Window Calculator

Rough opening vs window size

The rough opening (RO) is the framed hole — add 2 inches to both width and height of the window unit. A 36×48 window needs a 38×50 RO. The header spans the full RO width; size it by load (a 4-ft span typically uses a double 2×8, 6-ft needs a double 2×10). Always verify with the window manufacturer's installation guide.

Window calculator FAQ

What is the rough opening for a window?

Rough opening (RO) is the framed hole in the wall — typically 2 inches wider and 2 inches taller than the actual window unit. A 36×48 window needs a 38×50 rough opening. Always check the manufacturer's spec sheet, as it varies by brand and window type.

How much space should be between windows?

Structurally, leave at least one stud-bay width (16 inches) between window rough openings to maintain wall strength. From a design standpoint, 12 to 18 inches of wall between windows is common. Very large windows may require engineered headers spanning the combined opening.

How do I calculate the total glass area of a wall?

Multiply each window's width by its height to get the glass area per unit, then add them all together. This matters for load calculations, heat gain/loss estimates, and permit drawings. Remember the glass area is smaller than the rough opening — most window units lose 2 to 3 inches per side to the frame.

How many windows can fit in a standard wall?

A standard 8-ft wall typically fits 2 to 3 double-hung windows side by side at comfortable spacing. The limit depends on the wall length, header size, and structural load above. Code usually requires at least 10% of floor area in natural light for habitable rooms.