Window & Door Header Size Calculator
Get a rough built-up header size and the number of jack studs for a window or door opening from the span and what the wall carries.
How it works
pick the smallest doubled header whose max span ≥ opening width · jacks = 1 up to 4 ft, 2 up to 8 ft, else 3 (or engineer)
A header transfers the load above an opening out to the jack studs on each side. These rule-of-thumb spans are for a built-up header of two dimensional members (with plywood spacer) in SPF #2, a common single-story case; adding a floor above shortens what each size can carry, so the second load option steps every size down. Jack-stud count grows with span because more load lands on the sides. This is a sizing sanity check — the real answer comes from the header span tables in the current residential code for your building width, snow load and species.
Worked example
A 4 ft window in a one-story wall carrying roof and ceiling: the smallest doubled header spanning 4 ft is a double 2×6, with one jack stud each side. Add a floor above and the same 4 ft opening steps up to a double 2×8.
Sources: IRC Header Tables (R602.7)
Frequently asked questions
How accurate is a rule-of-thumb header size?
It gets you the right ballpark for a typical one- or two-story house, but real header sizing depends on the building width, roof and floor loads, ground snow and lumber species. Always confirm against the code header table (IRC Table R602.7) or have an engineer size it — this tool is a planning aid.
What are jack and king studs?
The king stud runs full height beside the opening and is nailed to the header; the jack (or trimmer) stud is cut to the header height and sits under the header end to carry it down to the plate. Wider or heavily loaded openings need two or three jacks on each side.
When do I need an engineered beam?
Once the span or load exceeds what doubled 2×12s can carry — typically past 6 to 8 ft depending on the load — you move to an LVL, glulam or steel beam sized by an engineer. Garage door and great-room openings almost always fall here.
Related calculators
Estimates only — add for waste and confirm every load-bearing decision against local building codes and a design professional.