Header Size Calculator
The size of a load-bearing header is a table lookup, not a guess. This tool sizes built-up and single-member wood headers straight from IRC 2021 Tables R602.7(1), R602.7(2), and R602.7(3) — by building width, ground snow, and what the header supports — and tells you the jack studs and full-height studs required at each end.
Spans are maximums: the smallest header that reaches your opening is selected. Table assumes No. 2 lumber of DF-L / hem-fir / Southern pine / SPF and 2-ply built-up framing (2× members nailed together) — verify local amendments and the actual lumber grade before framing.
What a header does
A header is the horizontal member that spans an opening in a bearing wall — a door, a window, a garage door. Everything the wall above used to carry now rides on it: roof and ceiling framing in an exterior wall, floor loads as well in interior and multi-story cases. The load has to travel around the opening, not through it.
The path is short. Roof and floor framing lands on the top plate, cripples carry it down to the top of the header, the header carries it across the opening, and jack studs carry each end down to the sole plate. That is why each table cell gives two things — a header size and a jack-stud count, the NJ column.
The studs around an opening are not interchangeable:
- King studs are full-height studs at each side of the opening, sole plate to top plate.
- Jack studs (trimmers) are short studs nailed flat against the king studs; the header bears on them and they carry its load down to the sole plate.
- Cripples are the short studs above the header, carrying the top plate down to it — and below the sill on a window.
Jack studs and cripples are not optional: the header needs the first, the top plate needs the second.
Reading the IRC R602.7 tables
Three tables cover prescriptive wood headers for No. 2 Douglas fir-larch, hem-fir, Southern pine, or SPF:
- R602.7(1) — exterior bearing walls. Building width 12 / 24 / 36 ft crossed with ground snow 30 / 50 / 70 psf. Each row is a header size (2-2×6 means two plies of 2×6, the standard built-up header); each cell is a maximum span plus a jack-stud count.
- R602.7(2) — interior bearing walls. The same width columns, no snow, two row sets: one floor only, or two floors.
- R602.7(3) — open porch headers. Roof snow 30 / 50 / 70 psf at porch depths of 8 and 14 ft (measured from the building face to the header centerline), plus a floor-above case.
Interpolation is where people go wrong. The footnotes allow it in three places only: between the 12 / 24 / 36-ft width columns, between 30 and 50 psf ground snow, and between porch depths of 8 and 14 ft. Everything else is a hard lookup — spans are never interpolated between sizes, and above 50 psf the table sends you to the 70 psf column rather than interpolating. Nothing here reaches past 36 ft of width, 70 psf, or two floors — beyond that it is an engineered design.
The 0.70 lateral-bracing reduction
Every span assumes the top of the header is braced sideways by perpendicular framing — joists or rafters crossing over it. Where cripples bear directly on the header with nothing bracing it, the footnote limits 2×8, 2×10, and 2×12 header spans to 70% of the tabulated value.
That is not a small haircut. A 2-2×10 spans 6-10 (82 in) at a 24-ft building width and 30 psf when braced, but only 82 × 0.70 = 57.4 in unbraced — which does not cover a 6-ft opening. It is the difference between a header that passes inspection and one that does not, decided by how you stack the wall rather than the lumber you bought.
Built-up headers vs single-member headers
A built-up header is two or more 2× members nailed together on edge — the 2-ply 2×10 or 2×12 over most sliding-door and garage openings. Table R602.7(1) also prints single-member rows, 1-2×6 through 1-2×12, which R602.7.1 permits only with a specific detail: a single flat 2× member, or a wall plate, not less in width than the studs, at the top and bottom of the header, face-nailed with 10d box nails (3″ × 0.128″) at 12″ on center.
The single-member route is not a shortcut. At a 24-ft width, 30 psf, roof and ceiling only: a single 1-2×12 spans 5-5 (65 in) with 2 jack studs each end, while a built-up 2-2×8 spans 5-9 (69 in) with 1 — the built-up header out-spans the single member and needs less end support.
Worked example — 6-ft opening, exterior wall
A 6-ft 0-in opening — a 72-in clear opening — in an exterior bearing wall of a 24-ft-wide building, 30 psf ground snow, roof and ceiling only, built-up header with the top braced. Walk the built-up rows at 24-ft width and 30 psf:
- 2-2×4 → 3-1 (37 in) — too short
- 2-2×6 → 4-7 (55 in) — too short
- 2-2×8 → 5-9 (69 in) — too short
- 2-2×10 → 6-10 (82 in) — covers 72 in. Stop here.
2-2×10, 2 jack studs each end. The span you found is a maximum, not a target. Two things move that answer:
- Snow. Same opening and building at 50 psf: 2-2×10 drops to 5-11 (71 in), which misses 72 in, so the header becomes 2-2×12 (6-11 / 83 in, 2 jack studs each end). The opening did not change; the load did.
- Bracing. Same 30 psf case with the top unbraced: 2×8, 2×10, and 2×12 spans are cut to 70%, so 2-2×10 (82 in) shrinks to 57.4 in and 2-2×12 (97 in) to 67.9 in — neither reaches 72 in. The search walks on to 3-2×10, 103 in × 0.70 = 72.1 in, with 1 jack stud each end.
Full-height studs are a separate count. Table R602.7.5 sets the minimum at each end of an exterior header from the header span: 1 stud for a span of 4 through 8 ft under the typical ≤115 mph Exposure B column, 2 from 10 ft up, and 2 to 4 studs under the higher wind column (less than 140 mph Exposure B / less than 130 mph Exposure C).
Common mistakes
- Sizing off the rough opening instead of the header span. The header spans the clear opening between the jack studs, not the rough opening.
- Running the 30 psf column because you never measured snow. That column applies only where ground snow is under 30 psf and roof live load is 20 psf or less.
- Using the interior-bearing table on an exterior wall. R602.7(2) has no snow in it, and an exterior wall carries roof load.
- Forgetting the jack studs. The size and the NJ count come off the same cell — a 2-2×12 with 1 jack stud where the table says 2 is not the framing the table describes.
- Treating a 2-ply as a 1-ply. A built-up 2-2×10 and a single 1-2×10 are different rows with different spans.
- Assuming the header top is braced. With cripples bearing on the header and no perpendicular framing over it, 2×8, 2×10, and 2×12 spans are cut to 70%.
Header size FAQ
What size header do I need for a door or window opening?
Read Table R602.7(1) for an exterior bearing wall: pick your building width column (12 / 24 / 36 ft, interpolating between them) and ground snow (30 / 50 / 70 psf), find the row for what the header supports, then take the smallest header whose maximum span covers your clear opening. Each cell also gives the jack studs required at each end.
Does ground snow load change the header size?
Yes. At a 24-ft building width with roof and ceiling above, a 2-2×10 spans 6-10 (82 in) at 30 psf but only 5-11 (71 in) at 50 psf, so a 6-ft opening that took a 2-2×10 becomes a 2-2×12.
What is the 0.70 lateral-bracing factor for headers?
The tables assume the top of the header is braced by perpendicular framing above it. Where cripples bear directly on the header with nothing bracing it, the footnote limits 2×8, 2×10, and 2×12 header spans to 70% of the tabulated value.
What is the difference between a built-up and a single-member header?
A built-up header is two or more 2× members nailed together on edge — the usual 2-ply header. A single-member header is one flat 2× member or wall plate, permitted by R602.7.1 only where a plate not less in width than the studs runs at the top and bottom of the header, face-nailed with 10d box nails (3″ × 0.128″) at 12″ on center.