Door Header Size Calculator

Background.

A header is the beam over a door or window that picks up whatever the wall would have carried if the opening were not there and hands it sideways to the jack studs at each end. Get it right and nobody ever thinks about it again. Get it wrong and the first symptom is a door that binds in its frame, followed by cracked drywall above the corners, followed by a sagging line in the ceiling. In a bearing wall, an undersized header is a collapse mechanism rather than a finish defect, which is why this is one of the few residential framing decisions that a building inspector will actually measure.

This calculator answers the question the way a framer asks it: I have an opening this wide, carrying this much roof or floor, so what size header do I need and how many jack studs go under each end? It walks a 2x6, 2x8, 2x10 and 2x12 in turn at the ply count you choose, runs three checks on each one, and reports the shallowest section that passes all three — plus the bearing check that sets the jack stud count. That is a genuinely different question from "how far can this beam span", which is what a span calculator answers; if you already know the member and want its reach, use the wood beam span page instead.

The three checks are the ones the code's own span tables are derived from. Bending asks whether the extreme fibres of the header are overstressed at midspan, where the moment is w L² / 8. Horizontal shear asks whether the member will fail along the grain near the supports, where the vertical shear is w L / 2. Deflection asks whether the header will sag visibly, which is a serviceability question rather than a strength one, and is what the familiar L/240 and L/360 limits control. On a residential header, bending governs almost every time, deflection occasionally, and shear essentially never — which is worth knowing before you spend an afternoon worrying about the wrong one.

A fourth check has nothing to do with the header itself. Wood is far weaker across the grain than along it, so the real question at each end is whether the bearing surface is long enough to keep the plate from being crushed. That is why the answer is not just a size but a size and a jack stud count: each jack stud gives you 1.5 in of bearing length, and the required length is the end reaction divided by the compression perpendicular to grain value and the built-up width.

What this page is, precisely, is an ANSI/AWC NDS-2018 allowable-stress check on built-up sawn dimension lumber. What it is not is a reproduction of IRC Table R602.7(1). Those tabulated spans are copyrighted and are not reproduced anywhere here; instead this page runs the engineering check behind them, using the published design criteria of Pacific Lumber Inspection Bureau Technical Report No. 8, whose stated criteria are consistent with that table. The two can disagree by a size, and where your jurisdiction is on the prescriptive path the table governs.

So read the result as a sanity check and a shopping list, not as a permit. It covers sawn lumber only, on a simple span, under a uniformly distributed load, with the top of the header laterally braced. It will not size engineered lumber or steel, it does not accept a point load from a girder or post landing on the header, and it says nothing about the studs, plates and footings that carry the reaction down to the ground. Adopted code editions differ by jurisdiction and local amendments govern. A licensed structural engineer or architect has to confirm the member and sign off before work proceeds.

What is door header size calculator?

A door header — a lintel in British usage, a girder when it runs under a floor rather than over an opening — is a horizontal member spanning a framed opening in a wall. In North American light-frame construction it is normally built up from two, three or four plies of nominal 2x lumber nailed together, sometimes with a plywood spacer so the assembly finishes flush with the wall thickness. Each ply is 1.5 in thick dressed, so a two-ply header is 3 in wide and a three-ply is 4.5 in.

The header sits on jack studs (also trimmer studs) that are cut short to fit under it, and those in turn bear on the bottom plate, the floor framing and eventually the foundation. King studs run full height alongside, holding the assembly in position. The header's design span is measured between the faces of the jack studs, which is the rough opening plus 1.5 in at each end — 3 in more than the opening you measured.

Two numbers describe the shape of the member and do all the work. The section modulus S = b d² / 6 governs bending: because depth is squared, a 2x10 is roughly 1.6 times stronger in bending than a 2x8 of the same width, while a second ply only doubles it. The moment of inertia I = b d³ / 12 governs deflection, where depth is cubed, so depth is even more decisive for sag than for strength. This is why adding depth beats adding plies whenever the headroom exists, and why a header that fails is almost always too shallow rather than too narrow.

How to use this calculator.

1. 01Measure the rough opening — the framed hole, not the door or window unit. A 3-0 door is usually about 38 in of rough opening and a 6-0 patio door about 74 in. The page adds 1.5 in of jack-stud bearing at each end for you.
2. 02Work out the tributary width: half the span of the joists, rafters or trusses that land on this wall, plus any overhang they carry. For a 28 ft wide house with a ridge down the middle it is 14 ft. It is half the framing span, never the whole building width.
3. 03Enter the live or snow load on that area. Use the ROOF snow load, not the ground snow load — ASCE 7 flat-roof snow is pf = 0.7 Ce Ct Is pg, so 30 psf on the ground is 21 psf on a warm unobstructed roof. Roof live load is 20 psf; a bedroom floor is 40 psf.
4. 04Enter the dead load. TR-8 uses 20 psf for roof plus ceiling and 10 psf for a light-frame floor, and allows 121 lb/ft for the wall sitting on the header — that goes in the extra dead load field, because it does not come through the tributary area.
5. 05Pick the governing load combination. It sets the load duration factor CD: 1.15 for snow, 1.25 for roof live, 1.00 for a floor. Then pick the deflection limit — L/240 for roof and ceiling only, L/360 once a floor is carried.
6. 06Choose the species from the grade stamp and the number of plies your wall thickness allows. Read the answer, then read the note beside it: it tells you which of the three checks governed, how close to the limit you are, and what the page did not check.

The formula.

Everything starts from the design span. The header bears on the jack studs, so L = rough opening + 2 × 1.5 in. A 74 in opening gives a 77 in span, which is 6.4167 ft. The built-up width is b = plies × 1.5 in, so a two-ply header is 3 in wide, and the dressed depth d is 5.5, 7.25, 9.25 or 11.25 in for a nominal 2x6 through 2x12 (PS 20 dry surfaced sizes). From those two, S = b d² / 6 and I = b d³ / 12, which are NDS-2018 equations 3.3-4 and 3.3-3.

The load is w = tributary width × (dead psf + live psf) + the two per-foot terms. In the worked example that is 14 × (20 + 21) + 121 = 695 lb per foot, of which 294 lb/ft is snow. The moment is M = w L² / 8 = 3,576.96 ft-lb and each end reaction is R = w L / 2 = 2,229.79 lb.

Bending compares f_b = M / S against the adjusted design value F_b′ = Fb × CD × CF × Cr. Douglas Fir-Larch No. 2 has a reference Fb of 900 psi in NDS-2018 Supplement Table 4A. CD is the load duration factor from Table 2.3.2 — 1.15 for a snow combination. CF is the size factor from the same table's adjustment factors, and it is where depth pays twice: 1.3 at 6 in deep, 1.2 at 8 in, 1.1 at 10 in and 1.0 at 12 in. Cr is the repetitive member factor, which NDS grants as 1.15 only to members "not less than 3 in number", so a two-ply header gets 1.0. For the 2x10 that makes F_b′ = 900 × 1.15 × 1.1 = 1,138.5 psi against an actual 1,003.32 psi — 88.13 % used, and the section passes.

Shear compares f_v = 1.5 V / A against F_v′ = Fv × CD, which is NDS-2018 equation 3.4-2 written as 3V/2bd. With V = 2,229.79 lb and A = 27.75 in², f_v is 120.53 psi against 207 psi allowed: 58.23 % used. The page deliberately declines the NDS 3.4.3.1 allowance to ignore uniformly distributed load within a depth of the supports, so this figure errs high.

Deflection uses the live or snow load alone: Δ = 5 w L⁴ / (384 E I). With E = 1,600,000 psi and I = 197.86 in⁴, Δ = 0.0354 in against an allowable L/240 = 0.3208 in, which is 11.04 % used. Note that the load duration factor never appears here: NDS Table 2.3.2 footnote 1 says CD does not apply to E. A snow header and a floor header of the same section sag identically under the same live load.

Bearing is the last check and the one people skip. The required bearing length is R / (Fc⊥ × b) = 2,229.79 / (625 × 3) = 1.189 in, and since each jack stud provides 1.5 in, one jack stud per end is enough. CD does not apply to Fc⊥ either, so a longer-duration load does not buy you a shorter bearing. Hem-Fir, with Fc⊥ of 405 psi against Douglas Fir-Larch's 625, needs two jack studs at exactly the same reaction — a good illustration that the species matters at the ends as well as in the middle.

One recorded conflict, because it changes answers. TR-8's design criteria use Cr = 1.1 for a two-ply header; the NDS text grants 1.15 only at three members or more and offers no 1.1. This page follows the NDS text, which is about 4.65 % more conservative on span. Reproducing TR-8's own table with its 1.1 gives a two-ply 2x10 a 7 ft 2 in span at 695 lb/ft, exactly as tabulated; with the NDS 1.0 the same member reaches 6 ft 10 in. So this page will sometimes ask for one size more than the prescriptive table. That is the direction to err.

A worked example.

Frequently asked questions.

References& sources.

1. [1]American Wood Council — ANSI/AWC NDS-2018 Supplement, Design Values for Wood Construction (2020-08-27 web printing). Table 4A reference design values for visually graded dimension lumber 2"–4" thick: Douglas Fir-Larch No. 2 Fb 900, Fv 180, Fc⊥ 625, E 1,600,000 psi; Hem-Fir No. 2 850 / 150 / 405 / 1,300,000; Spruce-Pine-Fir No.1/No.2 875 / 135 / 425 / 1,400,000. Table 4A Adjustment Factors: size factor CF for Fb of 1.3 / 1.2 / 1.1 / 1.0 at 6 / 8 / 10 / 12 in deep, and repetitive member factor Cr = 1.15 for members "not less than 3 in number". Table 4B Adjustment Factors, Size Factor: "Appropriate size adjustment factors have already been incorporated in the tabulated design values for most thicknesses of Southern Pine and Mixed Southern Pine dimension lumber" — which is why Southern Pine is not offered as a species here. Open access, retrieved 2026-07-30.
2. [2]American Wood Council — ANSI/AWC NDS-2018 with Commentary, Chapter 2. Table 2.3.2 Frequently Used Load Duration Factors CD: 0.9 permanent, 1.0 ten years (occupancy live), 1.15 two months (snow), 1.25 seven days (construction), 1.6 ten minutes (wind/earthquake). Footnote 1: load duration factors shall not apply to E, Emin, or to Fc⊥ based on a deformation limit. Open access, retrieved 2026-07-30.
3. [3]American Wood Council — ANSI/AWC NDS-2018 with Commentary, Chapter 3. Eq. 3.3-3 (I = bd³/12) and Eq. 3.3-4 (S = bd²/6); Eq. 3.4-2 (f_v = 3V/2bd for a rectangular bending member); 3.4.3.1(a), the allowance to ignore uniformly distributed load within a depth d of the supports, which this page declines; 3.3.3.3 (CL = 1.0 when the compression edge is supported throughout its length and the ends are held against rotation); 3.10.2 (bearing perpendicular to grain on the net bearing area, f_c⊥ ≤ F_c⊥′). Open access, retrieved 2026-07-30.
4. [4]Pacific Lumber Inspection Bureau — Technical Report No. 8, Span Tables for Headers and Girders for Domestic Species, 1 July 2021. Its introduction states the design criteria, calculations and tabulated values are consistent with AWC's Wood Frame Construction Manual and ICC's 2018 IRC R602.7(1) and R602.7(2), and that basic wind speeds above 130 mph leave the prescriptive provisions for the WFCM or ICC 600. Design criteria: 20 psf roof dead, 121 plf wall dead, 10 psf floor dead, ΔLL = L/240 supporting only roof and ceiling and L/360 otherwise, CD 1.25 / 1.00 / 1.15 by combination, Cr 1.0 / 1.1 / 1.15 / 1.15 at 1 / 2 / 3 / 4 plies. Table 2 gives No. 2 Douglas Fir-Larch spans and jack stud counts; footnote e carries the 0.70 multiplier for an unbraced header top. This is the source of the independent check that reproduces five of its tabulated spans to within half an inch. Open access, retrieved 2026-07-30.
5. [5]International Code Council — 2021 International Residential Code §R602.7 Headers and Table R602.7(1), Girder Spans and Header Spans for Exterior Bearing Walls. This is the prescriptive route that governs where it is adopted, and the table this page deliberately does not reproduce. Gated: ICC Digital Codes is free to read in a browser with an account but refused automated retrieval (HTTP 403) on 2026-07-30, so no value on this page is attributed to it. Adopted editions vary by jurisdiction — 2018, 2021 and 2024 are all in force somewhere — and local amendments govern.
6. [6]American Lumber Standard Committee / US Department of Commerce — Voluntary Product Standard PS 20, American Softwood Lumber Standard. Dressed dry surfaced sizes used here: nominal 2x lumber is 1.5 in thick, and 2x6 / 2x8 / 2x10 / 2x12 are 5.5 / 7.25 / 9.25 / 11.25 in deep. Print standard, not retrieved online; the same dressed depths are confirmed indirectly by the PLIB TR-8 span reproduction above, which only matches to the inch if these depths are correct.