Recessed lighting layout takeoff

Downlight layouts are almost always decided by spacing and almost never checked against light level, which is why so many rooms end up either gloomy or lit like a showroom. The illuminance row asks the question the other way round: for the fittings you intend and the room you have, how many of the fittings' rated lumens are there per unit of floor area? Enter the fitting's own output from its data sheet: a bare lamp's lumens are more than the same lamp gives from inside a can. That is an upper bound. It counts every lumen as reaching the floor, before the room's surfaces and ageing take their share, and in an ordinary room about half of it arrives at the working plane. Around 150 lux (14 fc) maintained suits a living room, 300 (28 fc) a kitchen work surface and 500 (46 fc) a desk, so read that row against roughly twice those — and a room maintained at 800 (74 fc) is not a better room, it is an uncomfortable one.

The lumen method row takes those losses off: how many of these fittings the target level needs, and the grid that keeps every pitch within the fitting's spacing-to-height ratio. The row works over a working plane at worktop height, 0.85 m (2 ft 9 in). That ratio and the utilisation factor belong on the fitting's data sheet, and on this page the row always uses the calculator's placeholders — a ratio of 1.25, a utilisation factor of 0.6 and a maintenance factor of 0.8 — so set them on the line once it is in your project. A rule of thumb — ceiling height divided by two, or a grid at 1.2 m (4 ft) — would be a fabricated spacing.

Average illuminance is also an average. It says nothing about where the light lands, and a layout with the right total in the wrong places gives a bright floor and a dark worktop. Use the number as a sanity check on the fitting count, then place them against what the room is for.