What this page is, and what it refuses to be
Every span table, every engineered lumber selector and every structural engineer starts the same conversation: how long is the span, how wide a strip of building does this member carry, and what does that strip weigh. Those three things produce a load in pounds per lineal foot and a reaction in pounds at each end. That is the arithmetic this page does, and it does it carefully.
What it does not do is turn those numbers into a member size, because that step is not arithmetic. It depends on species and grade of the actual lumber, on moisture content, on how many plies you have and how they are fastened to each other, on the deflection limit that applies to what is above and below, on the load duration factor for the load type, on whether the member is laterally braced, and on which edition of which model code your jurisdiction adopted and how it amended it. A page that produced a size from a span and a load would be discarding most of the inputs that decide the answer. Take the numbers below to the table your building department works from, to a manufacturer selector for an engineered member, or to an engineer.
Tributary width, which is where the errors live
Tributary width is the width of the strip of building that a member carries. For a header under a floor, joists land on the wall from one side, and each joist delivers half its load to each of its two ends. So the header takes half the joist span. If joists land from both sides, the header takes half of each, and the tributary width is the average of the two spans.
A concrete case. Joists span 14 feet from an exterior wall to a center beam, and the header is in that exterior wall. Tributary width is 7 feet. At 15 psf dead and 40 psf live that is 55 psf times 7 feet, which is 385 pounds per lineal foot. Over a 12 foot header that is 4,620 pounds, and each end is carrying 2,310 pounds down through the jack studs.
Two mistakes recur. The first is using the joist spacing instead of the joist span, which understates the load by an order of magnitude. The second is forgetting a second story: a header in a wall that has a floor and a roof above it carries the tributary width of both, and the wall between them. Use the wall load field for anything landing on the member that is not captured by a single tributary strip.
| Situation | Tributary width |
|---|---|
| Joists from one side only, span S | S / 2 |
| Joists from both sides, spans S1 and S2 | (S1 + S2) / 2 |
| Rafters from one side, horizontal run R | R / 2, measured level, not along the slope |
| Interior beam under a floor split by it | Half of each adjacent joist span |
Roof loads are measured on the horizontal projection, not along the slope. A 6 in 12 rafter with a 12 foot run delivers a tributary strip of 6 feet, not 6.7. The slope length matters for how much lumber and sheathing you buy, which is what the roof framing lumber calculator is for, but the load is spread over the ground the roof covers.
Dead, live and snow
Dead load is the weight of the construction: framing, sheathing, floor finish, drywall below, roofing. Live load is what occupancy puts on it, and design values differ by room use, so a bedroom, a living area and a deck are not the same figure. Snow is a local number that your building department publishes for your address, and in mountain and lake-effect areas it can dominate everything else.
The defaults in the fields are common working values for orientation, not design values for your project. If you are doing anything real, get the loads from the same document that will give you the span table, so that the load and the table agree with each other.
Follow the load all the way down
The reaction number is the most useful output on this page and the most ignored. A correctly sized header delivers half the load it carries to each end. Those ends are usually jack studs, and a pair of jack studs has a finite crushing capacity where they bear on the plate. Below them the load goes into the floor framing, then into a post or a bearing wall, then into a footing sized for the soil under it. Every one of those has to take the number in the reaction row. Removing a wall and putting in a beam without checking what is under the beam ends is one of the more common ways a renovation goes wrong quietly, months later, as a sagging floor rather than a collapse.
Once you know the reaction, the footing under it is a bearing area problem, and the concrete for it comes off the concrete slab calculator. If the load is coming from a floor you are also framing, the floor joist calculator gives you the spans to feed back into the tributary field here.
Permits
Cutting a beam into a bearing wall is structural work and needs a permit essentially everywhere. In many jurisdictions it also needs a stamped drawing, particularly for anything beyond a simple prescriptive header in an exterior wall. That requirement is not a formality: the plan reviewer is the last person in the chain who looks at whether the load path actually reaches the ground.
Questions people ask
Will this tell me what size header to use?
No. It is built specifically not to. Member sizing depends on the species and grade of the lumber in front of you, its moisture content, the number of plies and how they are fastened, the deflection limit that applies, the load duration factor, lateral bracing and the span table your jurisdiction adopted and possibly amended. A span and a load are two inputs out of ten. What this page gives you is the load side of the problem worked correctly, so that when you open the table or call an engineer you are not guessing at the tributary width. A beam is not sized from a web page.
How do I work out the tributary width?
Take the full span of the joists or rafters that land on the member and halve it, because each of those pieces delivers half its load to each end. If framing lands from both sides, halve each span and add the halves. Joists spanning 14 feet to a center beam give a 7 foot tributary width at the exterior wall. The common error is using the joist spacing instead of the span — 16 inches instead of 14 feet — which understates the load by roughly a factor of ten and produces a header that looks fine on paper and sags in service.
Why does the calculator just add dead, live and snow together?
Because it is the conservative and transparent thing to do at this stage, and because applying real load combinations properly would require making assumptions about which code edition applies. Codes combine loads with factors, and the governing combination is frequently not everything at full value simultaneously — snow and floor live load at their full design values at the same instant is unlikely for most geometries. The straight sum shown here will be equal to or higher than the design load in most residential cases. Treat it as a starting figure to hand to a table, not as a design load you have justified.
What is the reaction number for?
It is what each end of the member pushes down with, and it is the number that tells you whether what is underneath is adequate. A 12 foot header at 385 pounds per foot puts about 2,310 pounds on each end. That has to pass through the jack studs without crushing the plate, through the floor framing below, into a post or bearing wall, and into a footing that the soil can carry. People size the beam carefully and then land it on a single jack stud sitting over the middle of an unsupported floor joist. Following the reaction to the ground is the part of the job that gets skipped.
Can I use this for a deck beam or a ridge beam?
For the load side, yes, with care about which loads apply. A deck beam takes the tributary width from the deck joists and uses the deck live load rather than an interior floor value. A ridge beam in a roof without ceiling ties carries a tributary strip of the horizontal run each side and needs the snow load for your location, and it is a member that people frequently underestimate because a structural ridge beam and a non-structural ridge board look identical on a drawing. Both cases produce a load figure here and both cases need that figure taken to a table or an engineer, not to a rule of thumb.