★ REFERENCE TABLE · CONSTRUCTION · DOORS
A 36 by 80 inch door needs a 38 by 82.5 inch rough opening — two inches wider, two and a half inches taller. That allowance leaves room for the frame, shims, and a level, plumb fit. The chart below lists the standard interior and exterior sizes; the calculator gives the rough opening for any door size you enter, standard or not.
The rough opening is the hole in the wall before the frame goes in — bigger than the door, on purpose. The extra two inches of width take the two jambs, and the extra two and a half inches of height take the head jamb plus room to level the frame. Frame to the door size and the door will not fit; frame to the rough opening and it will, with shims to spare. Enter the door slab size and get the opening to frame.
Print this chart for the job site. Every rough opening below follows the same rule: width + 2 inches, height + 2.5 inches. Interior doors run 24 through 36 inches wide; exterior doors are usually 32 or 36 inches wide. Heights of 80, 84, and 96 inches cover nearly every home.
| Door size (in) | Use | Rough opening width (in) | Rough opening height (in) |
|---|---|---|---|
| 24 x 80 | Interior, closet / bath | 26 | 82.5 |
| 28 x 80 | Interior, bedroom / bath | 30 | 82.5 |
| 30 x 80 | Interior, bedroom | 32 | 82.5 |
| 32 x 80 | Interior / exterior | 34 | 82.5 |
| 36 x 80 | Interior / exterior, entry | 38 | 82.5 |
| 32 x 84 | Interior / exterior, tall | 34 | 86.5 |
| 36 x 84 | Exterior / entry, tall | 38 | 86.5 |
| 32 x 96 | Interior / exterior, 8 ft | 34 | 98.5 |
| 36 x 96 | Exterior / entry, 8 ft | 38 | 98.5 |
Rough opening width = door width + 2 inches. Rough opening height = door height + 2.5 inches. This is the standard framing allowance: about an inch of shim and jamb space on each side, and two and a half inches at the head for the head jamb, shimming, and leveling. The rule applies to pre-hung and slab doors alike, because the frame — not the slab — sets the fit. Always check the door manufacturer's specs before framing: some pre-hung units call for a slightly different opening, and exterior units with thick thresholds or sills may need more height. Measure the slab, not the old frame, and measure twice — a rough opening framed too small is a wall to rebuild.
Setup: a 36 by 80 inch door.
Line 1 — rough opening width. 36 + 2 = 38 inches.
Line 2 — rough opening height. 80 + 2.5 = 82.5 inches.
Line 3 — the answer. Frame a 38 by 82.5 inch rough opening.
30 by 80 inches. Rough opening 32 by 82.5 inches — the standard bedroom door opening.
32 by 84 inches. Rough opening 34 by 86.5 inches — a tall door keeps the same 2-inch width allowance and adds the same 2.5 inches of height.
36 by 96 inches. Rough opening 38 by 98.5 inches — an 8-foot entry door, framed exactly the same way.
| Output | Formula | What it means |
|---|---|---|
| Rough opening width | Door width + 2 in | 38 in at the defaults — the stud-to-stud width to frame. |
| Rough opening height | Door height + 2.5 in | 82.5 in at the defaults — the floor-to-header height to frame. |
The rule is the standard framing allowance used across the trade, and it is exact for the numbers you enter — but it is a rule of thumb, not a substitute for the manufacturer's spec sheet. Some pre-hung units, exterior doors with sills, and fire-rated assemblies call for different openings; if the spec sheet disagrees with this chart, the spec sheet wins. The calculator assumes a level floor at the opening: a sloping floor changes the height you actually frame, so measure from the high side. It also assumes standard jamb thickness; unusually thick jambs or applied trim may need more width. Error policy: missing, non-numeric, or out-of-range inputs are reported as errors rather than calculated.
Bright Side Kit. "Door Size Chart & Rough Opening Calculator." https://brightoolbox.com/door-size-chart/ Accessed October 24, 2026.The hole in the wall before the frame goes in. For a 36 by 80 inch door it is 38 by 82.5 inches — bigger than the door on purpose, to leave room for the frame, shims, and leveling.
Two inches wider and two and a half inches taller. That is the standard framing allowance: about an inch per side for jambs and shims, and 2.5 inches at the head for the head jamb and leveling.
38 by 82.5 inches. Width: 36 + 2 = 38. Height: 80 + 2.5 = 82.5. This is the most common entry-door opening in American homes.
32 by 82.5 inches. Width: 30 + 2 = 32. Height: 80 + 2.5 = 82.5. This is the standard bedroom-door opening.
Widths of 24, 28, 30, 32, and 36 inches, at heights of 80, 84, or 96 inches. Closets and baths take the narrow end; bedrooms take 30 or 32 inches; entries take 36 inches.
Usually 32 or 36 inches wide, at 80, 84, or 96 inches tall. A 36 by 80 inch door is the standard entry door; 36 by 96 inches is the common 8-foot entry.
Yes, always, before framing. The +2 / +2.5 rule is the trade standard, but some pre-hung units and exterior doors with sills call for a different opening. If the spec sheet disagrees, the spec sheet wins.
The door will not fit square, and shimming cannot fix a missing inch. You must widen the opening or order a smaller door. Framing slightly large is forgiving — shims fill it; framing small is not.