Header size is the smallest built-up lumber whose IRC table span covers the opening: a 9 ft garage door in a 24 ft wide house with a 30 psf snow load needs a 3-2x12 header, good for 10 ft 1 in. Enter your opening above and the calculator gives the smallest header, the jack and king studs at each end, and the whole header span chart.
How the header size is found
IRC 2021 Table R602.7(1) lists the longest span of each built-up header in an exterior bearing wall, for No. 2 Douglas fir-larch, hem-fir, Southern pine and spruce-pine-fir (one set of numbers covers all four). The span depends on what sits on the wall (roof and ceiling only, or also one or two floors, center-bearing or clear-span), the building width measured across the ridge (12, 24 or 36 ft), and the ground snow load (30, 50 or 70 psf). Each cell also names the jack studs (NJ) needed under each end. Full-height king studs come from Table R602.7.5.
Span at your width = span at the lower width + (span at the upper width − span at the lower width) × (your width − lower width) ÷ 12 ft.
The calculator takes the shallowest header with the number of plies your wall wants (two 2× plies with a ½ in plywood spacer for a 2x4 wall, three plies for a 2x6 wall) whose span covers the opening, and only then adds plies.
Worked example: a 9 ft garage door
A 9 ft 0 in opening in an exterior wall of a house 24 ft wide, carrying roof and ceiling only, ground snow load 30 psf, 2x4 walls:
- 2-2x12 spans 8 ft 1 in, which is short by 11 in.
- 3-2x12 spans 10 ft 1 in and passes, with 2 jack studs at each end.
- The wall needs 3 king studs at each end for winds up to 140 mph (Table R602.7.5, 9 ft rounds up to the 10 ft row).
Garage door header size
A garage door header carries the same roof and ceiling load as any other exterior header, so the sizes above apply. For a house 24 ft wide with a 30 psf snow load, roof and ceiling only:
| Garage door opening | Smallest header | Max span | Jack studs each end |
|---|---|---|---|
| 8 ft | 2-2x12 | 8 ft 1 in | 2 |
| 9 ft | 3-2x12 | 10 ft 1 in | 2 |
| 10 ft | 3-2x12 | 10 ft 1 in | 2 |
| 12 ft | none in the table | ||
| 16 ft | none in the table | ||
| 18 ft | none in the table |
Sawn lumber runs out at about 10 ft here. A 16 ft or 18 ft double garage door is a glulam, LVL or steel header sized by the manufacturer or an engineer: try the LVL beam calculator for a first size or the steel beam calculator. Tap a garage door chip above and the tables update for that width.
What changes the header size
Under the same 9 ft opening and 30 psf snow, changing one thing at a time:
- Building width: a 12 ft wide building needs 2-2x10 (9 ft 0 in), 24 ft needs 3-2x12 (10 ft 1 in) and 36 ft needs 4-2x12 (9 ft 10 in).
- Snow load: 30 psf uses the 3-2x12, 50 psf needs a 4-2x12 (10 ft 0 in), and 70 psf has no sawn-lumber answer.
- A floor above: with a center-bearing floor added, the answer becomes a 4-2x12 (9 ft 8 in); with a clear-span floor or two floors above, no lumber header spans 9 ft.
- Top of the header not braced: for 2×8 and deeper, the code multiplies the span by 0.7. Turn the brace option off to see the effect.
Header on an interior wall or an open porch
Interior bearing walls and open porch beams use Tables R602.7(2) and R602.7(3). The beam span calculator covers both and draws the framed wall. Nonbearing walls can use a flat 2x4 header up to 8 ft (R602.7.4).
Next questions
- Frame the rest of the wall with the stud calculator and the framing calculator.
- Sizing a door for the opening: see the garage door size calculator.
- Joists above the header: the floor joist span calculator.
- All the span tools are in the framing and lumber calculators.
Tables like R602.7(1) are for ordinary houses with uniform loads. Point loads from a beam or post, buildings wider than 36 ft and ground snow above 70 psf need a designed header. Have an engineer or your building department verify the size before you cut into a bearing wall.