Beam Calculator
Estimate the load a beam carries, its tributary width, and the built-up beam material you need.
What you need
6.0ft
Tributary width — Half the joist span each side. This is the figure span tables ask for.
- Total load on the beam (lb per linear ft)
- 300.0
- Total load carried (lb)
- 3,600.0
- Load at each support (lb)
- 1,800.0
- Ply boards to buy
- 3
- Total ply material
- 36.0 ft
- Board feet in the beam
- 41.6
What each post and footing beneath the beam has to carry.
Estimates for planning. Confirm quantities with your supplier before ordering — see the assumptions below for the figures used.
How to use the beam calculator
Sizing a beam properly is an engineering job, and this calculator does not pretend otherwise. What it does do is work out the numbers an engineer or span table needs from you — tributary width and total load — and cost the built-up beam material once you know the size.
What this calculator assumes
Every estimate depends on figures that vary by supplier and conditions. These are the ones used here, so you can sanity-check the result against your own quote.
- This calculator does NOT size beams. It computes tributary width and load, which are the inputs a span table or engineer needs from you. Getting a beam wrong is a structural failure, not a material overrun.
- Tributary width is taken as half the total joist span you enter — the standard simplification for a beam with joists landing from both sides. A beam carrying joists from one side only has a tributary width of half that span.
- Live and dead loads are typical code values in pounds per square foot. Your jurisdiction's figures govern, and snow loads vary enormously by location.
- Point loads from posts, columns or walls above are not included, and they change beam sizing completely.
- Load at each support tells you what the posts and footings must carry. Footing size then depends on soil bearing capacity.
- Any load-bearing beam almost certainly needs a permit and often an engineer's stamp. Have the size checked before you buy.
Frequently asked questions
What is tributary width?
The width of floor or roof area that feeds its load into a beam. For a beam with joists landing from both sides, it is half the joist span on each side added together — so joists spanning 12 feet total give a 6 foot tributary width. Multiply that by the load per square foot to get pounds per linear foot.
How do I calculate the load on a beam?
Add the live load and dead load in pounds per square foot, then multiply by the tributary width in feet. That gives pounds per linear foot. Multiply by the beam span for the total load, and halve it for the load at each support. A 40 psf live plus 10 psf dead load over a 6 foot tributary width is 300 lb per linear foot.
Can I size a beam myself?
For a small, simple, non-critical case you can work from the prescriptive span tables in the residential code — those exist precisely so that common situations do not need an engineer. Anything with a point load, an unusual span, an opening in a load-bearing wall, or a roof above needs a professional. The cost of an hour of an engineer's time is trivial next to a sagging floor.
What is a built-up beam?
Several dimensional boards nailed or bolted face to face to act as one beam — three 2x10s making a 3-ply beam, for instance. It is common because the pieces are easy to handle, but the plies must be fastened to a specific pattern to work as a unit, and engineered lumber such as LVL is often stronger for the same depth.
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