Door Header Size Calculator
Header size estimate for a door or window opening — an estimate, not an engineering design.
Estimate only — this is not an engineering sign-off
This tool applies a published code equation or standard beam formula to the numbers you enter. It does not know your building, your local code amendments, your soil, your connections, or your load path. The result is a planning estimate for sizing conversations and material take-offs only.
- Every design value (loads, allowable stresses, factors) is your input — a wrong value in gives a wrong answer out.
- Uniform load only — a post or beam landing on the header voids this estimate.
- Tributary width and adjusted allowable stresses are your inputs and drive the result.
- Sizes the header only — jack/king studs, bearing and the load path down are not checked. Cross-check IRC Table R602.7.
- No load path, connection, lateral, uplift, bearing, notch, hole, fire, or serviceability check beyond the one stated formula.
A licensed engineer or your local building official must review and stamp the final design before you build or order structural members.
Result appears here
Fill in the fields and press Calculate.
Formula, calculation, assumptions and sources
Uniform load w = tributary width (ft) × total load (psf)
For each candidate depth d, compute the allowable simple span (bending, shear, deflection — as for a beam)
Pick the smallest 2x depth whose allowable span ≥ the opening width
Run the calculation to see each step with your own numbers.
6 ft opening, 2x4 wall (2-ply), 14 ft tributary, 55 psf total (snow case), DF-L Fb' 1,000 / Fv' 180 / E 1.6M, L/240.
Load = 14 × 55 = 770 plf
2-ply 2x8 (3 in × 7.25) bending span ≈ 4.8 ft — not enough for a 6 ft opening
2-ply 2x10 (3 in × 9.25) bending span ≈ 6.1 ft ≥ 6 ft → suggested header: 2-ply 2x10
Confirm against IRC Table R602.7 and an engineer.
- Uniformly distributed load only. A concentrated load from a post, girder or beam bearing near mid-span requires a specific engineered header — do not use this.
- Tributary width is your input: for a header in a bearing wall it's roughly half the joist/rafter span plus any overhang; for a wall parallel to the framing above it may be near zero (non-bearing).
- Allowable stresses are your input, already adjusted (load duration CD — 1.15 for snow, 1.25 for roof live; size factor; wet service). Snow vs floor loading changes CD and the answer.
- Checks bending, shear and deflection of the header only. Not checked: the jack (trimmer) studs and their bearing, the king studs, cripples, uplift, or the wall's capacity to carry the reactions down.
- Cross-check against IRC Table R602.7(1)/(2) girder and header spans, which is the code-accepted prescriptive method for common cases.
- What size header for a 6-foot opening?
- In a typical single-storey house carrying roof and ceiling only, a 2-ply 2x8 or 2x10 usually does a 6-ft opening — but it depends entirely on how much roof, snow and floor load lands on that wall. Under two floors plus roof it could be a 2x12 or an engineered beam. Check the IRC header table and confirm with your building department.
- Do I need jack studs on both sides?
- Yes — a header bears on jack (trimmer) studs at each end, with king studs full-height alongside. Wider openings and heavier loads need two or more jacks per end. This calculator sizes the header, not the studs that carry it.
Beam mechanics
Simply-supported header under uniform load: M = wL²/8, V = wL/2, Δ = 5wL⁴/384EI. Standard mechanics; deflection limits per IRC Table R301.7.
Prescriptive cross-check
IRC Table R602.7(1) and R602.7(2) (girders and headers); AWC Wood Frame Construction Manual. Allowable stresses per AWC NDS Supplement.
About the Door Header Size Calculator
The Door Header Size Calculator works through the common built-up 2x header depths — 2x6, 2x8, 2x10, 2x12 — and picks the smallest one whose estimated allowable span covers your opening, for the uniform load you enter. It sizes the header only; the jack and king studs that carry it, the bearing, and the load path down are separate. A point load from a post or girder near mid-span voids the estimate — that needs a specific engineered header.
What you enter
- Rough opening width (header clear span)
- Tributary width carried (ft) — Half the span of the joists/rafters bearing on this wall, plus overhang. e.g. a 28 ft-wide house, header in an exterior wall ≈ 14 ft.
- Total load carried (psf) — Sum of the loads above: roof/snow + any floors + their dead loads. e.g. 40 psf snow + 15 psf roof/ceiling dead = 55; add ~50 psf per floor above.
- Allowable bending stress Fb' (psi) — NDS reference Fb × factors (CD for snow = 1.15, CF, CM…). DF-L No.2 2x with CD 1.15 ≈ 900 × 1.1 × 1.15 ≈ 1,138; SPF lower.
- Allowable shear stress Fv' (psi) — NDS Fv × CD. DF-L ≈ 180, SPF ≈ 135.
- Modulus of elasticity E (psi) — DF-L No.2 ≈ 1,600,000; SPF No.2 ≈ 1,400,000.
- Header build-up
- Deflection limit
What you get back
- Smallest adequate header depth — Nominal 2x depth (7.25, 9.25, 11.25 in = 2x8, 2x10, 2x12). 0 = nothing in the list works; use an engineered beam.
- Uniform load on the header — plf = tributary width × total load psf.
- Allowable span of that header — Should be ≥ your opening width.
The formula, a worked example, the assumptions behind it and the sources are shown in full below the calculator.
Related calculators
- Wood Beam Span Calculator — Allowable span estimate for a sawn-lumber beam under a uniform load — an estimate, not an engineering design.
- Framing Calculator — Stud count and lineal feet for a wall at a given stud spacing.
- Floor Joist Calculator — Floor joist count and a span estimate — an estimate, not an engineering design.
- Lumber Calculator — Lineal feet and piece count of dimensional lumber for a job.
Part of the Framing calculators.