Door Header Size Calculator

How a door header carries the load above the opening

A door header is the horizontal beam that sits across the top of a door opening. Walls are built from vertical studs, but a door opening leaves a gap, so the weight of the wall, floor and roof above has to travel around the hole. The header catches that load and sends it down the studs on each side of the door.

The size of the header is set by two things: how wide the opening is, and whether the wall is carrying other loads. A wide opening needs a deeper beam, and a wall that holds up floors or the roof needs a beefier one than an interior partition that holds little. Get either wrong and the wall can sag, crack the drywall, or worse.

This calculator turns a door width into the rough opening, the header length, and the span, then matches that span to a standard double-2x member. You can check every number by hand with simple addition.

From door width to rough opening to header

Two small adds turn a door slab width into the lumber you buy. First, the rough opening is the door width plus 2 inches, which leaves 1 inch of shim space on each side so the door frame sits plumb. Second, the header length is the rough opening plus 3 inches, because the header rests on a 1.5-inch jack stud (also called a trimmer) on each side.

So a 36-inch door becomes a 38-inch rough opening and a 41-inch header. The span the header carries is the header length itself, 41 inches, or about 3.4 feet. A non-load-bearing wall with a span under 4 feet is carried by a double 2x4.

The table below runs the same math for the door widths you find in homes. Every header length is the door width plus 5 inches (the 2-inch shim plus the 3-inch jack studs).

Single door vs double door: the span nearly doubles

A single door and a double door of the same style differ by one slab width, but the header span grows by the full width because both sides of the opening add the same shims and jack studs. A 36-inch single door spans 3.4 feet; a 72-inch double door of the same style spans 6.4 feet.

The jump matters because the sizing steps are based on span. A single door might land in the double-2x4 range while the double door lands in the double-2x8 or double-2x10 range, so the lumber bill and the framing labor both go up.

An 84-inch double door (a pair of 42-inch slabs) needs an 89-inch header, a 7.4-foot span, which on a load-bearing wall calls for a double 2x10.

Load-bearing walls need a taller, doubled header

On a wall that carries floor or roof load, the same span needs a deeper member. Builders use a simple lookup: pick the row for your wall type, then the column for your span, and read the header. The thresholds below follow common prescriptive framing practice for single-story load paths.

For spans beyond 8 feet, or for two-story loads, a double 2x12 or a factory-built laminated header is normal, and you should have the plan checked by a professional. The numbers here are a planning aid, not a stamped drawing.

Why the jack studs matter

The header does not float. It rests on a jack stud (trimmer) at each end, and that jack stud sits on the floor or a sill, carrying the load straight down. The 3 inches added for the header length is exactly the two 1.5-inch jack studs. If a framer forgets the jack studs and nails the header to the wall sheathing, the opening has no real support and the wall above can drop.

This is also why the header length equals the rough opening plus 3 inches, not the door width plus 3. The king stud defines the rough opening edge; the jack stud sits inside it. Skipping either support shrinks the real bearing and changes the span the header must cover.

Common sizing mistakes

The same few errors show up on job sites again and again. Watch for them before you cut lumber.

• Forgetting the 2-inch shim. Measuring the slab and calling that the rough opening leaves no room to plumb the frame.
• Forgetting the 3-inch header add. Buying a board equal to the rough opening leaves nothing for the jack studs.
• Using a single 2x. A header must be doubled so the pair acts as one beam; a single board flexes and the door binds.
• Ignoring load-bearing walls. Treating an exterior wall like a partition undersizes the header by a full step.
• Guessing past 8 feet. Openings wider than 8 feet need an engineer, not a bigger guess.

What a double 2x6 header is

Lumber is named for its rough-sawn size, not its finished size. A 2x4 is 1.5 inches by 3.5 inches, and a 2x6 is 1.5 inches by 5.5 inches. When a plan calls for a double 2x6 header, it means two 1.5-by-5.5-inch boards nailed or bolted together, side by side, with the wide faces vertical so the beam is 11 inches tall and 1.5 inches thick.

Doubling the board roughly doubles the stiffness, which is why the same span that needs a double 2x4 on a partition needs a double 2x6 on a load-bearing wall: the deeper pair resists bending better. The depth, not the count, does most of the work, so stepping up the size is the right fix when the wall is heavier.

Worked numbers you can check

Run the math by hand for the two most common cases. For a 36-inch door on a non-load-bearing wall: 36 + 2 = 38 inches of rough opening, 38 + 3 = 41 inches for the header, and 41 / 12 = 3.4 feet of span, so a double 2x4 does the job.

For a 72-inch double door on a load-bearing wall: 72 + 2 = 74 inches of rough opening, 74 + 3 = 77 inches for the header, and 77 / 12 = 6.4 feet of span, which calls for a double 2x10. The extra 36 inches of door width turned a 3.4-foot span into a 6.4-foot one, exactly one sizing step up.

If you are matching an existing opening, measure the slab width, not the frame, and add 5 inches total for the header. The International Residential Code and the American Wood Council publish the span tables that builders follow, and your local inspector is the final word on what size is allowed.