Daylighting Factor Calculator

Net Glazing Area is the dominant driver of Daylight Factor across its full 0.1-5,000 sq ft declared range -- more than Glass Transmittance, Sky Angle Factor, Average Surface Reflectance, or any of the room-dimension inputs -- but that dominance comes entirely from how wide its own range is, not from carrying more weight in the formula. The BRE formula multiplies Net Glazing Area, Glass Transmittance, and Sky Angle Factor together, so near this calculator's defaults the three are in an exact tie: a +10% nudge to any one of them alone (Net Glazing Area, Glass Transmittance, or Sky Angle Factor) raises Daylight Factor from 1.06% to 1.16%, identically, since a proportional change to any factor in a product changes the product by the same proportion. Net Glazing Area only pulls ahead once you sweep each input across its own full declared range -- its 0.1-5,000 sq ft span (50,000x) dwarfs Glass Transmittance's 0.1-0.95 (9.5x) and Sky Angle Factor's 0.1-1 (10x) span, so the swept range of Daylight Factor ends up far wider for Net Glazing Area even though none of the three has any special weight in the formula itself. Room size works against daylighting, not for it: raising Room Width, Room Depth, or Ceiling Height each lowers Daylight Factor across their whole declared ranges, because all three feed the Total Surface Area denominator in the BRE formula without adding any window area of their own -- a bigger room with the same window just dilutes the same daylight over more floor, wall, and ceiling.

Average Surface Reflectance moves the opposite way: lighter, more reflective interior finishes raise Daylight Factor across the full range, since more of the light that does enter keeps bouncing around the room instead of being absorbed. Window-to-Floor Ratio is a simpler, more literal comparison -- it only cares about Net Glazing Area relative to floor area, so Average Surface Reflectance and Sky Angle Factor never move it at all, no matter how much daylight actually reaches the workplane. Estimated Daylight Autonomy is a capped linear readout of Daylight Factor (roughly DF x 20, capped at 100%), so once Net Glazing Area alone pushes Daylight Factor to about 5% at this calculator's other default values, Daylight Autonomy has already hit its 100% ceiling and stops climbing even if more glass is added.