Door Header Size Calculator

A door header size calculator is a tool that recommends the size of the horizontal beam, called a header, needed above a door opening. It uses the door's width and whether the wall carries structural load, then suggests a size such as "Double 2x8" or "Single flat 2x4".

What is a door header?

A door header, also called a lintel, is a horizontal beam placed above a door opening. It carries the weight of the wall, ceiling, and sometimes the roof above the opening, and passes that weight down to the studs on either side of the door. Without a header, cutting a doorway into a wall would remove the wood that was holding up everything above it.

A header rests on jack studs, shorter studs cut to fit under each end of the header. Full-height king studs run alongside the jack studs from floor to top plate. Short cripple studs fill the gap between the top of the header and the top plate above.

How to calculate door header size

Header size depends mainly on two things: the width of the opening and whether the wall is load-bearing.

A load-bearing wall holds up weight from the floor or roof above it. A non-load-bearing wall, sometimes called a partition wall, only holds up itself.

For non-load-bearing walls, the International Residential Code (IRC), section R602.7.3, allows a single flat 2x4 above any opening up to 8 feet (96 inches) wide. "Flat" means the 2x4 lies on its wide face instead of standing on edge.

For load-bearing walls, the required size comes from IRC Table R602.7(1), which lists the longest span each header size and ply count can carry. The table changes with how many floors and how much roof the header supports, how wide the building is, and the local ground snow load, because heavier loads need bigger beams.

Wider openings also need bigger headers because of basic beam mechanics. For a beam carrying a uniform load, the maximum bending moment is M = wL²/8, where w is the load per inch of span and L is the span length. Because L is squared, doubling the span more than doubles the bending force the header must resist.

This calculator assumes the most common residential case: a single-story home where the header supports roof and ceiling only, a 20-foot-wide building, and a 30-pound-per-square-foot ground snow load. That is the lightest of the tabulated IRC conditions, so it gives the smallest header size that is still code-legal for that case. A house with more floors above, a wider footprint, or heavier snow load needs a larger header than this calculator shows. A builder should confirm the exact table row with a local building department or a structural engineer before framing.

Within that assumption, the calculator picks the smallest lumber size and ply count whose tabulated span is at least as long as the door's width.

Door header size chart

This table shows the header the calculator returns for a load-bearing wall, under its default assumptions, across its full input range of 12 to 144 inches.

For non-load-bearing walls, the calculator returns "Single flat 2x4" for any width up to 96 inches. Above 96 inches, it falls back to the same load-bearing table, since the IRC's flat-2x4 allowance only covers openings up to 8 feet.

Openings wider than a tripled 2x12 can span (beyond the calculator's 144-inch limit) need an engineered beam, such as laminated veneer lumber (LVL) or steel, sized by an engineer.

Worked examples

Example: a 60-inch-wide load-bearing opening. Under the calculator's default load case, a double 2x4 spans up to 42 inches, too short for a 60-inch opening. A double 2x6 spans up to 65 inches, which covers 60 inches, so the calculator returns "Double 2x6".

Frequently asked questions

What size header does a 32-inch door need?

In a non-load-bearing wall, a single flat 2x4 is enough for any door up to 8 feet wide, including a 32-inch door. In a load-bearing wall, a 32-inch opening needs a double 2x4.

Does a load-bearing wall always need a doubled header?

Yes, for openings in this calculator's range. Wider load-bearing openings need three plies (a "triple" header) once the span is too long for two plies, and very wide openings need an engineered beam instead of dimensional lumber.

What is the widest opening a 2x8 header can span?

Under the calculator's default assumptions, a doubled 2x8 spans up to 82 inches (6 feet 10 inches) in a load-bearing wall. A wider opening needs a double 2x10 or larger.

How do I know if a wall is load-bearing?

A wall is usually load-bearing if it runs perpendicular to the floor or ceiling joists above it, sits directly under a beam or another wall on the floor above, or is an exterior wall. When in doubt, ask a structural engineer or building inspector before cutting into it.

Why does a non-load-bearing door under 8 feet only need a single 2x4?

Because it carries almost no weight beyond the header's own weight and the wall material directly above it. The IRC allows a single flat 2x4 for openings up to 96 inches wide in walls that are not load-bearing.

Does this calculator account for snow load and building width?

No. It always uses the lightest tabulated case: roof and ceiling load only, a 20-foot-wide building, and 30 pounds per square foot of ground snow. A heavier snow load, a wider building, or extra floors above the header all call for a larger size than this calculator shows.

References

1. International Code Council. International Residential Code, Table R602.7(1) and section R602.7.3.
2. American Wood Council. National Design Specification (NDS) for Wood Construction.