There is one rule, it takes ten seconds, and it is right in most rooms:
Spacing between fittings = ceiling height ÷ 2. Distance from the wall = spacing ÷ 2.
An 8 ft ceiling gives 4 ft spacing and 2 ft from the wall. A 9 ft ceiling gives 4.5 ft and 2.25 ft. A 10 ft ceiling gives 5 ft and 2.5 ft.
Why ceiling height and not room size
A downlight emits a cone. The higher the fitting, the further that cone travels before it reaches the floor, and the wider the circle it makes when it gets there.
Spacing is really asking a different question: how far apart can two circles be and still overlap? Since the circle size is set by the height, so is the spacing. Room dimensions decide how many fittings you need, not how far apart they go.
Get it wrong in the wide direction and you get scalloping — bright pools with dark gaps between them, and a row of arcs up the walls.
Working an actual room
Take a 12 × 14 ft room with an 8 ft ceiling.
Spacing is 4 ft, wall offset is 2 ft.
Along the 12 ft wall: 12 − 2 − 2 = 8 ft of run, divided by 4 ft spacing = 2 gaps, so 3 fittings. Along the 14 ft wall: 14 − 2 − 2 = 10 ft of run, divided by 4 = 2.5 gaps. Round to 3 gaps and tighten the spacing to 3.3 ft, so 4 fittings.
That gives a 3 × 4 grid, 12 fittings. Always round to more fittings and closer spacing rather than fewer and wider — a slightly tight grid is invisible, a slightly loose one scallops.
| Ceiling | Spacing | From wall |
|---|---|---|
| 8 ft | 4 ft | 2 ft |
| 9 ft | 4.5 ft | 2.25 ft |
| 10 ft | 5 ft | 2.5 ft |
| 12 ft | 6 ft | 3 ft |
The wall offset does more work than people expect
Two feet from the wall is not arbitrary. It puts the edge of the cone on the wall surface rather than on the floor in front of it, which washes the wall — and a lit wall is the single biggest factor in whether a room reads as bright.
Push the fittings out to the middle of the room and the walls fall dark. The room reads as gloomy even when the floor is well lit, and people respond by adding more fittings in the middle, which makes it worse.
Where the rule breaks
Over a worktop or an island. Task areas are not general lighting. Fittings go directly over the working surface, typically 2–3 ft apart, ignoring the grid. Getting the grid right and then discovering it puts a fitting over the middle of nothing is the most common kitchen error.
With a beam angle far from normal. The rule assumes something like a 60° flood. A 24° narrow spot needs much tighter spacing; a very wide beam tolerates more. Check the fitting.
Very high ceilings. Above about 12 ft the rule gives spacing so wide that each fitting has to be very powerful, and the room ends up with a few intense sources and heavy shadows. Above that height, use fewer downlights and more of something else.
Sloped ceilings. Measure to the point directly above each fitting position, not to the ridge.
Fittings alone will not light a room
A grid of downlights is one layer. It lights the floor and it lights the tops of things. It leaves faces shadowed and the room slightly flat, because all the light arrives from the same direction.
Rooms that look good use downlights for the general layer and add something at eye level — table lamps, wall lights, a floor lamp — for the rest. This costs less than adding more downlights and does far more.
How many lumens across the grid
Spacing tells you where. It does not tell you how much. Work out the room total first from lumens per square foot by room, then divide by the number of fittings the grid produced — that gives the output each one needs.
The room lighting calculator does both at once: pick the room type and size and it returns the total lumens, the bulb count and the recommended color temperature together.