Formula
The spacing rule sets the center-to-center distance equal to the ceiling height and counts round(length ÷ spacing) by round(width ÷ spacing), at least one each way. The wall offset is half of the actual spacing once the count is known. The lumen target is foot-candles × floor area, divided by the lumens of one can, rounded up. The layout uses the larger of the two counts and picks a grid close to the room’s shape.
spacing count = round(length ÷ ceiling) × round(width ÷ ceiling)
lumen count = ceil(foot-candles × area ÷ lumens per can)
These foot-candle numbers are midpoints of the lower published ambient band: living and bedroom and dining 15 (summaries say 10–20), kitchen and bathroom 35 (20–50), hall 7.5 (5–10), office 40 (30–50), garage 30 as a planning figure. They are not a transcription of the IES illuminance selector, which was not copied. Bedroom reprints in particular run higher. Task light at a counter, a desk, or a mirror is extra. Multiplying foot-candles by area assumes every lumen reaches the work plane. It does not. A coefficient of utilization and a light-loss factor would both raise the lamp lumens. The diagram is the count, not a photometric plot.
Worked examples
12 × 16 ft kitchen, 8 ft ceiling, 35 fc, 800 lm cans. Area is 192 ft². The work plane wants 6,720 lumens, which is 9 cans. The spacing rule is round(16/8) × round(12/8) = 4 cans. Use 9. The lumen target is the tighter grid.
The same room as a bedroom at 15 fc. Lumens drop to 2,880, which is 4 cans of 800 lm, and that matches the spacing rule. A vanity light is not in that count.
Reference
Targets used when you pick a room. Edit the foot-candle field to override.
| Room | Foot-candles | Published band this midpoint sits in |
|---|---|---|
| Living room | 15 | 10–20 |
| Bedroom | 15 | 10–20 |
| Kitchen, general | 35 | 20–50 |
| Dining | 15 | 10–20 |
| Bathroom | 35 | 20–50 |
| Hall | 7.5 | 5–10 |
| Office or study | 40 | 30–50 |
| Garage | 30 | planning figure |
Questions
How far apart should recessed lights be?
A residential rule of thumb puts the cans about one ceiling height apart, with the first can half that distance off the wall. In an 8 ft room that is an 8 ft spacing. It is not an IES spacing criterion.
How many lumens does a 12 by 16 foot kitchen need?
At a general-ambient target of 35 foot-candles the work plane wants 35 × 192 = 6,720 lumens. Nine 800-lumen cans cover that. The spacing rule alone would drop four cans in the same room. The page uses the larger count. Task lights over a counter are extra, and the 35 ignores how much light the fixture actually delivers to the floor.
Are the foot-candle numbers from IES?
They are midpoints of published summaries of residential guidance, in the lower ambient band. The IES illuminance selector was not copied. Bedroom summaries in particular disagree, from 10–20 up to 20–50. This page uses 15 for a bedroom and says task light is separate.
Sources
- Spacing ≈ ceiling height, first can at half spacing from the wall. A residential rule of thumb, not an IES spacing criterion.
- Foot-candle midpoints of published summaries of residential ambient guidance. The IES illuminance selector (RP-11 and the current selector) was not transcribed. Bedroom summaries conflict. This page uses the lower ambient band and treats task light as extra.
- Lumens = foot-candles × area ignores coefficient of utilization and light-loss factor.