Lumen Method Lighting Calculator (Fixtures Needed)

How it works

N = E × A ÷ (lumens per luminaire × CU × LLF); RCR = 5 × h × (L + W) ÷ (L × W); CU from the luminaire's coefficient table (approximated here); LPD = N × W ÷ A

The lumen method estimates the average illuminance a room will receive from a regular grid of luminaires. Not every lumen a fixture emits lands on the desks: some is absorbed by walls and ceiling, more so in tall narrow rooms with dark surfaces, and the coefficient of utilization captures that fraction as a function of the room cavity ratio and reflectances (real values come from the manufacturer's photometric table). Light loss factors then account for lumen depreciation and dirt over the maintenance cycle. The method gives the count; a spacing criterion — roughly 1.3 times the mounting height for typical direct fixtures — checks that the grid is dense enough to be uniform, and dividing installed watts by area gives the lighting power density that energy codes cap at about 0.6–0.8 W/ft² for offices.

Worked example

A 40 × 30 ft office lit from 7 ft above the desks has a room cavity ratio of 2.0 and, with medium reflectances, a CU near 0.74. For 50 fc from 4,000-lumen LED troffers at an LLF of 0.8 it needs 25.3 fixtures — a 6 × 5 grid of 30 gives about 59 fc at 0.8 W/ft², spaced 6.7 × 6 ft, comfortably inside the 9 ft spacing limit.

Frequently asked questions