Work out window glass area instantly with clear inputs, formula shown and shareable results.
Glass area is the window opening less the frame on all sides and between panes, so more panes means more frame and less glass. The glazing ratio matters for both daylight and heat loss, since frame and glass have very different U-values.
Glass area
glass width = opening width - frame width x (panes + 1); glass height = opening height - 2 x frame
Glass and frame lose heat at different rates, and the whole-window U-value is an area-weighted average. Slim frames improve both daylight and thermal performance.
More than people expect. A 60 mm mullion in a 1.2 m window removes 5% of the glass area and creates a visible obstruction.