What this calculator does
Door / window header size works out minimum header depth for a rough opening, using the standard light-framing rule of thumb. Enter your own figures above and the answer updates as you type: nothing is fixed in the code, so the result reflects exactly the numbers you supply.
The formula this calculator evaluates is printed under the tool and explained below, so you can check the working by hand or reuse it in a spreadsheet.
The formula
The inputs explained
| Field | What to enter |
|---|---|
| Opening width (rough span) (ft) | A number, measured in ft. Starts at 6. |
| Plies of lumber (built-up header) | A number. Starts at 2. |
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How the answer changes with opening width (rough span)
Every other input is held at the calculator’s starting values while opening width (rough span) varies. Select any row to load that scenario into the calculator.
| Opening width (rough span) (ft) | Recommended header | Actual lumber depth | Header cross-section |
|---|---|---|---|
| 3 | 2×4, 2 plies | 3.50 in | 3.00 in × 3.50 in |
| 4.5 | 2×6, 2 plies | 5.50 in | 3.00 in × 5.50 in |
| 6 | 2×6, 2 plies | 5.50 in | 3.00 in × 5.50 in |
| 9 | 2×10, 2 plies | 9.25 in | 3.00 in × 9.25 in |
| 12 | 2×12, 2 plies | 11.25 in | 3.00 in × 11.25 in |
| 18 | 2×18, 2 plies | 17.25 in | 3.00 in × 17.25 in |