Inputs and interpretation
Obtain initial source lumens, coefficient of utilization (CU), light loss factor (LLF) and directional spacing limits for the chosen luminaire and room. CU depends on the room and mounting above the work plane; this calculator does not choose it from a fixture name or ceiling height.
The model supports positive CU and LLF no greater than 1. Use initial source lumens on the same basis as CU. Entering already maintained lumens and applying LLF again would count depreciation twice.
For the surface mode, provide the dimensions of the defined surface and either average illuminance or luminous flux already incident on it. A lamp or luminaire's source lumens are not automatically delivered to the surface.
Inputs and results stay in this page and are not saved or transmitted.
Method and examples
Room area A = length × width in m². Lumen-only quantity N = ceil(target lx × A ÷ (initial source lm per luminaire × CU × LLF)). For the resulting grid, estimated maintained average E = grid quantity × source lm × CU × LLF ÷ A.
In the surface conversion mode, average illuminance E = incident luminous flux Φ ÷ surface area A, and incident flux Φ = E × A. The input flux must already reach the defined surface; source lumens are not treated as delivered light.
Begin with enough columns and rows to stay within each supplied spacing limit. If that grid has fewer than N luminaires, increase columns toward a proportionate room grid, limited by the rows already required, then add enough rows to reach N. This deterministic proposal does not search every arrangement for a minimum fixture count.
Centre spacing is room dimension divided by its column or row count. Each edge offset is half that spacing. Position column i at (i − ½) × length spacing and row j at (j − ½) × width spacing, counting from 1. The opposite-wall offset is the same.
Worked example
For an illustrative 6 m × 4 m room, target 200 lx, 1000 initial source lm per luminaire, CU 0.6 and LLF 0.8, the lumen-only minimum is 10 luminaires. With 2 m maximum spacing in both directions, the proposed complete grid is 4 columns × 3 rows, or 12 luminaires, with an estimated 240 lx average.
Length spacing is 1.5 m and width spacing is about 1.333 m; wall offsets are 0.75 m and about 0.667 m. Reducing both spacing limits to 1 m gives a 6 × 4 grid. These example inputs are not residential illuminance recommendations or product specifications.
For a 6 m × 4 m surface, 200 lx requires 4,800 lm already incident on that surface. This does not state how a luminaire should deliver those lumens.
Limits and interpretation
This is an average-lumen model and a cell-centre geometry convention. It does not calculate point illuminance, shadows, reflectance/CU, glare, daylight, beam overlap, obstructions, sloped ceilings or asymmetric room shapes. A regular grid and a stated spacing limit do not prove adequate light at every point.
Obtain a luminaire-specific photometric layout when those conditions matter. Recessed-luminaire clearances, insulation compatibility, electrical circuit loading, emergency lighting and applicable installation requirements are outside this model. The preview draws up to 400 positions; larger valid grids retain full numerical results.
The surface mode is only area-average arithmetic for incident flux. It does not calculate point illuminance, beam distribution, source-to-surface delivery, reflectance, utilization, maintenance, glare, uniformity or a lighting target.
Common questions
Why can the grid quantity exceed the lumen-only number?
Both directional spacing limits and complete rows/columns constrain the proposal. The lumen-only number is a lower bound for its average-light arithmetic, not a full placement plan.
Can I leave CU or LLF blank and use 1?
Only enter 1 when supported for the calculation basis. The tool requires both fields because assuming no utilization or maintenance losses can overstate the expected average.
Does the preview show the light pattern?
No. The points show proposed luminaire centres. They are not beam outlines, illuminance contours or a uniformity assessment.
What does incident flux mean?
It means luminous flux already reaching the defined surface. Source lumens from a lamp or luminaire are not automatically incident surface flux.
How are surface lumens and lux related?
Average lux equals incident lumens divided by surface area in square metres. Conversely, incident lumens equal average lux multiplied by area.
Does this choose a lighting fixture?
No. The surface mode does not apply CU, light loss, beam distribution, fixture count or installation requirements.
Related tools
Sources and model scope
National Research Council Canada, G. R. Newsham and D. M. Sander, The effect of office design on workstation lighting: a simulation study, NRCC-45357, section 4.4, printed page 12 / PDF page 14, uses the Lumen Method with source lumens, CU, depreciation and area. Only that general relationship is used here; its office geometry, targets and partition corrections are not imported. The equal-cell grid is this tool's stated layout convention.
Canadian Conservation Institute, Light, ultraviolet and infrared, glossary: lux is lumen per square metre. No museum illumination target is used. Sources checked November 10, 2026.
Report a problem or suggest a source; inputs are not attached automatically.