Skylight Sizing Calculator

This calculator starts from a rule of thumb -- skylight area equal to about 5% of floor area yields roughly a 5% daylight factor -- then adjusts Skylight Area Needed for Ceiling Height, Roof Slope, and Latitude before dividing by a standard 2x4 ft skylight's area to recommend how many units to install. Achieved Daylight Factor is the one output built to always land back on whatever you entered for Target Daylight Factor: the same three adjustment factors (height, slope, and latitude) that scale up Skylight Area Needed also scale the ratio used to compute Achieved Daylight Factor, and they cancel out exactly, so Ceiling Height, Roof Slope, and Latitude are all inert to this particular output even though they clearly move Skylight Area Needed itself.

Roof Slope in particular can move Skylight Area Needed substantially at steep pitches, since the calculator divides by the cosine of the slope angle (floored at a minimum divisor of 0.5, equivalent to a 60-degree pitch) to account for a sloped skylight capturing less direct overhead light than a flat one at the same nominal area. Peak Heat Gain is a flat 300 BTU/hr per square foot of Skylight Area Needed, intended as a rough cooling-load flag, not a substitute for a full solar heat gain calculation with the actual glazing's SHGC.