Lighting Layout Calculator Formula

Understand the math behind the lighting layout calculator. Each variable explained with a worked example.

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Formulas Used

Fixtures Needed

fixtures_needed = lumens_per_fixture > 0 && cu_factor > 0 && maint_factor > 0 ? ceil(total_lumens_needed / (lumens_per_fixture * cu_factor * maint_factor)) : 0

Total Lumens Required

total_lumens = total_lumens_needed

Estimated Wattage (LED)

watts_estimate = lumens_per_fixture > 0 && cu_factor > 0 && maint_factor > 0 ? ceil(total_lumens_needed / (lumens_per_fixture * cu_factor * maint_factor)) * lumens_per_fixture / 100 : 0

Variables

VariableDescriptionDefault
room_lengthRoom Length(ft)20
room_widthRoom Width(ft)15
target_fcTarget Foot-Candles(fc)50
lumens_per_fixtureLumens per Fixture(lm)3200
cu_factorCoefficient of Utilization0.6
maint_factorMaintenance Factor0.8
room_areaDerived value= room_length * room_widthcalculated
total_lumens_neededDerived value= room_area * target_fccalculated

How It Works

Lumen Method for Lighting Design

N = (Area x FC) / (Lumens per Fixture x CU x MF)

Where FC is the target foot-candle level, CU is the coefficient of utilization (depends on room shape and reflectance), and MF is the maintenance factor (accounts for dirt and lamp aging).

Worked Example

20x15 ft office at 50 fc using 3,200-lumen fixtures, CU=0.6, MF=0.8.

room_length = 20room_width = 15target_fc = 50lumens_per_fixture = 3200cu_factor = 0.6maint_factor = 0.8
  1. 01Area = 20 x 15 = 300 sq ft
  2. 02Lumens needed = 300 x 50 = 15,000 lm
  3. 03Effective per fixture = 3200 x 0.6 x 0.8 = 1,536 lm
  4. 04Fixtures = 15,000 / 1,536 = 9.77, round up to 10

Frequently Asked Questions

What foot-candle level do I need?

General office: 30-50 fc. Retail: 50-75 fc. Warehouse: 10-30 fc. Kitchen: 50-75 fc. These values come from the Illuminating Engineering Society (IES) recommendations.

What is the coefficient of utilization?

CU represents how effectively light reaches the work surface. It depends on fixture type, room dimensions, and surface reflectances. Typical values range from 0.4 to 0.8.

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