About this tool
The Recessed Lighting Layout Calculator is a free, in-browser tool that works out how many recessed can lights (downlights) a room needs and how to space them in an even grid. It uses the classic rule of thumb that the maximum spacing between fixtures equals the ceiling height divided by two — so an 8-foot ceiling gets lights roughly every 4 feet. Enter the room length and width, the ceiling height, and the room type, and it returns the fixture grid, spacing, wall offset, and a lumen target for balanced ambient light. All math runs locally in your browser.
The method computes the spacing criterion S = ceiling height ÷ 2, then the number of fixtures along each wall = room dimension ÷ S (rounded up), so total cans = rows × columns. Fixtures are set in from the walls by about half the spacing (S ÷ 2) so the edge of the room is not left dark. A lumen check sizes the light level: recommended total lumens = floor area × a per-square-foot target (living rooms ~20, kitchens ~35, bathrooms ~40, offices/task ~50 lumens/ft²), and dividing by the fixture count gives the lumens each can should deliver.
As a worked example, a 16 ft × 12 ft living room with an 8-foot ceiling has S = 4 ft, so it needs 4 fixtures along the length and 3 across the width — a 4 × 3 grid of 12 cans, offset 2 ft from the walls and spaced 4 ft apart. At 20 lumens/ft² the 192 ft² room wants about 3,840 total lumens, or roughly 320 lumens per fixture. Treat the grid as a starting point and adjust for task zones, islands, and wall-wash accents.