15×18 bathroom:
16 lights
16 fixtures A 15×18 ft bathroom (270 sq ft) needs approximately 16 recessed lights at 800 lumens each — about 11,731 lumens in total — laid out 4 across by 4 deep.
Space them 4.5 ft apart along the 18 ft wall and 3.8 ft apart along the 15 ft wall, starting 2.3 ft from the end walls.
The ceiling plan
Drawn to scale, with each fixture at its computed position and the pale circles showing roughly where each one lands on the working plane.
Same room, different fixtures
If you have already chosen a fitting, read across from its lumen output.
| Fixture output | Rough equivalent | Fixtures needed | Suggested grid |
|---|---|---|---|
| 450 lm | 40 W incandescent | 25 | 5 × 5 |
| 650 lm | 50 W incandescent | 20 | 5 × 4 |
| 800 lm | 60 W incandescent | 16 | 4 × 4 |
| 1000 lm | 75 W incandescent | 12 | 4 × 3 |
| 1200 lm | 100 W incandescent | 9 | 3 × 3 |
Fewer, brighter fixtures give more contrast; more, dimmer fixtures give flatter, more even light. Both rows deliver the same total.
How the 16 was calculated
| Target illuminance for a bathroom | 200 lux | published range 200–300 lux |
|---|---|---|
| Floor area | 25.1 m² | 270 sq ft |
| Mounting height above the working plane | 8 ft | 8 ft ceiling, measured to the floor |
| Room index | 1.02 | (L × W) ÷ (Hm × (L + W)) |
| Utilisation factor | 0.53 | from the room index, light surfaces |
| Maintenance factor | 0.8 | dirt and lamp depreciation |
| Lumens required | 11,731 lm | 200 × 25.1 ÷ (0.53 × 0.8) |
| Fixtures at 800 lm | 16 | delivering about 218 lux |
Even light on the face at the mirror matters more than ceiling brightness. Fixtures must carry an IP rating appropriate to their zone.
Other ceiling heights
| Ceiling | Fixtures | Grid | Spacing | Max spacing |
|---|---|---|---|---|
| 8 ft | 16 | 4 × 4 | 4.5′ × 3.8′ | 10.4′ |
| 9 ft | 16 | 4 × 4 | 4.5′ × 3.8′ | 11.7′ |
| 10 ft | 16 | 4 × 4 | 4.5′ × 3.8′ | 13′ |
| 12 ft | 20 | 5 × 4 | 3.6′ × 3.8′ | 15.6′ |
Questions
How many recessed lights do I need for a 15x18 bathroom?
About 16 fixtures at 800 lumens each, arranged 4 across by 4 deep. That gives roughly 218 lux across 270 square feet, against a target of 200 lux for a bathroom.
How many lumens does a 15x18 bathroom need?
Roughly 11,731 lumens in total. That comes from 200 lux over 25.1 m², divided by a utilisation factor of 0.53 and a maintenance factor of 0.8 — the light that never reaches the working plane has to be paid for up front.
How far apart should the lights be in a 15x18 room?
4.5 ft apart along the 18 ft wall and 3.8 ft apart along the 15 ft wall, with the outer row about 2.3 ft in from the end walls — half a spacing. With a 8 ft ceiling the maximum sensible spacing is around 10.4 ft before the floor starts to show scallops.
Does the ceiling height change the answer?
Yes, in two ways. A higher ceiling spreads each fixture's light over a wider area, so fixtures can sit further apart — but it also drops the light level reaching the floor, so a taller room generally needs more total lumens. At 8 ft the room index works out at 1.02, giving a utilisation factor of 0.53.
Computed with the lumen method — see how it works and where it is weak. Planning guidance only, not an electrical design. Fixture counts are a starting point — follow your local wiring rules and use a licensed electrician for the installation.