Calculator Power
Downlight calculator
Work out how many downlights a room needs and how far apart to space them, using room size, ceiling height and the lumen output of the fitting you have chosen.
Sets a starting light level, which you can change below.
The longer side, in metres.
The shorter side, in metres.
A 100° beam lights about 5.72 m across at floor level, so the 2.5 m spacing overlaps.
A starting point, not a rule: Australian lighting retailers publish general-lighting figures from about 100 to 250 lux. Change it to suit how you use the room.
From the fitting's specs. Most general-purpose LED downlights are 400 to 1,200 lm, and the count moves with it.
Downlights
4 fittings
20 m² × 150 lx = 3,000 lm ÷ 800 lm = 4 fittings2 × 2 grid, spaced 2.5 × 2 m, half that at the walls
Light neededlm = area × target luxHow manyn = total lm ÷ lm per fittingSpacinggap = room side ÷ rows, half at the wallsBeamspread = 2 × height × tan(beam ÷ 2)
How the calculation works
Take the floor area of the room, multiply it by a target for how much light it should get, and divide by the output of the downlight you plan to use. That gives you a number of fittings. The spacing then comes from laying that number out evenly across the ceiling.
The formulas
-
Light needed
Area in square metres, length times width.
-
How many downlights
Round up rather than down. A part fitting is not an option.
2400 lm ÷ 800 lm = 3 fittings
Where those targets actually come from
This is the part most downlight calculators skip. The room-by-room lux figures you see quoted are industry convention, not a rule. AS/NZS 1680 is the interior and workplace lighting series, and it sets recommended illuminance for tasks and workplaces rather than a schedule of levels for the rooms of a house. The figures Australian lighting retailers publish for general lighting sit in a range of roughly 100 to 250 lux, and different sources land in different places inside it.
So treat the number the calculator starts you with as a sensible default, not a target you have to hit. A kitchen bench wants more light than a bedroom, and how much more is a judgement about how you use the room.
Spacing follows the beam, not a magic number
Two things decide how far apart the fittings go: the beam angle of the downlight and the height of the ceiling. At a 100 degree beam, the pool of light at floor level is about 2.4 times the ceiling height, so a wide beam in a high ceiling covers more floor per fitting than a narrow one in a standard ceiling. Identical rooms with different fittings do not want identical layouts.
The convention for the edges is to leave the perimeter gap at about half the spacing between fittings. Push the outer row too close to the walls and you get bright scallops on the plaster with a dim middle. Push it too far in and the edges of the room go dark.
Picking the fitting before the count
Because the count divides by the output of one downlight, the fitting decides the layout. A 450 lumen fitting and a 1200 lumen fitting in the same room are a difference of several holes in the ceiling. If you are replacing halogens, start with GU10 vs MR16, and if you are working from a wattage rather than a lumen figure, convert it first with the lumens to watts calculator.
Frequently asked questions
How do I calculate how many downlights I need?
Multiply the room's floor area in square metres by a target in lumens per square metre, then divide by the lumen output of the downlight you plan to use. Round up rather than down.
How many downlights per room in Australia?
There is no per-room number, because it depends on the area, the ceiling height and the output of the fitting. A small bedroom with 800 lumen downlights usually lands on three or four; a large open-plan kitchen and living area can need ten or more.
What is the rule of thumb for downlights?
The useful one is about beam spread rather than a fixed count: at a 100 degree beam, the light reaching the floor covers roughly 2.4 times the ceiling height, and the gap to the wall sits at about half the spacing between fittings.
How many downlights per m2?
Work in lumens rather than fittings. General lighting targets published in Australia run from about 100 to 250 lux, and how many fittings that takes depends entirely on how bright each one is.
Still need a hand?
We answer most messages within one business day. If your question is about an existing order, have your order number ready. It speeds things up.
Contact us