Header size calculator

The header over a door or window isn't sized by the opening alone. For an exterior bearing wall, how wide the building is decides how much roof and ceiling that header carries — and that sets the depth. Pick the opening and the building width; get the smallest built-up header that clears, with the jack-stud count.

01 How the table works

A header span isn't a formula you run — it's a code lookup. IRC Table R602.7(1) fixes the maximum clear span for each built-up header size by the supporting load condition and the building width. The deeper the header, the farther it spans. The wider the building, the shorter the same header spans, because more roof and ceiling bears on it.

This page holds the roof-and-ceiling, 30 psf, No. 2, two-ply rows for the three building-width columns the 2018 and 2021 IRC print — 12, 24, and 36 ft. The single span per size is the governing value across Douglas fir-larch, hem-fir, southern pine, and spruce-pine-fir, so it holds whichever of the four your yard stocks. The jack-stud count beside each span (NJ) is how many trimmers carry each end.

Two-ply spans, roof and ceiling only, 30 psf, No. 2 grade:

A doubled 2×10 nearly halves in reach across those three widths — 9 ft on a narrow building, under 6 ft on a wide one — for the same two boards. A header is three inches thick and nine and a quarter deep as a doubled 2×10, which is neither two of anything nor ten of anything, but the table rates the real section all the same.

02 Worked example

Take a 6 ft (1.83 m) rough opening for a patio slider in an exterior bearing wall, roof and ceiling above, on a 24 ft (7.32 m) wide building.

Walk the column from the top. A doubled 2×8 spans 5 ft 9 in (1.75 m) at a 24 ft width — short of the 6 ft opening. A doubled 2×10 spans 6 ft 10 in (2.08 m) with two jack studs each end, so that's the smallest header that clears. Cut the header board to the opening plus two 1½ in jacks: 6 ft 0 in + 3 in = 6 ft 3 in (1.91 m).

Now put the same 6 ft door in a 12 ft (3.66 m) wide building. The doubled 2×8 reaches 7 ft 7 in (2.31 m) there and carries the opening with one jack each end. Same door, same lumber, two inches of header depth saved — because the narrower building drops less load on the span. Set the widget to 12 ft and it lands on the 2×8; switch to 24 ft and it moves you to the 2×10.

03 When this calculator is wrong

This is one column of a much larger table, and the column is the first thing to check. These spans are for a header carrying roof and ceiling only. The moment a floor bears on that wall above the opening — a second storey, or a center-bearing floor landing on the wall — a different, shorter-span block of R602.7(1) governs, and the roof-only number over-reaches. Size a two-storey wall off the one-storey column and the header is undersized. The exception is a single-storey exterior wall with nothing but roof and ceiling above, which is exactly what this column is for.

Where the numbers here stop applying:

• Building width is the input most tools skip. A calculator that asks only for the opening can't size an exterior header, because the same header spans a different distance on a 12 ft and a 36 ft building. A doubled 2×10 goes from 9 ft 0 in to 5 ft 9 in across that range. Measure the building width — exterior face to exterior face — not just the hole.
• An unbraced header top loses 30%. Footnote f: where the top edge isn't laterally braced by perpendicular framing — a dropped header, or one with cripple studs bearing on it rather than joists or trusses tying its top — the tabulated span for a 2×8, 2×10, or 2×12 is multiplied by 0.70. A doubled 2×10 at a 12 ft width falls from 9 ft to 6 ft 4 in. Untick the braced box and the widget applies it. The 2×6 row isn't reduced.
• Only 2-ply is carried here. Three-ply and four-ply built-up headers span farther and are a separate part of the table. For a wide opening that a doubled 2×12 won't make, a tripled 2×12 or an engineered beam (LVL, PSL) is the next step — read the full table or a manufacturer's chart.
• Heavier snow is a different column. The 30 psf block also covers ground snow under 30 psf where the roof live load is 20 psf or less. Above that — 50 and 70 psf ground snow — the spans shorten, and the column on this page over-predicts.
• Interior bearing walls use Table R602.7(2), and local amendments govern the permit. An interior girder carries floor load from both sides and sizes off its own table. And the span your jurisdiction's adopted code shows — plus any engineered design — is the one that passes inspection, not a generic chart.

04 What to do with the result

Frame the size that clears with margin, not the one that just barely makes the span. Then order and cut to the real members, because the built-up header is smaller than its name twice over. A doubled 2× header is 3 in thick — two 1½ in plies (PS 20) — which is half an inch shy of a 3½ in 2×4 wall, so framers pack it out with a ½ in plywood or OSB spacer between the plies to bring the assembly flush with the wall face. The depth that sets the span is the dressed one too: a "2×10" header is 9¼ in deep, not ten.

Cut the header board to the opening plus two jack thicknesses — opening + 3 in — so it bears fully on both trimmers, and install the number of jack studs the result shows at each end. For any case this column doesn't cover — a floor bearing above, snow over 30 psf, an opening past a doubled 2×12 — pull the exact row from your adopted code table or have the beam confirmed by the AHJ or an engineer before you frame it.

05 Common questions

What size header do I need for a 6 ft opening?

For an exterior bearing wall carrying roof and ceiling only at 30 psf, No. 2 grade: on a 12 ft-wide building a doubled 2×8 clears 6 ft (it spans 7 ft 7 in); on a 24 ft-wide building you need a doubled 2×10 (6 ft 10 in), because the wider building loads the header more. Building width decides it, not the 6 ft alone.

How far can a double 2×10 header span?

Roof and ceiling only, 30 psf, No. 2 grade, two-ply: 9 ft 0 in (2.74 m) on a 12 ft-wide building, 6 ft 10 in (2.08 m) at 24 ft, and 5 ft 9 in (1.75 m) at 36 ft (IRC Table R602.7(1)). An unbraced top cuts each by 30%.

How many jack studs does a header need?

One or two per end, set by the span and load — the NJ column in the table. Short headers take one jack (trimmer) each end; longer and more heavily loaded ones take two. Where the table calls for one, an approved framing anchor on the full-height stud may stand in for the jack.

Does the building width really change the header size?

Yes, for an exterior bearing wall. The header carries the roof and ceiling load that falls on it, and that tributary load grows with building width. The same doubled 2×10 that spans 9 ft on a 12 ft-wide building drops below 6 ft on a 36 ft one. A calculator that asks only for the opening can't give a correct exterior header.

Do non-bearing walls need a header?

Not a load-bearing one. An interior partition that carries no load above the opening needs only a flat 2× or a single member to back the finish and hold the rough opening — not a sized header. This table is for bearing walls. If a wall carries roof, ceiling, or floor load, it bears, and the opening needs a header.