Recessed lights for a 18 × 30 ft bathroom with a 9 ft ceiling
| Layoutacross the 18 ft dimension × along the 30 ft dimension | 4 × 7 |
|---|---|
| Spacing (rule of thumb)half of the 9 ft ceiling | 4.5ft |
| Actual spacing to mark outafter rounding to whole fixtures | 4.5 × 4.3ft |
| First light from the wallhalf a spacing — closer washes the wall, further leaves dark corners | 2.3 × 2.1ft |
| Room area | 540ft² |
| Target light levelgeneral light — the mirror needs light from the sides, not above | 30fc |
| Total lumens needed | 16,200lm |
| Lumens per fixtureabout 8 W each in LED | 579lm |
Notes
- The spacing rule is ceiling height ÷ 2, which gives 4.5 ft here. Rounding to whole fixtures moves it to 4.5 × 4.3 ft — mark the ceiling off the real numbers, not the rule.
- 579 lumens per can is the figure to shop by. A 4″ LED downlight typically runs 600–800 lm and a 6″ runs 800–1,200 lm, so you could drop to 4″ cans or dim these down.
- Bathroom is a task space. 30 fc is the general level — countertops, desks and mirrors still need their own light. Ceiling cans alone put the light behind you and your own shadow onto the work.
- Put them on a dimmer. It costs almost nothing at install, and it is the difference between a ceiling that works at dinner and one that works at 11pm.
- Check the joists before marking. A 4 × 7 grid rarely lands clear of framing on the first try, and moving a can 6 inches to miss a joist is invisible once the trim is on.
The grid
Spacing for recessed cans is ceiling height divided by two. A 9 ft ceiling gives 4.5 ft, which across a 18 × 30 ft room works out to a 4 × 7 grid — 28 lights.
Rounding to whole fixtures moves the real spacing to 4.5 × 4.3 ft. Mark the ceiling off those numbers, not off the rule of thumb. The first row sits 2.3 ft from the wall — half a spacing. Closer and you get a bright band down the wall; further and the corners go dark.
What to buy, not just how many
540 ft² at 30 footcandles needs 16,200 lumens in the room, which is 579 lumens per fixture — around 8 W in LED.
That is the number to shop by, and it is the one most layout calculators leave out. A 4″ LED downlight typically runs 600–800 lm; a 6″ runs 800–1,200 lm.
Before you cut
Find the joists first. A 4 × 7 grid rarely lands clear of framing on the first attempt, and shifting a can six inches to miss a joist is invisible once the trim is on. Put the whole layout on a dimmer while the wiring is open — it costs almost nothing then and cannot be added cheaply later.
Nearby sizes
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Read more
- LED Dimmer Compatibility: The Four Checks Before You BuyCompatibility is not a mystery, it is four specifications. Check them before buying and you avoid the trial-and-error most people go through.
- Why LED Lights Flicker When Dimmed — and the Fix OrderThe dimmer is usually not broken and the bulbs are usually not faulty. They were designed for each other's opposites, and there are four fixes worth trying in order.
- CRI 90 vs 80: What the Ten Points Actually Cost YouTen points on a hundred-point scale sounds marginal. It is the difference between a room where the colors look right and one that feels vaguely tired.
- Color Temperature Chart: Every Kelvin Value and Where It GoesThe scale runs backwards from intuition and the useful range is narrower than the chart suggests. Here is every value, what it reads as, and where it belongs.