Header size calculator
Determine the right header size for door and window openings based on IRC span tables.
| Header | Max span | Your opening | Status |
|---|---|---|---|
| Double 2x4 | 2.5 ft | 3.0 ft | Too short |
| Double 2x6 | 4.5 ft | 3.0 ft | OK |
| Double 2x8 | 6.0 ft | 3.0 ft | OK |
| Double 2x10 | 7.5 ft | 3.0 ft | OK |
| Double 2x12 | 8.5 ft | 3.0 ft | OK |
Header Span Table — IRC R602.7
Maximum allowable header spans for exterior bearing walls supporting one story, roof, and ceiling. Values assume standard residential loading (ground snow load ≤30 psf). For two-story support or heavy snow loads, reduce spans or consult a structural engineer.
| Header Size | 20' bldg | 24' bldg | 28' bldg | 32' bldg | 36' bldg |
|---|---|---|---|---|---|
| Double 2x4 | 3'-0" | 2'-6" | 2'-6" | 2'-0" | 2'-0" |
| Double 2x6 | 5'-0" | 4'-6" | 4'-0" | 3'-6" | 3'-6" |
| Double 2x8 | 7'-0" | 6'-0" | 5'-6" | 5'-0" | 4'-6" |
| Double 2x10 | 8'-0" | 7'-6" | 7'-0" | 6'-6" | 6'-0" |
| Double 2x12 | 9'-0" | 8'-6" | 8'-0" | 7'-6" | 7'-0" |
How to Determine Header Size for Doors and Windows
A header is a horizontal structural member that spans above a door, window, or other opening in a framed wall. It transfers the load from above — roof, ceiling, floor, or another story — around the opening to the jack studs (trimmers) on each side. Getting the header size right is critical: too small and it sags or fails, too large and you waste lumber and reduce insulation space.
Header Sizing Rules of Thumb
For most residential construction, these rules of thumb work for single-story exterior bearing walls:
- 2x6 header span — up to about 4-5 feet (standard windows)
- 2x8 header span — up to about 5-7 feet (patio doors, wide windows)
- 2x10 header span — up to about 7-8 feet (large openings)
- 2x12 header span — up to about 8-9 feet
- LVL header — required for spans beyond 9 feet or heavy loads
Window Header Size
For window headers, measure the rough opening width — this is the clear distance between the trimmers, not the window frame size. A typical 36" window has a 38-1/2" rough opening, needing about a 3'-3" header span. A double 2x6 handles this easily in most applications.
Door Header Size
Standard interior door headersare 38" rough opening for a 36" door. Exterior doors are commonly 38" (36" door) or 74" (double 72" door). For a single exterior door in a bearing wall, a double 2x6 is typical. For double doors or wider, size up to 2x8 or 2x10.
Garage Door Header Size
Garage door headersspan 8 to 18 feet and almost always require engineered lumber (LVL or glulam). A standard single garage door (9') might work with a double 2x12, but a double garage door (16') definitely needs an LVL beam. Always consult a structural engineer for garage door headers — the span and load are too critical for rules of thumb.
Bearing vs. Non-Bearing Walls
Non-bearing walls (interior partitions) only support their own weight. A single flat 2x4 works as a header for openings up to about 8 feet. No structural calculation is needed — the header just holds the drywall above the opening.
Bearing wallscarry loads from above and require properly sized headers per IRC Table R602.7. The header is typically doubled dimensional lumber with a 1/2" plywood spacer to match the 3-1/2" wall thickness.
FAQ
What size header for a 3-foot window opening?
A double 2x6 is sufficient for a 3-foot opening in a single-story bearing wall for most building widths up to 36 feet. For non-bearing walls, a flat 2x4 works.
What size header for a 6-foot sliding door?
A double 2x8 or 2x10 depending on building width and stories supported. For a 24-foot-wide single-story house, a double 2x8 handles a 6-foot opening. For wider buildings or two-story support, use 2x10 or consult an engineer.
What size header do I need for a 16-foot garage door?
Dimensional lumber won't span 16 feet. You need an engineered beam — typically a multi-ply LVL (3-ply or 4-ply 1.75" x 14" or deeper). Always have a structural engineer specify the garage door header.
Can I use a single 2x for a header in a non-bearing wall?
Yes. Non-bearing (partition) walls don't carry structural loads. A single flat 2x4 works for openings up to about 8 feet. For wider openings, use a flat 2x6.