Header size calculator
Enter the rough opening width and what the wall holds up. You get the smallest header from the IRC span tables that will carry it, the jack and king studs it needs at each end, every other size that works, and the full span table for your load.
Inputs
Result
| Header | Max span | Jacks each end |
|---|
Header span table
| Header | 12 ft wide | 24 ft wide | 36 ft wide |
|---|
Spans in feet and inches, with the number of jack studs needed at each end in brackets. From 2021 IRC Tables R602.7(1), exterior bearing walls, and R602.7(2), interior bearing walls; No. 2 Douglas fir-larch, hem-fir, Southern pine and spruce-pine-fir.
How it is worked out
The IRC's header table gives the longest span each header can carry for five load cases, three ground snow loads and three building widths, with the number of jack studs needed under each end. The calculator looks up your column, interpolating between building widths as the table's note c allows, and lists every header that spans your opening. It picks the one needing the fewest jack studs, then the least lumber, among those that fit in the wall.
A 6 ft window in a one-storey house 24 ft wide, with ground snow of 30 psf or less: a 2-2x10 header spans 6 ft 10 in and needs two jack studs at each end. It is 6 ft plus 2 x 2 x 1-1/2 in long: 6 ft 6 in.
Reading the header sizes
- 2-2x10 means two 2x10s, nailed together with a 1/2 in spacer between them so they are as thick as a 2x4 wall (3-1/2 in).
- 3-2x10 is three 2x10s, 4-1/2 in thick, which needs a 2x6 wall.
- 1-2x8 is a single 2x8 on edge, which leaves room for insulation in the rest of the wall thickness.
Jack studs and king studs
Jack (trimmer) studs carry the header down to the bottom plate. The number at each end comes from the table: wider openings and heavier loads need two or three. Where only one is needed, the table's note d allows an approved framing anchor to the king stud in place of the jack.
King studs run full height beside the jacks and resist the wind pushing on the wall around the opening. IRC Table R602.7.5 sets how many at each end of a header in an exterior wall:
| Header span up to | Wind under 140 mph, Exp. B, or 130 mph, Exp. C | Wind 115 mph or less, Exp. B |
|---|---|---|
| 4 ft | 1 | 1 |
| 6 ft | 2 | 1 |
| 8 ft | 2 | 1 |
| 10 ft | 3 | 2 |
| 12 ft | 3 | 2 |
| 14 ft | 3 | 2 |
| 16 ft | 4 | 2 |
| 18 ft | 4 | 2 |
Spans between those listed use the next larger row. Where a framing anchor replaces a jack stud, use the first column whatever the wind speed.
What the table does not cover
Interior bearing walls use Table R602.7(2), which the calculator includes for walls carrying one or two floors; interior walls carrying roof loads need a design. Beyond that, the tables do not cover openings wider than the largest header in the column, buildings over 36 ft wide, ground snow over 70 psf, or point loads from girders or posts above. Those need an engineered header, often LVL, sized by the manufacturer's tables or a designer. If cripple studs bear on the header so its top is not braced, 2x8, 2x10 and 2x12 headers carry only 70% of the table span (note f).
To count the studs for the whole wall, kings and jacks included, use the stud calculator.
Questions
What size header do I need for a 6 foot opening?
In a one-storey house 24 ft wide with ground snow up to 30 psf, a 2-2x10 header with two jack studs at each end, from IRC Table R602.7(1). With a floor above it bearing on a center wall, a 2-2x12 with two jacks.
What size header for a 3 foot door?
For a 36 in door, about a 38 in rough opening, a 2-2x6 header with one jack stud at each end carries a roof and ceiling in a building 24 ft wide with snow up to 30 psf, and a floor above as well where the floor bears on a center wall.
How far can a double 2x12 header span?
On an exterior wall carrying only the roof and ceiling, with ground snow up to 30 psf: 10 ft 7 in for a building 12 ft wide, 8 ft 1 in at 24 ft and 6 ft 10 in at 36 ft. Under a floor bearing on a center wall, 8 ft 6 in, 6 ft 8 in and 5 ft 8 in.
What size header for a 16 foot garage door?
No 2x lumber header in IRC Table R602.7(1) spans 16 ft; the largest, 4-2x12, reaches 15 ft 3 in for a 12 ft wide building under roof only. Garage door headers that wide are usually LVL or glulam, sized from the manufacturer's tables.
How many jack studs do I need?
The table gives it with each header: one per end for short headers under light loads, up to four for wide headers under two floors and heavy snow. One jack may be replaced by an approved framing anchor.
How many king studs do I need at a header?
In an exterior wall, IRC Table R602.7.5 gives 1 king stud per end for headers up to 4 ft, 2 up to 8 ft, 3 up to 14 ft and 4 up to 18 ft where the design wind speed is under 140 mph in Exposure B. Where it is 115 mph or less in Exposure B, 1 up to 8 ft and 2 up to 18 ft.
How long should a header be?
The rough opening width plus 1-1/2 in for each jack stud at each end. With one jack each side, the opening plus 3 in.
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