Free Window & Door Calculator – Rough Opening & Trim

Window & Door Rough Opening & Trim Guide

The Window & Door Calculator does two related jobs: it totals the rough opening area you are framing and it estimates the linear feet of casing (trim) needed to finish those openings. Enter the rough opening width and height in inches for your windows and doors, along with how many of each you have. The rough opening is the framed hole, sized larger than the window or door unit itself to leave room for shimming, leveling, and the unit's frame, so use the opening dimensions, not the bare glass or slab size. Choose how many sides of the wall get trim: interior only (one side) or both interior and exterior (two sides). The area figure helps you size headers, flashing, and house-wrap cut-outs and quickly tells you how much wall is glazing versus solid framing. The casing figure feeds your trim board order. Group openings by size so the counts stay accurate, then enter a trim stick length, commonly 8, 10, 12, or 16 ft, to convert linear feet into boards.

Rough opening area = (width in / 12) x (height in / 12) x count. A 36 x 60 inch window equals 3 ft x 5 ft = 15 sq ft; five of them total 75 sq ft. Casing differs by opening type. Windows are trimmed on all four sides, so the perimeter is 2 x (width + height); a door is trimmed on only three sides (two legs plus the head, no sill), so its perimeter is width + 2 x height. Casing linear feet = perimeter (ft) x count x sides, with a built-in 10% waste for miter cuts. For one 36 x 60 inch window, 2 x (36 + 60) = 192 in = 16 ft; trimmed on two sides with 10% waste that is 16 x 2 x 1.1 = 35.2, rounded to 36 linear feet. A 36 x 80 inch door is (36 + 160)/12 = 16.33 ft per side, two sides plus waste, about 36 linear feet as well.

Casing is the figure most people underestimate, because every opening has four mitered corners (three on a door) and each miter wastes a few inches you cannot reuse, which is why a 10 to 15% waste factor is standard rather than generous. Mixing up the unit size with the rough opening is the other frequent error: a rough opening is typically about 1/2 inch larger than the window unit on each side, and roughly 2 inches wider and 2.5 inches taller than a door slab to fit the jamb. Convert linear feet to boards by dividing by your stick length and rounding up, and prefer longer sticks for long runs to minimize butt joints. Building codes shape these openings: the International Residential Code (IRC) sets minimum egress dimensions for bedroom windows (a clear opening area and minimum width and height) and requires safety glazing near doors and floors per the referenced standards, so verify egress and tempered-glass requirements before finalizing any opening size.