Door Size Calculator
Estimate Door Size quantities, sizes, and project costs for home and construction work.
This door size calculator finds the standard rough opening dimensions needed for framing a given door size, accounting for the extra clearance required around the door frame for proper installation and shimming.
Select your door type and size, and the calculator returns the recommended rough opening dimensions.
Worked Calculation Examples
| Scenario | Result | Calculation Step |
|---|---|---|
| 30 in × 80 in interior door | Rough opening ≈32 in × 82.5 in | Width: 30 + 2 = 32 in. Height: 80 + 2.5 = 82.5 in. |
| 36 in × 80 in exterior door | Rough opening ≈38 in × 82.5 in | Width: 36 + 2 = 38 in. Height: 80 + 2.5 = 82.5 in. |
Rough Opening vs. Finished Door Size
A rough opening is larger than the actual door size to allow room for the door frame, shims, and adjustment during installation - a typical interior door rough opening adds about 2 inches to the door's width and about 2-2.5 inches to its height, though exact allowances vary by door and frame manufacturer.
Standard Door Size Conventions
Interior doors commonly come in standard widths (like 24, 28, 30, or 32 inches) at a standard height of 80 inches, while exterior doors are most commonly 36 inches wide - using standard sizes when possible avoids the added cost and lead time of custom door orders.
How to Use the Door Size Calculator
- Open the Door Size Calculator and enter the values requested in the input fields.
- Check the units, percentages, dates, or time periods before reading the answer.
- Review the instant result and adjust any value to compare another scenario.
- Use the formula, example, and FAQs below to understand how the door size calculator works.
Frequently Asked Questions
How much bigger should a rough opening be than the door itself?
A typical rough opening adds about 2 inches to the door's width and 2-2.5 inches to its height to allow room for the frame and shims, though exact allowances vary by manufacturer specifications.
What is a standard interior door size?
Common standard interior door widths are 24, 28, 30, and 32 inches, typically at a standard height of 80 inches, though sizes vary somewhat by region and building age.
Why use a standard door size instead of custom?
Standard sizes are readily available and less expensive than custom orders, which require longer lead times and higher costs - using standard sizing when the opening allows for it usually saves both time and money.
How do I use this door size calculator?
Enter the known values, review the units or settings, and the calculator updates the result instantly. The formula and example on this page show how the answer is produced.
What does the Door Size Calculator calculate?
Estimate Door Size quantities, sizes, and project costs for home and construction work. It is designed for fast browser-based calculations without sign-up, downloads, or manual spreadsheet setup.
What formula does this door size use?
The formula is: Rough Opening = Door Size + Framing Allowance (≈2 in width, ≈2-2.5 in height). Standardized door sizing conventions in the US developed alongside the broader standardization of residential construction materials in the mid-20th century, driven by the economics of mass-produced lumber and manufactured door units - building to these standard sizes remains significantly cheaper than custom millwork precisely because it lets suppliers manufacture at scale.
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Last updated: September 27, 2026.