Window Natural Light Area Calculator

Window size for natural lighting

Under Brazilian code NBR 15575, window area has to reach at least 1/6 of the floor area for ventilation. Plenty of municipal codes carry that same 1/6 ratio over to natural lighting in habitable rooms. Run A_window = factor · A_floor with a factor between 0.15 and 0.20 and a 12 m² bedroom lands somewhere around 1.8-2.4 m² of clear glazing.

Hitting the minimum area is only part of the story. How good the daylight actually feels comes down to where the window sits (a higher one pushes light deeper into the room), which way the room faces (in the southern hemisphere, north-facing rooms catch more winter sun), and how much light the glass lets through. A daylight factor of 2% reads as good; 5% is excellent.

Applications

NBR 15575 housing performance, municipal codes (e.g. São Paulo COE), LEED EQ Daylight credit, NBR 15215 daylighting, public school CODE 21.

FAQ

Does kitchen need a window? Not always. Some codes let you swap it for mechanical exhaust, so check your local Plano Diretor / COE first.

Skylight counts toward the 1/6 rule? It does. In tropical latitudes a skylight actually pulls in more daylight per m² than a vertical window does.

How to handle a service area with only an air shaft? The light shaft (poço de iluminação) has to meet the minimum cross-section set by the COE and run up past the slab.