What the Recessed Light Calculator actually calculates
Fixture count is room area times target lux divided by lumens per downlight, rounded up. The separate spacing guideline is ceiling height times 0.75. The engine does not construct a grid or check the count against that spacing; utilization losses, maintenance, beam angle, task-plane height, and glare are not included.
The calculation path
Target lumens determine fixture count; spacing is a separate ceiling-height heuristic, not a checked grid.
Inputs that can change the answer
- Room length: Enter room length in m.
- Room width: Enter room width in m.
- Target lux: Enter target lux in lux.
- Lumens per downlight: Enter lumens per downlight in lm.
- Ceiling height: Enter ceiling height in m.
Checks that stay outside the formula
Use beam-angle photometric data for high ceilings or demanding task areas.
A practical order for entering the data
- Enter room length, width, and the target average lux.
- Enter the lumen output of one downlight and the ceiling height in metres.
- Use the whole fixture count and spacing guideline as starting checks, then lay out a grid using the actual beam data.
Project walkthrough: Lighting: from lumen estimate to fixture layout
Project walkthrough: Build a layered kitchen lighting plan around tasks
Engine check using the prefilled sample
Lighting example: room length = 5 m, room width = 4 m, target lux = 200 lux, lumens per downlight = 750 lm and ceiling height = 2.7 m. Treat these as demonstration room and fixture values, then recalculate with the dimensions and rated output of the fittings you are considering.
Results for these inputs
- Target lumens
- 4,000 lm
- Downlights required
- 6 lights
- Guideline maximum spacing
- 2.03 m