Enter your room and ceiling height and pick a rule: ERCO’s upper limit for even light, or Kichler’s tighter residential rule of thumb. You get the number of downlights, the spacing and the distance from the walls, drawn to scale.
The two downlight rules differ; Kichler’s is the tighter, residential one. Wall-washer mode lights one wall evenly.
Measure to the underside of the ceiling the downlights will sit in.
ERCO measures from the height of a table or worktop, not the floor. Enter your own; there is no default.
ERCO rule. Lighting maker ERCO gives a rough guide for even general lighting: downlights can be spaced up to 1.5 times their height above the working plane (the height of a table or worktop, not the floor), and the first row sits half that spacing from the wall. Example: with a 2.7 m ceiling and a 0.8 m worktop, h is 1.9 m, so spacing up to about 2.85 m and about 1.4 m to the wall — an upper limit for evenness, not a target. The calculator takes h = ceiling height − working-plane height and a maximum spacing of 1.5 × h. With the first light half a spacing from each wall, a row of lights fills the room exactly when the spacing is the length divided by the number of lights, so the calculator rounds length ÷ (1.5 × h) up to a whole number of lights, then spacing = length ÷ lights and wall distance = spacing ÷ 2 — the same across the width. That rounding is our arithmetic on ERCO’s rule, not part of it.
Kichler rule. US lighting brand Kichler's residential rule of thumb is tighter: start downlights no less than 2 ft (about 0.6 m) from the walls, and space them at roughly the ceiling height divided by two, never more than 6 ft (about 1.8 m) apart. The calculator uses a spacing of ceiling height ÷ 2, capped at 1.83 m (6 ft), and keeps 0.61 m (2 ft) from each wall: lights per row = (length − 2 × 0.61) ÷ spacing, rounded up, plus one, and the actual spacing is then (length − 2 × 0.61) ÷ (lights − 1).
Wall-washers. To wash a wall evenly, ERCO's rule of thumb sets wallwashers back from the wall by about one third of the room height, and spaces them no more than 1.3 times that setback apart — for a 2.7 m ceiling, roughly 0.9 m from the wall and up to about 1.2 m apart. The calculator uses setback = ceiling height ÷ 3, maximum spacing = 1.3 × setback, and the number of fittings = wall length ÷ maximum spacing, rounded up.
All three are starting points. A lamp's beam angle is the angle at which its light intensity has dropped to half (50%) of the intensity at the centre of the beam — so a 24° spotlight gives a tight pool and a 60° downlight a wide one, and the pool gets wider the further the light is from the surface. For how bright the room should be, use the lux and lumen calculator.
| Factor | How it changes the result |
|---|---|
| Ceiling height | Higher ceilings allow wider spacing under both rules, up to Kichler’s 1.8 m cap. |
| Working-plane height | ERCO measures from the table or worktop, so a higher working plane means closer spacing. |
| Which rule | ERCO’s is an upper limit for evenness; Kichler’s tighter residential rule gives more lights. |
| Beam angle | A narrow beam makes a tighter pool of light, so the same spacing can leave dark gaps. |
| Furniture and tasks | Worktops, sofas and beds matter more than a perfect grid — shift lights to where people work. |
Rules of thumb from named lighting makers and ST’s recommended lux table — not a Malaysian design standard. The fitting’s beam angle and datasheet change the answer.
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