What this calculator does
The carpenter rule of thumb is that the header depth in inches matches the maximum span in feet. A 2×6 spans up to 6 feet, a 2×8 up to 8, and so on.
It is a rule for light loads only: roof load with no floor above, in a single-storey situation. Anything carrying a floor, a second storey or a point load from a beam needs a properly engineered header, and the rule will undersize it. Treat the answer as a starting point for a conversation, not as a specification.
The formula
The span in feet is matched to the smallest nominal lumber depth whose number equals or exceeds it. The actual depth is reported alongside, since a nominal 2×6 measures 5.5 inches. The cross-section assumes the plies are sandwiched with spacer material to match the wall thickness, which is standard practice in timber framing.
| Term | Meaning |
|---|---|
| Header | The beam over an opening, carrying the load that the removed studs would have taken. |
| Rough span | The clear width of the opening, which is what the header must bridge. |
| Plies | Layers of lumber laminated together. Two is standard in a 90 mm wall. |
| Nominal size | The size lumber is sold as. A 2×6 actually measures 1.5 × 5.5 inches. |
The inputs explained
| Field | What to enter |
|---|---|
| Opening width (rough span) (ft) | Rough opening width in feet, measured between the jack studs. |
| Plies of lumber (built-up header) | Layers of lumber. Two is standard; more plies add capacity but must fit the wall thickness. |
When to use it
Sizing a header for an internal door
A non-loadbearing or lightly loaded internal opening is where the rule is most defensible.
Checking existing work
Comparing an installed header against the rule shows whether it is plausibly adequate.
Preparing for an engineer
Knowing the rule-of-thumb answer makes the conversation with a designer more productive.
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.
What header does each span need?
A range of opening widths with the recommended header.
| Opening width | Recommended header | Actual lumber depth | Maximum span at this depth |
|---|---|---|---|
| 4 ft | 2×4, 2 plies | 3.50 in | 4 ft |
| 6 ft | 2×6, 2 plies | 5.50 in | 6 ft |
| 8 ft | 2×8, 2 plies | 7.25 in | 8 ft |
| 10 ft | 2×10, 2 plies | 9.25 in | 10 ft |
Questions
Is this rule safe to build from?
Only for light roof loads with nothing above, and only where local code permits rule-of-thumb sizing. Any header carrying a floor, a second storey or a concentrated load needs engineering. The rule is a sanity check, not a design.
Why is a 2×6 not 6 inches?
Because nominal sizes describe the rough-sawn dimension before drying and surfacing. A 2×6 finishes at 1.5 by 5.5 inches. All the depths reported here are the actual finished dimensions, which is what matters structurally.
Do more plies let me span further?
Somewhat, but depth matters far more than width. Bending stiffness scales with the cube of depth and only linearly with width, so doubling the plies helps much less than going one size deeper. Depth is almost always the better move.
What about engineered beams?
LVL and glulam carry considerably more than sawn timber of the same depth and are the normal answer for longer spans. They come with manufacturer span tables, which supersede any rule of thumb and should be used where the loads are real.
For the studs around the opening, see the wall stud calculator. For timber beam spans, see the timber beam span calculator.