Lighting Fixture Count and Spacing Estimator
Estimate a minimum fixture count and review an even-row spacing example from room area, target average illuminance, and luminaire data.
Lighting details
Use a value from the luminaire photometric data or a lighting calculation. The displayed value is an editable example.
Enter the design factor for expected lumen depreciation, dirt, and other light losses. The displayed value is an editable example.
Set a rough row count. The estimator rounds fixtures per row up to cover the minimum count.
Minimum fixture count
The even layout uses equal center-to-center spacing and half-spacing at room edges. It may add fixtures beyond the minimum count.
About this estimator
Enter room dimensions, a project target for average illuminance, luminaire output and design factors, and a rough fixture row count. The estimator calculates the minimum count and an even-row layout example with center-to-center spacing.
The example values are editable inputs, not room-specific design recommendations.
Method and assumptions
Room area = length × width. Effective lumens per fixture = fixture lumens × coefficient of utilization × light-loss factor. Minimum fixture count = round up (room area × target footcandles ÷ effective lumens per fixture).
For the layout example, fixtures per row = round up (minimum count ÷ entered row count). Center spacing along the room length = room length ÷ fixtures per row. With more than one row, spacing between rows = room width ÷ row count. A single row is centered across the room width. This assumes a rectangular room, equal spacing, and half-spacing between the outside fixtures and room edges.
The average-illuminance estimate represents an idealized layout with uniform overhead spacing and one luminaire type. It does not account for sloped ceilings, daylight, task lighting, or changes in fixture output across the room. The example layout may use more fixtures than the minimum count.
Interpret the result
Use the count and spacing as early planning estimates. The spacing is geometric; it does not use a luminaire spacing criterion or predict uniformity. Check the selected fixture's photometric data, mounting height, room surface reflectance, glare, obstructions, and the target for the task and space before setting a final layout.
FAQs
Does this choose the target illuminance for a room?
No. Enter the target for the room and task from the project requirements or a qualified lighting design. The example value is editable and is not a recommendation.
What is the coefficient of utilization?
It estimates the fraction of luminaire output that reaches the work plane, based on the luminaire and room characteristics. Use photometric data for the selected fixture.
What should I use for the light-loss factor?
Use a project value that accounts for expected lumen depreciation, dirt, and other light losses. The factor depends on the fixture, maintenance plan, and environment.
Does the spacing estimate guarantee even lighting?
No. It shows equal geometric spacing for the row count you enter. Fixture photometry, mounting height, room surfaces, glare, and obstructions determine the actual lighting distribution.
Sources
- U.S. Department of Energy FEMP, Energy-Efficient Indoor Environmental Quality Assessment Guide — lumen-method inputs, average illuminance estimate, and idealized uniform-layout assumptions.
- Use the selected luminaire's current photometric report for its coefficient of utilization, spacing criterion, and light-loss assumptions.