Door Header Size Calculator

A header is sized from the span of the opening and the load bearing down on it. First find the load: take half the building width as the tributary width, multiply by the total pressure above — roof, snow and any floors — which gives the load per foot on the header. Then check candidate members against that load.

The header is treated as a simple beam spanning the opening, with a maximum bending moment of the load times the span squared, divided by eight. The shallowest member that keeps both the bending stress and the deflection within the limits is the one to use.

For example, a 3 ft opening in a 28 ft building carrying a roof plus one floor at 30 psf snow works out to about 1,330 plf, and a doubled 2×8 in Douglas Fir-Larch passes at about 76% of allowable. This calculator does the whole check and picks the size.

It depends on the load above the door, not just the width. For a typical 3 ft door in a 28 ft wide building carrying a roof plus one floor, with a 30 psf snow load, a doubled 2×8 in Douglas Fir-Larch #2 passes at about 76% of allowable, with bending governing.

If the door is in a non-load-bearing interior wall, a much smaller header or even a single flat 2x is enough. If it carries two floors or a wide building, you may need a 2×10, a 2×12 or an engineered LVL.

So a 3 ft door is usually a doubled 2×8 or 2×10 in practice, but the load decides. This calculator sizes it from your building width, snow load and what the header supports.

The load comes from the floor and roof area that bears on the wall above the opening. Half the building width — the tributary width — frames into that wall, so the header carries that strip. The total pressure is the roof dead load, taken at about 15 psf, plus the ground snow load, plus about 50 psf for each floor above.

Multiply the tributary width by the total pressure and you get the uniform load per foot along the header. Dividing by twelve converts it to pounds per inch for the beam check.

For example, a 28 ft building gives a 14 ft tributary width; at 95 psf — 15 roof plus 30 snow plus 50 for one floor — that is about 1,330 plf. This calculator builds the load up from your inputs.

Bending is about strength — whether the header will break. It compares the maximum moment against the member’s capacity, which is its bending strength (Fb) times its section modulus. If the bending use is over 100%, the header is overstressed.

Deflection is about stiffness — how much the header sags under load. Even a header that is strong enough can sag too much, cracking the finishes above the opening. The sag is compared against a limit of the span over 240 for a roof, or the span over 360 when floors are carried.

The governing check is whichever gives the higher percentage. Short, heavily loaded headers are usually governed by bending; long spans are often governed by deflection. This calculator reports both and flags which one governs.

Yes — laminated veneer lumber (LVL) is much stronger and stiffer than sawn lumber, so it spans farther in the same depth. A 2.0E LVL has a bending strength of about 2,600 psi against roughly 900 to 1,000 psi for #2 sawn species, so where a doubled 2×12 sawn header runs out, an LVL of the same depth often still passes.

LVL also comes in a 1.75 inch ply thickness, so a two-ply LVL header is 3.5 inches wide, matching a 2×4 wall. It is the usual choice for wide openings and heavy loads.

This calculator includes LVL 2.0E as a material option, so you can compare it directly against SPF, Douglas Fir-Larch and Southern Pine for your span and load.

Each end of a header sits on a jack stud, also called a trimmer, which carries the header’s load down to the plate and foundation, and is nailed to a full-height king stud beside it. A narrow opening usually needs one jack and one king at each end; wider openings or heavier loads need two or more jacks per end to carry the reaction.

The number of jacks depends on the end reaction and the bearing area — the wood has to be able to carry the load without crushing. The building code gives the required number for common cases.

This calculator sizes the header member itself; it assumes adequate bearing and the right number of jack and king studs. Confirm the stud count and bearing against the code or an engineer.

Not for strength. An interior wall that carries no floor or roof load above it only needs something to frame the top of the opening and give the drywall and door jamb something to attach to — often a single flat 2x or a doubled 2x on edge is plenty.

A wall is load-bearing if it supports floor joists, roof trusses or another wall above; those need a sized header. If you are not sure whether a wall is load-bearing, treat it as if it is until you can confirm.

This calculator is for load-bearing headers. For a true non-load-bearing opening, the header is a framing detail rather than a structural one.

No. This calculator is a planning aid that estimates a header size from a simplified uniform load and a simple-beam check. It assumes a uniform load, adequate end bearing and jack and king studs, and no point loads, wind or seismic effects.

Real projects are governed by the prescriptive header tables in the building code (IRC R602.7) or by an engineered design, which account for load combinations, point loads, concentrated reactions and connections that a quick estimate does not.

Use this to get a sense of the size and compare options, then confirm the final header, bearing, connections and load path against the current code and your building official or a licensed engineer.