How many downlights does a room need?
Work from the light level the room needs, not from a rule about square footage. Here is the arithmetic, with a worked example.
Almost every “how many downlights” answer online is a rule of thumb that ignores what the room is for. The arithmetic is not hard, and it takes two minutes.
Start with the light level, not the fixture
Each kind of space has a target illuminance in lux. A bedroom is comfortable at about 150 lux, a kitchen wants 300, an office 400, and detailed work 500 or more.
Convert lux to total lumens
Multiply the floor area in square metres by the target lux, then add about 25 per cent for losses — fittings, wall colour, and the way output falls over the life of the fixture.
Divide by the output of one fixture
A 9W COB gives roughly 800–900 lumens; a 12W gives about 1,100. Divide the total by that number and round up.
A worked example
A 12 x 14 ft bedroom is 15.6 square metres. At 150 lux that is 2,340 lumens, or about 2,900 with the allowance. Divided by 850 lumens per fixture, that is four downlights — plus bedside lamps for reading.
Then check the spacing
Space fixtures about 1.2 times the height between the fixture and the surface being lit, and keep them half that distance from the walls. Even spacing matters more than the exact count.
Where the rule of thumb fails
Dark paint, a deep room, a high ceiling or a lot of glass all push the number up. So does replacing a 38° beam with a narrower one.
Frequently asked questions
Is it better to have more low-wattage fixtures or fewer bright ones?
More, lower-output fixtures give flatter, more comfortable light. Fewer bright ones are cheaper to install but create bright patches and glare.
How far from the wall should the first downlight be?
About half the spacing you are using between fixtures u2014 typically 2 to 2.5 ft on a domestic ceiling.