Rough Opening Calculator — Windows & Exterior Doors

For windows, frame the rough opening 1/2 inch wider and 1/2 inch taller than the unit call size as a default (1/4 inch per side plus top clearance), then confirm the manufacturer sheet — vinyl nailing-fin units often want 1 inch total width clearance. For prehung exterior doors, the rough opening calculator adds 2 inches to the slab width (3/4 inch jamb plus 1/4 inch shim per side) and 2.5 inches to the slab height (head jamb plus threshold bed). A 36×80 exterior door therefore gets a 38 × 82.5 inch rough opening. Oversizing beyond the spec by more than 1/4 inch per side forces shim stacks wider than 3/8 inch, which is where sagging casings and latched-door misalignment begin.

Frame 38 inches wide by 82.5 inches high for a standard 36×80 prehung exterior door. The 2 inches of added width covers the 3/4 inch jamb on each face plus 1/4 inch of shim per side; the 2.5 inches of added height covers the 3/4 inch head jamb, 1/4 inch of shim, and roughly 1.5 inches of threshold assembly sitting on the subfloor. If your door ships with a 1-1/4 inch inswing threshold or you are setting the door on a raised porch pan, add that depth to the 82.5 inches rather than burying the slab in the opening.

A 38 inch rough opening in a single-story exterior 2×4 wall bearing roof load uses a doubled 2×6 header (2-ply 2×6, 5.5 inches deep) with one jack stud per side. In a 2×6 exterior wall the same span uses 2× 2×6 with plywood filler to reach the 5.5 inch cavity depth. The header table above steps up to 2× 2×8 at 48 inches, 2× 2×10 at 66 inches, and 2× 2×12 at 78 inches. Two conditions force an upgrade to engineered LVL: the opening carries a second story or a roof valley load, or the clear span exceeds 8 feet — at that point header depth is a beam calculation, not a rule of thumb.

The king stud runs the full wall height from bottom plate to top plate and anchors the sheathing and drywall edge; the jack (trimmer) stud is cut short, fits inside the kings, and carries the header ends down to the bottom plate. Every rough opening needs at least one jack per side bearing the header, and one king per side tying the assembly into the wall run. On openings wider than 6 feet, or in high-wind and seismic zones, the calculator and most inspection checklists double the jacks — two per side — so the header bearing area never exceeds the allowable compression across a single 2× end grain.

Yes — cripples above the header are what stop the top plate from sagging into the opening between the kings. Frame one cripple for every 16 inches (or your wall's O.C. layout) of rough opening width, starting with the longest cripple flush to the jack line and stepping down to the top plate. Windows also need cripples below the rough sill to carry the unit's weight into the subfloor, typically one per 16 inches of sill span. A rough opening framed without cripples may pass a visual check but fails a level-string inspection, because within one heating season the plate line dips visibly at the opening.

Yes, and it is expensive to ignore. Extra width is fixable: add a second jack stud and move the king outward, or rip-filler a 2× strip to the jamb face and re-nail — but extra height is the dangerous one, because the header bearing height cannot be recovered without re-framing. If the rough opening is more than 1/2 inch over spec in height, the correct fix is to add a sill plate under the jacks (for doors) or lower the rough sill (for windows) rather than shimming the unit up on plywood pads. This rough opening calculator keeps you out of that situation by printing the RO before you cut the first stud.

The rough opening width and height math does not change — clearances are driven by the unit, not the wall. What changes is everything vertical around it: pre-cut studs are no longer usable (92-5/8 inch pre-cuts are an 8-foot-wall standard, while 9-foot walls use 104-5/8 inch pre-cuts), cripple lengths grow, and headers in load-bearing tall walls may need an extra ply because the tributary wall weight above the opening increases. For a 9 foot wall with a 36×60 window, frame the same 37 × 60.5 inch RO but verify the header against a 9-foot tributary height before reusing the single-story table above.