Header Size Calculator
Determine the required header size for window and door openings based on span and load. Enter values for instant results with step-by-step formulas.
Formula
Required S = M / Fb where M = wL^2/8
The required section modulus S is calculated from the maximum bending moment M (which equals the distributed load w times span L squared divided by 8) divided by the allowable bending stress Fb. Deflection is checked against L/240 limit. The smallest standard header size that satisfies both criteria is recommended.
Worked Examples
Example 1: 6-Foot Window in One-Story Exterior Wall
Problem: Size a header for a 6-foot window opening in a one-story exterior wall. Roof load = 30 psf, tributary width = 12 feet, Douglas Fir-Larch.
Solution: Total load = (30 + 15) * 12 = 540 PLF = 45 PLI\nMax moment = 45 * (72)^2 / 8 = 29,160 in-lb\nRequired S = 29,160 / 850 = 34.31 in3\nDouble 2x10: S = 3.0 * 9.25^2 / 6 = 42.78 in3 (OK)\nCheck deflection L/240: adequate for double 2x10\nBearing reaction = 540 * 6 / 2 = 1,620 lbs per side\n2 jack studs per side provide adequate bearing
Result: Required Header: 2-2x10 Douglas Fir | 2 jack studs per side | Reaction: 1,620 lbs/side
Example 2: 8-Foot Garage Door in Two-Story Wall
Problem: Size a header for an 8-foot garage door opening supporting two stories plus roof. Roof = 30 psf, floor = 40 psf, tributary = 14 feet.
Solution: Total load = (30 + 40 + 25) * 14 = 1,330 PLF = 110.8 PLI\nMax moment = 110.8 * (96)^2 / 8 = 127,656 in-lb\nRequired S = 127,656 / 850 = 150.2 in3\nDouble 2x12: S = 3.0 * 11.25^2 / 6 = 63.28 in3 (NOT OK)\nTriple 2x12: S = 4.5 * 11.25^2 / 6 = 94.92 in3 (NOT OK)\nLVL or steel beam required for this application
Result: Solid lumber insufficient - requires LVL beam or steel header | 3 jack studs per side
Frequently Asked Questions
What is a header and why is it needed above windows and doors?
A header is a horizontal structural beam installed above window and door openings in load-bearing walls to transfer the weight from above the opening to the jack studs on either side. Without a header, the loads from the roof, upper floors, and wall framing above the opening would have no path to the foundation, potentially causing structural failure, sagging, and damage to the window or door frame. Headers redistribute concentrated loads to the trimmer (jack) studs, which carry those loads down to the bottom plate and foundation. In non-bearing walls, headers are technically not required structurally but are commonly installed using flat 2x4 stock for consistency and to provide a nailing surface for trim and casing materials.
What size header do I need for common window and door openings?
For standard residential construction supporting one story plus a roof, common header sizes are: openings up to 4 feet typically use a double 2x6 (two 2x6 boards nailed together). Openings from 4 to 6 feet generally require a double 2x8. Openings from 6 to 8 feet need a double 2x10. Openings from 8 to 10 feet require a double 2x12. Openings wider than 10 feet often need triple or quadruple members, engineered lumber (LVL or PSL), or a steel beam. These guidelines assume Douglas Fir or equivalent species, No. 1 or No. 2 grade lumber, and typical residential loading conditions. Two-story loads, heavy roof loads, or wide tributary areas require larger headers that should be verified by engineering calculation.
What is the difference between a solid header and a built-up header?
A solid header uses a single piece of lumber or engineered wood product the full width of the wall cavity. A built-up header consists of two or more pieces of dimensional lumber fastened together with plywood spacers between them to make up the wall thickness. Built-up headers using two 2x members with a 1/2-inch plywood spacer fill a standard 2x4 wall (3.5 inches total), while 2x6 walls require three 2x members or alternative configurations. Engineered lumber headers (LVL, PSL, or glulam) offer higher strength-to-depth ratios and are available in various widths to match wall thicknesses. Solid engineered headers are preferred for heavy loads because they provide uniform strength across their width, while built-up headers may have gaps between plies that reduce load sharing efficiency.
How many jack studs and king studs are needed for a header?
Jack studs (also called trimmer studs) directly support the header ends and transfer the concentrated load to the bottom plate. King studs run full height from bottom plate to top plate and are nailed to the jack studs to provide lateral stability. The International Residential Code requires the following minimum jack studs per side based on opening width: 1 jack stud for openings up to 4 feet, 2 jack studs for openings from 4 to 8 feet, and 3 jack studs for openings from 8 to 10 feet. Each jack stud should have a corresponding king stud. Wider openings and heavier loads may require additional support. The bearing capacity of the jack studs must be verified to ensure they can support the header reaction without crushing the wood fibers at the bearing point.
How does tributary width affect header sizing?
Tributary width is the perpendicular distance from the header to the midpoint between the header and the next parallel structural support. It determines how much floor, ceiling, or roof area loads into the header. For a simple gable roof, the tributary width equals half the building width for headers in walls parallel to the ridge. For headers in gable-end walls, the tributary width calculation is more complex due to the triangular load distribution. Increasing the tributary width linearly increases the load on the header, potentially requiring a significantly larger header size. A header with a 6-foot tributary width carries half as much load as one with a 12-foot tributary width, which can make the difference between needing a double 2x8 versus a double 2x12 for the same opening span.
What is the maximum opening width without a structural header?
In non-bearing walls, openings of any width technically do not require structural headers because no significant load transfers through the wall above. However, building codes and standard practice call for a flat 2x4 or 2x6 header even in non-bearing walls for frame integrity and to provide a nail base for trim. In bearing walls, very small openings under 2 feet wide may not require a full structural header under some code interpretations, as the cripple studs above can redistribute the load. However, most building officials require headers at all bearing wall openings regardless of size. The maximum span for the smallest practical header (double 2x4, which functions as a double 2x4 on edge) is approximately 3 feet under light residential loads. Any opening wider than 3 feet in a bearing wall should have a properly engineered header.