Tributary Load Calculator
Before any span table can be used, the load going into it has to be right, and the step that goes wrong is the tributary width. A beam carries half the span on each side of it, not the whole span — every joist delivers half its load to each end. Use the full spans and the load doubles, which sizes a beam for a building twice the size. Forget one side and it halves, which is the dangerous direction. This calculator gets the area right and stops there deliberately: it gives you the load, not the member.
- Load on the member
- 10,560 lb 192.0 ft² of tributary area × 55 psf. Carries half the span on each side, along its own length.
- Tributary width
- 12.00 ft Half of 12 ft on one side plus half of 12 ft on the other. Each joist or rafter delivers half its load to each end, so a member picks up half the span either side of it — never the full span.
- If you used the full spans
- 21,120 lb Twice the real load. It is the most common error in this calculation, and it leads to members and footings sized for a building that does not exist. The opposite error — forgetting one side entirely — halves it again and is more dangerous.
- Tributary area
- 192.0 ft² 12.00 ft of width along 16 ft of member.
- Uniform load along the member
- 660 plf Pounds per linear foot — the figure span tables are indexed on. A table asking for "total load in plf" wants this number, not the psf you started with.
- Dead against live
- 15 + 40 psf Dead load is the structure itself and anything permanently attached — framing, sheathing, flooring, roofing, drywall below. Live load is people, furniture and snow. They are separated because deflection under long-term dead load includes creep, while live load deflection does not, and span tables limit the two differently.
- An exterior wall carries more than what is above it
- Half the joist span A wall looks like it carries only the roof or floor directly over it. In fact every joist or rafter bearing on it delivers half its span, so an exterior wall under 16 ft joists carries 8 ft of floor even though the floor appears to sit beside it rather than on it.
- Stacking levels
- 1 level A post at the bottom of a stack carries everything above it. Roof, second floor and first floor all arrive at the same footing, and the loads add without any reduction for a residential structure of this size.
- This does not size anything
- Load only Deliberately. Turning a load into a beam depth, a post size or a footing dimension requires span tables for the specific species, grade and deflection limit, or an engineer. What this page gives you is the correct load to take into that step — and a wrong load makes every table lookup after it wrong too.
Half, on each side
A joist spanning between two supports delivers half its total load to each end. That is true of any simply supported member and it is the whole basis of tributary area.
So a beam under the middle of a floor picks up half the span on its left and half the span on its right. If the joists span twelve feet to one side and ten to the other, the tributary width is six plus five, or eleven feet — not twenty-two.
Multiply that width by the length of the beam and you have the area of floor it carries. Multiply by the load in pounds per square foot and you have the load on the beam.
The full-span version of this mistake doubles the answer, which produces a beam and a footing sized for a building twice the size — expensive, and usually discovered only when the excavation looks absurd. The opposite mistake, counting only one side, halves the answer, and that one does not announce itself at all.
Posts collect what the beam distributes
A post under a beam carries half the beam span on each side of it, in the same way the beam carries half the joist span on each side.
That has an unintuitive consequence: an intermediate post in the middle of a beam run carries as much as both end supports combined. Two twelve foot beam spans meeting at a post give it twelve feet of beam to carry, while each end support has six.
It is the reason the middle footing in a row is often the one that has settled. Sized identically to the ends by eye, it is carrying twice the load.
The footing below that post carries everything the post does, plus the post and the footing itself. On a multi-storey stack, the roof, the upper floor and the lower floor all arrive at the same point and add without reduction in a house this size.
Two loads, kept apart
Dead load is the permanent weight of the structure — framing, sheathing, flooring, roofing, insulation and the ceiling below. Live load is what moves: people, furniture, snow on a roof.
They are kept separate rather than added into one number because they behave differently. Wood creeps under sustained load, so long-term deflection under dead load is larger than the instantaneous figure, while live load deflection is not. Span tables reflect this by limiting total-load and live-load deflection differently.
Typical residential figures are ten to fifteen pounds per square foot dead for a floor, and forty live for living areas or thirty for sleeping rooms. Roof live or snow load varies enormously by region and is set locally.
Getting the split wrong matters less than getting the total wrong, but a table asking for the two separately is asking for a reason.
What this is based on
- Tributary area — half the span on each side, from simple beam reactions
- Typical residential dead and live loads: 10–15 psf dead, 40 psf living area live, 30 psf sleeping rooms
A load calculation only. It does not size beams, posts or footings, which requires span tables for the specific species, grade and deflection limit, or a structural engineer. Snow, wind and seismic loads are set locally and are not included. Point loads from posts above, and loads from stairs or concentrated equipment, must be added separately.
Frequently asked questions
What is tributary width?
Half the span on each side of the member. A joist delivers half its load to each end, so a beam picks up half the span to its left plus half the span to its right — never the full span on either side.
Why does using the full span double the load?
Because it counts every joist twice — once at each end. The other support is carrying the other half. Using full spans sizes members and footings for a building twice the size.
Does an exterior wall carry much load?
More than it looks. Every joist or rafter bearing on it delivers half its span, so a wall under 16 foot joists carries 8 feet of floor even though the floor appears to sit beside it rather than on it.
Why does the middle post carry so much?
It picks up half the beam span on each side of it, so an intermediate post carries as much as both end supports combined. It is why the middle footing in a row is so often the one that settles.
Can I size a beam from this?
No, and that is deliberate. Turning a load into a member size needs span tables for the specific species, grade and deflection limit, or an engineer. This gives you the correct load to take into that step — and a wrong load makes every lookup after it wrong too.