Stick-Built Curtain Wall Calculator

Total Glass Area is the one output in this calculator that Mullion Spacing and Transom Spacing never move at all, across their entire declared ranges. That's not an oversight -- it falls straight out of the geometry: this calculator treats mullions and transoms as zero-width grid lines, so however many bays the facade gets carved into, the individual panel widths and heights always divide the full Facade Width and Facade Height exactly, and panel count times panel area cancels straight back to Facade Width times Facade Height every time. Tightening the mullion grid changes how many panels you get and how big each one is -- never how much total glass the facade needs. That identity relies on there always being at least one bay along each axis, though, which is why bay count is floored at 1 rather than left to round down to zero when a requested spacing is coarser than the facade itself (a 1 m wide facade with 1.5 m mullion spacing, for instance, still gets a single full- width bay, not zero bays and zero glass). When that flooring kicks in, Spacing Infeasible flags it (1) and Spacing Note names which axis's requested spacing didn't fit evenly, so the calculator never silently reports a coarser grid than what was actually asked for.

Wind Load and mullion/transom spacing matter for a different output, Min. Mullion Ix -- the minimum bending stiffness a mullion profile needs to keep wind-load deflection within the L/175 limit this calculator applies (a standard curtain-wall serviceability check). Transom Spacing enters that formula cubed (it sets the mullion's unsupported span), so a modest increase in floor-to-floor height raises the required stiffness fast; Design Wind Load enters linearly but ranges across two full orders of magnitude (100-10,000 Pa), so at the far ends of its declared range it can move the required stiffness by more than Transom Spacing does, so which of the two has the bigger say flips depending on where in their declared ranges you're standing rather than holding a fixed pecking order. Mullion Spacing, meanwhile, works the opposite direction on Mullion Lines: a wider spacing always means fewer vertical mullions across the same facade width, never more.