Cut a hole in a load-bearing wall and the load that was travelling down through those studs has to go somewhere else. A header is the beam over the opening that carries it sideways to each end, and the jack studs are what carry it down from there. This calculator works out the load arriving at that opening, the reaction at each end, and how many jack studs the bearing area calls for.
Arb Digital publishes this as a preliminary and teaching tool. It deliberately does not tell you what header to use. Doing that would mean either reproducing a span table or prescribing a structural member, and neither is something a web page should do. What it does is the part that gets skipped: turning a building geometry into an honest load, so that whatever you take to a span table, a building department or an engineer starts from the right number.
What This Header Size Calculator Does
It computes the tributary load on a rough opening. Tributary area is the idea that each supporting element carries the load from the area of floor or roof nearest to it. For a bearing wall carrying a structure that spans between two walls, the tributary width is half the span, because half the load goes each way. Multiply that width by the area loads acting on it and you have a uniformly distributed load in pounds per foot along the header. Multiply by the opening width and you have the total load; halve that and you have the reaction at each end.
From the reaction it computes the bearing area required, which is the reaction divided by the compression perpendicular to grain design value you supply, and divides that by the bearing area one jack stud provides to give a jack stud count. It also reports the maximum bending moment and shear for the same loading, since those are the numbers a beam check needs.
What it does not do is check anything. It reports demand. Whether a given member can carry that demand is a separate calculation, and our wood beam span calculator is the page that performs it: enter the tributary width and total load from here, along with the design values for your species and grade, and it runs bending, shear and deflection.
How to Use It
- Measure the rough opening. This is the clear width, not the door or window unit size.
- Enter the building width the wall supports. That is the span of the roof or floor structure between bearing walls, and the tool takes half of it.
- Pick the load-bearing condition. It decides which area loads are summed. A wall carrying two floors and a roof carries far more than one carrying a roof alone.
- Enter the loads from your code and your actual construction. None of them are published here, and every one of them changes the answer directly.
- Take the result somewhere it can be checked. A span table, a building official, an engineer, or the wood beam span calculator with your own design values.
The Formula and How It Is Calculated
Tributary width = building width ÷ 2. Uniform load w = tributary width × the sum of the area loads for the condition chosen, plus any wall load per unit length. Total load W = w × opening width. Reaction R at each end = W ÷ 2, which for a uniformly loaded simple span is also the maximum shear.
Maximum moment M = wL² ÷ 8 for a uniformly loaded simply supported span. Required bearing area = R ÷ Fc⊥. Jack studs = the required bearing area divided by the bearing area of one jack stud, rounded up to a whole number.
Worked example against the loaded values. A wall supporting a 28 foot building width has a tributary width of 14 feet. With a roof live load of 30 psf, a roof dead load of 15 psf and a ceiling dead load of 10 psf, that is 55 psf, so w = 14 × 55 = 770 pounds per foot. Over a 6 foot opening the total load is 4,620 pounds and each end reaction is 2,310 pounds. The maximum moment is 770 × 36 ÷ 8 = 3,465 pound-feet. At an Fc⊥ of 625 psi the bearing area required is 2,310 ÷ 625 = 3.70 square inches. One 1.5 by 3.5 inch jack stud gives 5.25 square inches, so a single jack stud satisfies bearing at each end — which says nothing at all about whether the header itself is adequate.
Why This Page Will Not Tell You to Use a 2x10
Span tables for headers exist, they are useful, and they belong in codes and in publications by the bodies that maintain the underlying design values. They are not something to retype into a calculator, for three reasons.
First, they are conditional. A header span table is valid only for a specific species and grade group, a specific set of load assumptions, a stated building width range, a stated snow load range, a number of floors supported, and often a limit on the number of jack studs. Strip those conditions off and the table becomes a set of numbers that look authoritative and mean nothing.
Second, they are jurisdictional. Which edition of which code applies is decided locally, through the code adopted in your area, published for much of the United States via the ICC Digital Codes library, and amended by states and municipalities on top of that. The design values behind them come from the National Design Specification for Wood Construction and its supplement, published by the American Wood Council. A table retyped from one edition is wrong in the next.
Third, and most simply: a header is a structural member. If it fails, a wall and everything above it comes down. The person who decides what goes over an opening should be a licensed engineer, or a building official applying a prescriptive table under the conditions it was written for. The load is the part you can and should work out honestly in advance, and that is what this page is for.
Bearing Is a Real Check, and It Is Often the One That Bites
Jack studs are not decoration and their number is not a convention. The reaction from the header has to pass into the jack studs through a bearing surface, and wood is much weaker across the grain than along it. Compression perpendicular to grain is what governs where a header sits on a plate or a stud, and it is why a heavily loaded header on a single jack stud crushes the plate under it long before anything breaks.
Notice what happens as the opening widens. The load per foot does not change with opening width — it depends on the building width and the loads above. But the total load and therefore the reaction rise in direct proportion to the span. Double the opening and you double the reaction, so you double the bearing area required. That is why wide openings need multiple jack studs at each end, and why a garage door opening is a different structural problem from a window in the same wall.
The load does not stop at the bottom of the jack studs either. It continues through the wall plates, down through the floor structure and into the foundation, and every one of those interfaces is another bearing check. A concentrated load landing mid-bay on a floor below is a common and serious oversight. Our floor joist calculator covers the floor framing side and the beam load calculator handles reactions and moments for the members that carry it further down.
The Assumptions Behind Every Number Here
This calculation is a simple model and it is worth being explicit about where it stops being true.
It assumes the header is a simply supported single span with a uniformly distributed load. A point load landing on the header — a girder truss, a post from above, a beam bearing part way along — produces a completely different moment diagram and is not covered.
It assumes the tributary width is half the building width, which holds for a structure spanning simply between two bearing walls. Hip roofs, intersecting roofs, cantilevers, offset bearing walls, girder trusses and any framing that concentrates load onto one wall all break that assumption, usually in the unconservative direction.
It sums unfactored area loads. It applies no load combinations, no duration factors and no safety factors of its own. Whether the loads you enter should be combined, and how, is set by the code you are building under, and that is one of the things a proper design does that this page does not.
It says nothing about deflection, which frequently governs a header long before strength does, particularly over a wide opening carrying a floor. It also says nothing about lateral bracing, uplift, or what happens to a header in a shear wall, all of which are real design considerations. Where the roof load itself is the question, our snow load calculator applies the published method to a mapped ground snow load, and the section modulus calculator gives the geometric property a bending check needs.
Arb Digital builds free calculators and interactive tools that earn search traffic for engineering, timber frame and construction businesses. Browse the full library, or tell us what your customers keep asking you to work out.
Browse Free Tools Talk To Arb DigitalCommon Mistakes to Avoid
- Treating a load figure as a size — this page reports demand only, and no output here means a header is adequate.
- Using half the building width when the framing does not do that — hips, girder trusses and offset bearing walls concentrate load onto one wall.
- Ignoring point loads — a post or a girder landing on the header changes the moment completely and this model does not represent it.
- Skipping the bearing check — wood crushes across the grain, and a wide opening on a single jack stud is where that shows up first.
- Forgetting the load path below — the reaction continues through plates, floor framing and foundation, and each interface is another check.
Related Free Tools From Arb Digital
Pair this with the wood beam span calculator to run bending, shear and deflection on a member, the beam load calculator for reactions and moments, the floor joist calculator for the framing that lands on the wall, the snow load calculator for the roof load itself and the section modulus calculator for section properties. The full free online tools hub lists every calculator we publish.
Frequently Asked Questions
No, and it never will. Prescribing a structural member or reproducing a span table is not something a web page should do. It gives you the load, the reactions and the bearing demand, which is what a span table, a building official or an engineer needs as a starting point.
That page checks a member you describe against bending, shear and deflection using design values you supply. This page works out the load arriving at a rough opening in the first place, along with the end reactions and the bearing area. They are two halves of the same job.
Because a roof or floor spanning between two bearing walls sends half its load to each. That assumption breaks with hip roofs, girder trusses, cantilevers and offset bearing walls, all of which can put far more than half onto one wall.
It is the design value for compression across the grain, which governs where a header bears on a plate or a stud. It depends on species, grade and conditions of use and is published in the NDS Supplement or by the manufacturer for engineered lumber. It is an input here for that reason.
Because the reaction rises in direct proportion to the opening width while the load per foot stays the same. Twice the opening is twice the reaction and twice the bearing area required, so the bearing surface has to grow with it.
No. It models a uniformly distributed load on a simply supported single span. A post, a girder truss or a beam landing on the header produces a different moment diagram entirely, and that case needs an engineer.
The tool flags that condition rather than calculating it. A header in a non-bearing wall carries only itself and whatever wall sits above the opening, but whether a wall is genuinely non-bearing is a question about the framing above it, and it is often answered wrongly.
No. The tool sums the unfactored area loads you enter and applies nothing of its own. Load combinations, duration factors and the treatment of live against dead load are set by the code you are building under and are part of a proper design.
This tool computes tributary load and bearing demand from figures you supply, for education and preliminary work only. It publishes no span table and no design value table, it does not size a header, it checks nothing, and no output from it means a member is adequate. Headers and their supports are structural elements and must be designed by a licensed engineer or approved by the building department having jurisdiction under the adopted code.
