Lighting tools
Lumen & layout calculator
Size a lighting layout in seconds. Enter a room and a target light level and get the number of fixtures (or linear feet), a balanced grid layout with spacing, the achieved foot-candles, and power density — computed with the standard lumen method and, optionally, the real published output of any fixture in the LightKilo catalog.
Required light
75,000 lm
total delivered lumens for 35 fc over 1,200 ft²
You need
20 fixtures
suggested layout: 5 × 4 grid
Layout preview
Spacing ≈ 8 ft × 7.5 ft · edge offset ≈ 4 ft / 3.8 ft
- Fixture output
- 4,000 lm
- Achieved level
- ≈ 37 fc
- CU × LLF
- 0.70 × 0.80
Estimate only — the lumen method gives an average, not a point-by-point calculation. Verify with a photometric layout (IES files) before specifying.
How the lumen method works
The lumen method estimates the average maintained illuminance a set of fixtures will deliver across a room. It rearranges the core photometric relationship
fixtures = (foot-candles × area) ÷ (lumens × CU × LLF)
- Foot-candles — the maintained light level on the work plane (1 fc = 10.76 lux).
- Area — length × width of the space in square feet.
- Lumens — the delivered output of one fixture (or per foot, for linear runs).
- CU — coefficient of utilization: how much light reaches the work plane (≈ 0.5–0.8).
- LLF — light loss factor for depreciation and dirt over time (≈ 0.7–0.85).
The suggested grid balances fixture columns and rows to the room's proportions and reports the spacing and edge offsets. It's a starting point — confirm uniformity with a full photometric layout from the manufacturer's IES files.
Frequently asked questions
How do you calculate how many lumens a room needs?
The lumen method multiplies your target maintained illuminance (in foot-candles) by the floor area (in square feet), then divides by the coefficient of utilization (CU) and light loss factor (LLF): total lumens = (foot-candles × area) ÷ (CU × LLF). Dividing that by a fixture’s delivered lumens gives the number of fixtures.
What is a good foot-candle level for an office?
General open-office work is typically designed to about 30–35 maintained foot-candles on the work plane, private offices and conference rooms around 30, corridors about 10, retail sales floors around 50, and healthcare exam rooms up to 100. Always confirm against the current IES recommendation for the specific task.
What are CU and LLF?
The coefficient of utilization (CU) is the fraction of a fixture’s lumens that actually reaches the work plane, depending on room shape and surface reflectances (commonly 0.5–0.8). The light loss factor (LLF), or maintenance factor, accounts for lumen depreciation and dirt over time (commonly 0.7–0.85). Lower values mean you need more fixtures.
Is the lumen method accurate enough to specify a design?
The lumen method estimates the average illuminance across a space, not the level at any single point. Use it for early sizing and fixture counts, then verify uniformity and point-by-point values with a photometric layout built from the manufacturer’s IES files before finalizing a specification.
How do you calculate a continuous linear (per-foot) run?
For linear systems rated in lumens per foot, divide the total required lumens by the per-foot output to get the total run length in feet, then split that across rows to fit the room. This calculator switches to a run-length result automatically when you pick a per-foot fixture.