Window Header Size Calculator

Window and Door Headers are horizontal structural members that transfer the load above an opening down to the jack studs on either side. Choosing the wrong header size can lead to sagging, cracking, or structural failure.

How Header Sizing Works: The required header size depends on three main factors:

1. Span width, the width of the opening
2. Load condition, meaning what sits above the header (roof only, one floor, two floors)
3. Building material, solid lumber or engineered lumber (LVL) or steel

Standard Header Sizes (Solid Lumber, Doubled):

Header Construction:

• Standard headers use two pieces of dimensional lumber with 1/2" plywood spacer between them.
• This creates a header that matches the 3.5" depth of a 2×4 wall (or 5.5" for 2×6 walls).
• Example: a doubled 2×8 header = two 2×8 boards + 1/2" plywood = 3.5" total thickness.

Engineered Lumber (LVL): Laminated Veneer Lumber is significantly stronger than solid lumber. A single LVL beam can often replace a doubled solid lumber header. LVL is recommended for spans over 8–10 ft (2.4–3.0 m).

Non-Load-Bearing Walls: If the wall is non-load-bearing (does not support structure above), a flat 2×4 header is sufficient for any span width.

Practical Example: A 6-foot-wide window in an exterior wall with one floor above:

• Recommended header: doubled 2×8 (two 2×8 boards + 1/2" plywood spacer), which is the 4 to 6 ft row of the table above
• Jack studs: one on each side of the opening
• Cripple studs: short studs above the header to maintain stud spacing

Six feet sits exactly on a band boundary, and the table is inclusive at the top of each band.
A hair over six and you are into the next row, so measure the opening rather than the window.

Tips:

• Always verify whether a wall is load-bearing before choosing a header size.
• Local building codes may differ from general guidelines. Always check with your building department.
• For garage doors and other large openings (over 10 ft / 3 m), consult a structural engineer.
• Headers in 2×6 walls need a wider spacer (two layers of 1/2" plywood or 1" foam + 1/2" plywood).
• When in doubt, size up. An oversized header is never a problem, an undersized one is dangerous.

A note on where this table sits. These bands are the common carpentry rule of thumb, and they run a little generous against the prescriptive span tables in the IRC, which shorten fast as snow load and building width go up. Treat the answer as the size to price and to take to your building department, not as the size to nail up unchecked. Our door rough opening calculator uses a tighter ladder for the same reason, so the two pages will not always name the same board.