Header Size Calculator

How to Use the Header Size Calculator

This header calculator determines the minimum required header size for window and door openings in load-bearing walls. Enter the opening width (the rough opening dimension), select whether it's an exterior or interior bearing wall, input your building width, specify how many floors the header supports, and choose your roof snow load and wood species. The calculator computes the total load on the header and recommends appropriate header sizes from dimensional lumber or engineered options like LVL beams.

What is a Header and Why is it Needed?

A header is a horizontal structural beam installed above window and door openings in load-bearing walls. When you cut an opening in a wall for a door or window, you remove studs that normally carry loads from above. The header bridges across this opening, collecting loads from the studs above and transferring them to the king studs and jack studs on each side of the opening. Without an adequately sized header, the wall above the opening would sag, causing doors and windows to bind, drywall to crack, and potentially leading to structural failure. Headers are critical load-bearing members that must be properly sized for safety.

Load Calculation for Headers

Headers must support all loads from above within their tributary area. The tributary width is typically half the building width for exterior walls (assuming the roof or floor joists span the full building width) and varies for interior walls based on joist direction. Total load includes dead loads (weight of the structure itself) and live loads (occupants, furniture, snow). A typical residential floor contributes about 50 psf total load, while roofs vary from 30-60 psf depending on snow load. For a header supporting one floor and a roof, with a 14-foot tributary width and 6-foot opening, the total load might be 7,500-10,000 pounds or more.

Exterior vs Interior Bearing Walls

Exterior bearing walls typically carry higher loads than interior walls because they support half the roof or floor span. For a 28-foot-wide building, an exterior wall header has a tributary width of 14 feet (half the building width). Interior bearing walls may carry loads from both sides if they're located near the center, or they might carry minimal load if joists run parallel to the wall. This calculator assumes interior bearing walls carry half the tributary area of exterior walls. Non-bearing walls require minimal headers, often just a flat 2x4 or 2x6, because they only support the wall weight above the opening with no roof or floor loads.

Standard Header Construction

Traditional headers are built from dimensional lumber laid on edge. A common method uses two 2x members (2x8, 2x10, 2x12) with 1/2-inch plywood spacers between them to match the width of a 2x4 wall (3.5 inches). For 2x6 walls, you might use three 2x members or add additional spacers. The header sits on jack studs (also called trimmer studs) which transfer the load down to the bottom plate. King studs run full height beside the jack studs. The number of jack studs required on each side depends on the header load - heavier headers or wider openings may require double jack studs for adequate bearing area.

Engineered Lumber Headers

Laminated Veneer Lumber (LVL) and Glulam beams offer superior strength compared to dimensional lumber headers. A single 1.75-inch LVL can often replace a doubled 2x12 header, saving space and installation time. LVLs are manufactured from thin wood veneers bonded with adhesive, creating uniform strength without knots or defects. They're available in various widths and depths, and come in longer lengths than dimensional lumber. While more expensive per piece, LVLs can be cost-effective when considering labor savings and reduced material waste. Consult manufacturer span tables for specific LVL products, as capacities vary by brand and grade.

King Studs and Jack Studs

The opening frame consists of king studs (full-height studs flanking the opening), jack studs (shorter studs that support the header), a header, and cripple studs above the header. King studs provide nailing surfaces and lateral support. Jack studs transfer the header load to the foundation. For openings up to 6 feet with moderate loads, single jack studs typically suffice. Wider openings or heavier loads require double jack studs on each side to provide adequate bearing area. The jack studs must extend from the bottom plate up to the header, creating a solid load path. Some codes require direct bearing on foundation walls for heavily loaded headers.

Common Header Sizing Rules of Thumb

While this calculator provides precise recommendations, some general guidelines apply: For single-story buildings with roof only, a 2x6 header works for openings up to 4 feet, 2x8 for 5-6 feet, 2x10 for 7-8 feet, and 2x12 for 9-10 feet. Add one size for each floor above. These rules assume moderate loads and No. 2 grade lumber. For spans over 8 feet or two-story applications, consider LVL or Glulam beams. Always verify with calculations or span tables - undersized headers are a common framing error with serious consequences. When in doubt, going one size larger provides a safety margin and prevents callbacks for sagging headers.