The model, stated exactly
Rough-opening rule: rough width = finished width + 2 inches; rough height = finished height + 2.5 inches. The calculator inverts it: nominal finished width = rough width − 2 inches; nominal finished height = rough height − 2.5 inches. The nominal size is then snapped to the nearest standard finished size — widths 24, 28, 30, 32, 36 inches and heights 80, 96 inches, with ties going to the smaller size — and the rough opening that snapped size needs is reported back, so the fit is checkable: if the required rough opening equals the entered one, the fit is exact; otherwise the difference in each dimension is the shim or trim needed. Standard thickness is 1-3/8 inches for interior doors and 1-3/4 inches for exterior doors; thickness does not enter the rough-opening rule, but it decides which hinges, latches, and locksets fit. French and double doors are listed by total width (two slabs): a 60-inch French opening is two 30-inch slabs, 64 inches is two 32-inch slabs, and 72 inches is two 36-inch slabs, each 80 inches high.
The worked example, line by line
Setup: rough opening 34 × 82.5 inches.
Line 1 — nominal finished width. 34 − 2 = 32 inches.
Line 2 — nominal finished height. 82.5 − 2.5 = 80 inches.
Line 3 — nearest standard door. 32 × 80 is itself a standard size, so the recommendation is 32 × 80 inches.
Line 4 — rough opening check. 32 + 2 = 34 inches wide and 80 + 2.5 = 82.5 inches high — exactly the entered opening, so the fit is exact.
Try these
A wider opening: 38 × 82.5 inches. Nominal 36 × 80, standard 36 × 80 inches, required rough 38 × 82.5 inches — an exact fit, and the common exterior entry size.
A narrower opening: 30 × 82.5 inches. Nominal 28 × 80, standard 28 × 80 inches, required rough 30 × 82.5 inches — an exact fit, typical for a bedroom or bathroom passage.
A tall opening: 32 × 98.5 inches. Nominal 30 × 96, standard 30 × 96 inches, required rough 32 × 98.5 inches — an exact fit, and the tall entry-door look without a transom.
A closet opening: 26 × 82.5 inches. Nominal 24 × 80, standard 24 × 80 inches, required rough 26 × 82.5 inches — an exact fit.
An in-between opening: 35 × 82.5 inches. Nominal 33 × 80 is not a standard size; the nearest standard door is 32 × 80 inches, which needs a 34 × 82.5 inch rough opening — so the entered opening is 1 inch too wide and needs a 1-inch filler strip, or the next step is a custom door.
Standard door sizes reference table — print this table
Every rough opening in this table is computed with the same rule as the calculator: finished width + 2 inches, finished height + 2.5 inches. Measure the framed opening, find your finished size, and read off the rough opening it needs. Thickness: interior doors 1-3/8 inches, exterior doors 1-3/4 inches.
| Door type | Finished size (in) | Rough opening (in) | Thickness | Typical use |
|---|
| Interior passage | 24 x 80 | 26 x 82.5 | 1-3/8 | Closets, small passages |
| Interior passage | 28 x 80 | 30 x 82.5 | 1-3/8 | Bedrooms, bathrooms |
| Interior passage | 30 x 80 | 32 x 82.5 | 1-3/8 | Bedrooms, living areas |
| Interior passage | 32 x 80 | 34 x 82.5 | 1-3/8 | Main passages, accessibility-friendly |
| Interior passage | 36 x 80 | 38 x 82.5 | 1-3/8 | Wide passages, moving furniture |
| Closet (bifold) | 24 x 80 | 26 x 82.5 | 1-3/8 | Narrow closets |
| Closet (bifold) | 30 x 80 | 32 x 82.5 | 1-3/8 | Standard closets |
| Closet (bifold) | 36 x 80 | 38 x 82.5 | 1-3/8 | Wide closets |
| Exterior entry | 32 x 80 | 34 x 82.5 | 1-3/4 | Side and back entries |
| Exterior entry | 36 x 80 | 38 x 82.5 | 1-3/4 | Front entries |
| Exterior entry (tall) | 36 x 96 | 38 x 98.5 | 1-3/4 | Tall front entries |
| French / double | 60 x 80 | 62 x 82.5 | 1-3/8 | Two 30-inch slabs |
| French / double | 64 x 80 | 66 x 82.5 | 1-3/8 | Two 32-inch slabs |
| French / double | 72 x 80 | 74 x 82.5 | 1-3/8 | Two 36-inch slabs |
Output · formula · meaning
| Output | Formula | What it means |
|---|
| Nominal finished width | Rough width − 2 in | 32 inches at the defaults. |
| Nominal finished height | Rough height − 2.5 in | 80 inches at the defaults. |
| Recommended door | Nearest standard size | 32 x 80 inches at the defaults. |
| Required rough opening | Door + 2 in / + 2.5 in | 34 x 82.5 inches at the defaults — an exact fit. |
| Fit difference | Entered rough − required rough | 0 x 0 inches at the defaults; anything else is shim or trim. |
Who uses this
- The homeowner replacing an interior door: measures a 34 × 82.5 inch opening, gets a 32 × 80 inch door, and orders the right slab the first time instead of returning a 36-inch door.
- The DIYer framing a new closet: enters the planned opening before cutting studs and learns a 26-inch rough opening is what a 24-inch door actually needs.
- The contractor checking a framer's work: enters a 35-inch opening, gets a 1-inch-too-wide warning, and catches the filler-strip fix while the wall is still open.
- The buyer shopping a tall entry door: enters 32 × 98.5 inches and confirms a 30 × 96 inch door fits — no transom, no custom order, no surprise.
Accuracy & limitations
The +2 / +2.5 inch rule is the standard US framing allowance used by door manufacturers and framers, stated here as the planning rule rather than copied from any single source — and it assumes a pre-hung door with its own frame. Slab-only replacements reuse the existing frame, so the rough opening is irrelevant and the slab must match the existing frame's dimensions instead. The rule also assumes a level floor and plumb studs: an out-of-square opening, a sloping floor, or thick flooring installed after framing all change the effective height, so measure height from the finished floor, not the subfloor. Accessibility codes may require wider passages (a 32-inch or 36-inch door) regardless of what fits, and fire-rated or exterior doors have their own frame and threshold requirements that can change the rough opening — confirm with your local code and the door manufacturer's spec sheet before cutting or ordering. Very small openings (under 20 inches of rough width) fit no standard door and are reported as errors. Error policy: missing, non-numeric, zero or negative, or over-200-inch rough dimensions are reported as errors rather than calculated, so no result ever shows NaN.
Cite this calculatorBright Side Kit. “Standard Door Sizes Chart & Rough Opening Calculator.” brightoolbox.com, 2026.Bright Side Kit. "Standard Door Sizes Chart & Rough Opening Calculator." https://brightoolbox.com/standard-door-sizes-chart/ Accessed October 24, 2026.
Embed this calculatorPaste this into any web page to embed the live calculator. The “Powered by Bright Side Kit” link below it must stay intact.