The recessed lighting calculator that thinks like an electrician

It depends on room size, ceiling height, and how bright the room should be. The standard method (the one this calculator uses) is the IES lumen method: multiply the room area by its target foot-candles, then divide by each fixture’s lumens adjusted for how much light actually reaches the room. A 16′ × 12′ living room with a 9′ ceiling typically needs six 1,200-lumen fixtures; a kitchen the same size needs roughly twice that because kitchens call for 30–50 foot-candles instead of 10–20.

The quick rule of thumb is ceiling height divided by two — about 4 feet apart under an 8-foot ceiling. The precise answer multiplies the fixture’s spacing criterion (printed in its photometric data, usually 1.1–1.5) by the mounting height above the surface you’re lighting, then checks that beam footprints overlap. In practice that lands at 4–6 feet in kitchens and task areas and 5–8 feet in living spaces.

Half of your fixture spacing, up to about 3 feet. That keeps light at the edges of the room as even as light in the middle. Fixtures meant to wash the wall itself are the exception — place those 2–3 feet from the wall, spaced 2–3 feet apart.

6-inch fixtures put out more light (typically 900–1,400 lumens), so you need fewer of them for general room lighting — they’re the default for living rooms and bedrooms. 4-inch fixtures (600–900 lumens) trade output for a cleaner ceiling and tighter beam control, which suits kitchens, hallways, and task clusters. Many designs mix both.

A 60° beam is the all-purpose choice for 8–9 foot ceilings — at 9 feet it throws a 10-foot pool of light. Go narrower (24–40°) for accent lighting or tall ceilings, and wider (90–120°) for low ceilings or when you want maximum coverage from few fixtures. Narrow beams need tighter spacing to avoid dark stripes between fixtures.

Canless (wafer) LEDs are thin discs wired to a junction box — the easiest retrofit, and they fit shallow ceilings. Traditional canned housings take replaceable trims and lamps, which makes future changes easier. Both light a room identically if the lumen output matches; choose by construction constraints, and check IC and airtight ratings either way.

Yes, wherever ceiling insulation will touch the fixture — an IC (insulation contact) rating means it can’t overheat under insulation. Most modern LED downlights are IC-rated and airtight; verify it on the spec sheet before buying.

A 15-amp, 120-volt circuit safely carries about 1,440 watts of continuous load. Modern LEDs draw 9–15 watts each, so the breaker allows far more fixtures than any room needs — in practice you’ll be limited by your dimmer’s rated wattage and wiring layout, not the circuit. Every saved layout here includes a circuit-load check in its printable report.