculated
The lumen (lm) measures luminous flux — the total amount of light emitted by a source in all directions. The lux (lx) measures illuminance — the flux landing on one square metre of a surface. The relationship is simple: E = Φ / A. Doubling the area halves the illuminance; doubling the lumens doubles it. This assumes the light is evenly distributed over the area, which is a good approximation for a diffuse ceiling fixture in a small room but less accurate for a spotlight or a tall space.
Luminous efficacy (lm/W) tells you how efficiently a bulb converts electrical energy into visible light. Modern LED bulbs achieve 80–120 lm/W; incandescent lamps typically manage only 10–15 lm/W. The efficacy figure helps you compare running costs for the same light output.
The candela (cd) result assumes the source radiates equally in all directions (isotropic), using I = Φ / 4π steradians. Real light sources are directional, so treat the candela figure as a theoretical lower bound for a beam. Standard illuminance recommendations: living rooms 100–200 lux, offices and desktops 300–500 lux, workshops and fine-detail work 750–1000 lux.
Frequently asked questions
Multiply the floor area (m²) by the target lux level: about 150–200 lux for a relaxed living space, 300–500 lux for a study or kitchen worktop. For a 15 m² living room at 200 lux you need 3 000 lm — roughly four 800 lm LED bulbs.
Lumens measure the total light output of a source (how much light it emits). Lux measure the brightness falling on a surface (lumens per square metre). The same 1 000 lm torch produces 1 000 lux on a 1 m² target but only 100 lux if the beam spreads to 10 m².
Look for at least 70–80 lm/W for a good LED. Top-tier LED bulbs reach 100–130 lm/W. Fluorescent (CFL) lamps deliver 55–70 lm/W; halogens 15–25 lm/W; and incandescent lamps only 10–15 lm/W. Higher efficacy means less electricity for the same brightness.
Also known as
TG we-Calculate Editorial Team. (2026). Lumen Calculator — Illuminance & Luminous Efficacy [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/lumen-calculator
TG we-Calculate Editorial Team. "Lumen Calculator — Illuminance & Luminous Efficacy." TG we-Calculate. 2026. https://we-calculate.com/calculator/lumen-calculator.
TG we-Calculate Editorial Team, "Lumen Calculator — Illuminance & Luminous Efficacy," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/lumen-calculator
@misc{wecalculate_lumen_calculator, title = {Lumen Calculator — Illuminance & Luminous Efficacy}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/lumen-calculator}}, year = {2026}, note = {TG we-Calculate} }
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