Door Header Size Calculator

Door Header Size Calculator: Plan the Header Above a Door

A door header sits across the top of a framed opening. It gives the wall a path to carry weight around the opening and down to the floor. If you are laying out a new doorway, changing a door size, or checking a framing plan, the first useful measurement is the rough opening width. This free door header size calculator turns that width into a practical cut-length estimate after you add the bearing at each end. It also shows the total header depth and a basic stud count so you can organize a material list.

The result is a planning aid, not a structural design. There is no single header size for every door. The right member depends on what the wall supports, how wide the opening is, the lumber species and grade, building location, wind and snow conditions, and local code. Use an approved span table or have a qualified design professional confirm the actual header before you cut framing lumber. This is especially important for exterior walls, load-bearing walls, multi-story homes, and openings under roof or floor loads.

Enter the rough opening width from your door schedule or framing plan, choose inches or millimeters, and enter the bearing you intend to provide at both ends. The calculator adds those dimensions to find the header cut length. You can then choose a common nominal lumber width and the number of plies to see a simple assembled-size reference. You can print the result for a project note or take it to the lumber desk for a conversation about the approved design.

What Does a Door Header Do?

A wall opening interrupts the studs that would normally run continuously from the bottom plate to the top plate. The header bridges that gap. Loads above it move through the header toward the two sides of the opening, where jack studs (also called trimmer studs) support its ends. King studs run beside the jack studs and continue from the bottom plate to the top plate. Depending on the wall and the approved framing detail, blocking, cripple studs, straps, or other connections may also be needed.

A header is often made from dimensional lumber, engineered lumber such as LVL, or another material specified on the plans. A built-up wood header may use two or more boards with a spacer between them to match the wall thickness. The words ‘double 2x8’ describe two nominal 2-by-8 boards installed together; they do not by themselves prove that the header is suitable for a particular span. Nominal lumber names also differ from actual finished dimensions, so check the product size when planning a flush or tight fit.

Headers are sometimes called lintels, particularly in masonry and other construction systems. The terms are related, but the materials, support details, and design rules can be different. This calculator focuses on planning the length of a framed door header and comparing basic dimensional-lumber layouts. It does not size steel lintels, masonry lintels, garage-door beams, or engineered wood products.

How to Measure a Door Opening

Use the rough opening width, not the door slab width or the nominal door size. A door sold as 36 inches wide is not necessarily installed in a 36-inch rough opening. The required rough opening is usually listed by the door manufacturer or on the construction plans and makes room for the jamb, shims, and fitting clearance. If an existing opening is being modified, measure the clear distance between the inside faces of the supporting studs at the header location. Check the width at more than one point if the framing is old or out of square.

Next, confirm the bearing length at both ends. Bearing is the amount of header that rests on its support, typically a jack stud or a specified bearing assembly. The required bearing is set by the structural design and local rules; it is not a universal value. Enter the left and right bearing separately because real plans can call for different details. The tool adds the left bearing, rough opening, and right bearing to estimate the cut length. For example, a 38-inch opening with 1.5 inches of bearing on each side gives a 41-inch header length. That arithmetic does not confirm that the bearing or header is adequate.

Also check wall thickness, header height, ceiling height, door height, and any required clearance for trim or a transom. Header depth affects how much wall space remains above the door, but it does not establish structural capacity. Compare the planned header depth with the elevations and framing details before ordering lumber.

How to Use This Header Length Calculator

Start by selecting the measurement system. The calculator accepts inches or millimeters and converts internally so the final length stays consistent with your selection. Enter the rough opening width and the bearing on each side. Values must be positive, and the tool flags missing or invalid dimensions instead of presenting an incomplete result as a recommendation.

Choose a nominal lumber depth and the number of plies only if you are comparing a layout already shown in a plan or span table. The displayed assembled depth is an approximate actual dimension based on common North American surfaced lumber sizes: a nominal 2x4 is about 3.5 inches deep, a 2x6 about 5.5 inches, a 2x8 about 7.25 inches, a 2x10 about 9.25 inches, and a 2x12 about 11.25 inches. Local products can differ, and the calculator does not include a spacer, finish layer, or special connection plate. The face width of a built-up header depends on board thickness, not the selected depth.

The quick framing estimate counts two king studs, one jack stud per side, and any extra jack studs you enter for each side. This is a shopping and layout reminder, not a code prescription. A plan may call for multiple jacks, posts, straps, hold-downs, or a different load path. Read the framing schedule and structural notes before deciding how many pieces to install.

Header Length Formula and Worked Example

For a straight opening with bearing at both ends, the basic cut-length equation is: header length = rough opening width + left bearing + right bearing. All three measurements must use the same unit. If the opening is 914 mm and the plan gives 38 mm of bearing at each end, the estimated cut length is 990 mm. If your design uses a specified gap, hanger, or another support detail, follow that detail instead of this simple bearing calculation.

The estimated assembled depth is the actual depth of one board in the selected nominal size. Adding plies increases the header's width through the wall; it does not multiply its depth. For example, two nominal 2x8 boards remain about 7.25 inches deep, while their combined face thickness is roughly 3 inches before any spacer or finish. Always verify actual product dimensions and the connection between plies in the plans.

The cut length is best treated as a layout number. Before cutting, check the measured opening, end support, squareness, and whether the design expects the header to fit between king studs or extend to a different reference point. Some framing details describe header length as the rough opening plus bearing; others dimension it directly on a plan. If a dimensioned plan conflicts with a quick calculation, ask the designer or builder to resolve the difference.

What Determines the Structural Header Size?

Span is only one part of header design. A wider opening generally creates more demand, but the load above the wall matters just as much. A non-load-bearing partition may only support its own framing and finishes. An exterior load-bearing wall can carry roof, ceiling, and floor loads. A wall below another story may collect loads from several levels. Roof shape, tributary width, snow, wind, and the location of beams or posts can change what the header needs to carry.

Material properties matter too. Species, grade, moisture condition, lumber size, and whether the member is solid sawn or engineered affect allowable strength and stiffness. Deflection limits can control even when a member is strong enough not to break. The supports and connections must transfer the forces safely: the header needs suitable bearing, the jack studs need a continuous path to support below, and fasteners or connectors must match the approved detail.

Building codes and prescriptive span tables vary by jurisdiction and edition. A table may only apply to a specific house type, loading assumption, species, grade, number of plies, and snow-load range. Do not take a size from an online example and assume it applies to your building. The building department, permitted plans, manufacturer design tables, or a licensed structural professional can confirm the correct choice for your conditions.

Load-Bearing Wall or Non-Load-Bearing Partition?

A partition usually separates rooms and does not support the roof or floor structure. A load-bearing wall carries weight from framing above. Interior walls can be load-bearing, and exterior walls are often part of the building's load path, so location alone is not enough to decide. Roof trusses, joists, beams, and posts may direct loads in ways that are not visible from the room below.

Before widening an existing doorway, do not remove studs or cut an existing header until you know what they support. Look at the framing plans and the direction of joists, but treat a visual inspection as a clue rather than proof. If the wall supports a floor, roof, another wall, or a concentrated load, get the design reviewed and use temporary support where the approved work sequence requires it. A building permit or inspection may also be required.

For a confirmed non-load-bearing partition, the header detail may be simpler, but local code and the door installation instructions still matter. The framing must stay straight and stable so the jamb can be shimmed correctly and the door can swing without binding.

King Studs, Jack Studs, and Header Bearing

A king stud runs full height beside the opening. A jack stud sits inside it and supports the header end. In a basic single-door layout, there is often one king and one jack at each side, but the required number depends on the design, span, load, and wall construction. The calculator starts with this common planning count and lets you add extra jack studs to either side. It does not decide whether those studs or their connections are strong enough.

Bearing is the contact length between the header and its support. A longer bearing is not a substitute for checking the header itself, and the required amount may depend on the material, reaction, and code detail. Make sure the support below the jack studs can carry the load as well. A header can be correctly cut but still be part of an unsafe assembly if the load path stops at a weak floor or an unsupported point.

Some engineered headers use hangers or specified fasteners instead of a simple bearing arrangement. Do not combine a hanger detail with assumed bearing or swap one connector for another without approval. Follow the product listing and the structural notes for fasteners, edge distances, and installation.

Common Door Header Planning Mistakes

The most common measurement error is using the nominal door width instead of the actual rough opening. Another is adding bearing to the opening twice when the plan already gives a finished header dimension. Write down the dimensions and their reference points before doing any arithmetic. Measure in one unit system and check the result against the wall layout.

A second mistake is assuming that more boards automatically make a safe header. Extra plies affect capacity only when the material, fastening, bearing, and design support that assembly. Similarly, choosing a deeper board solely because it fits above the door does not prove it works. A header that is too shallow can deflect; one that is too deep can collide with a ceiling or leave too little room for the door height and trim.

Finally, do not ignore the supports below and connections around the opening. Header, jacks, kings, plates, fasteners, and foundation or floor support form one load path. Protect the wall from movement during a structural alteration and arrange an inspection if your jurisdiction requires one.

Frequently Asked Questions About Door Headers

Use the questions below as a quick guide when planning a framed doorway. The final size and details should come from the approved plan, applicable span table, or qualified design professional for your project.

• What size header do I need for a 36-inch door? The door's nominal width does not tell you the header size. Confirm the rough opening, span, loads, material, and required bearing in the plans or applicable span table.
• How long should a header be? For a simple bearing detail, add the rough opening width and the specified bearing at each end. Check plan dimensions before cutting.
• Can I use a double 2x8 over a door? Sometimes a design may specify it, but the label alone cannot confirm capacity. Span, loads, species, grade, and local requirements all matter.
• Does this calculator design a load-bearing header? No. It estimates cut length and framing quantities; it does not calculate load capacity or approve a structural member.
• What is header bearing? It is the length of header supported at each end. Use the dimension stated by your design or applicable code detail.
• How many jack studs do I need? A simple layout often has one at each side, but a structural plan may require more or a different support. The displayed count is only a material-planning prompt.
• Is a header the same as a lintel? The terms can describe a member spanning an opening, but lintel details often refer to masonry or other systems with different design rules.
• Can I replace a door header without a permit? Permit rules depend on your location and the scope of work. Ask your local building department before changing structural framing.