Lighting Calculation: The Lumen Method Step by Step

Instead of estimating a space's fixture needs by eye, lighting designers use a standard approach called the Lumen Method. It accounts for the fact that some light is lost on walls and ceilings, and that fixtures lose efficiency over time.

01The Core Formula

The required number of fixtures (N) is found by: N = (E × A) / (Φ × UF × MF). Here E is target illuminance (lux), A is area (m²), Φ is a fixture's lumens, UF is the utilization factor and MF is the maintenance factor.

• —E — Target illuminance: taken from the standard by space type (office 500 lux, restaurant 200 lux).
• —A — Area: the space's square meters.
• —Φ — Fixture lumens: the lumen value of the chosen product.
• —UF — Utilization factor: efficiency based on room geometry and surface reflectance (usually 0.4–0.8).
• —MF — Maintenance factor: efficiency loss over time (clean 0.85, normal 0.80, dirty 0.70).

02A Worked Example

Consider a 40 m² office: E = 500 lux, A = 40, Φ = 3000 lm, UF = 0.6, MF = 0.8. N = (500 × 40) / (3000 × 0.6 × 0.8) = 20,000 / 1,440 ≈ 14 fixtures. So this office needs about 14 fixtures of 3000 lumens each.