Joist Calculator
Count floor or deck joists for a given span and spacing, plus rim joists, blocking and hangers.
What you need
14
Joists needed — One per spacing interval, plus one to close the run.
- Rim joist boards
- 2
- Total joist material
- 168 ft
- Joist hangers
- 28
- Blocking pieces
- 13
- Floor area framed
- 192 sq ft
- Board feet of joist material
- 152.3
Two runs closing the ends of the joists, in 16 ft lengths.
One at each end of every joist.
Estimates for planning. Confirm quantities with your supplier before ordering — see the assumptions below for the figures used.
Shop this project
The supplies a joist job usually takes. Quantities come straight from your calculation above — check them against your supplier's pack sizes before ordering.
Your shopping list
- Dimensional lumber
- 14 joists
- Joist hangers
- 28
Materials
Dimensional lumber
Prep & extras
Joist hangers
Structural connectors
Hurricane ties, post bases and the nails rated for them.
Framing nails or screws
Tools & equipment
Level
Circular saw
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How to use the joist calculator
Joist counts come from the length of the run divided by the spacing — but the material list also needs rim joists, blocking and a hanger at every end. This calculator covers all of it. It counts joists; it does not size them, which is a span-table question.
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 counts joists. It does not size them. Allowable span depends on joist depth, species, grade, spacing and the load — always check a current span table or your local code.
- Joist count is one per spacing interval plus one to close the run, then rounded up with your waste allowance.
- Hangers are counted at two per joist. Face-nailed or ledger-supported framing may need fewer; engineered joists need manufacturer-specified hangers.
- Blocking is estimated as one row at mid-span, one piece per bay. Code typically requires blocking or bridging on spans over 8 ft, and at least every 8 ft on long spans.
- Beams, posts, footings, ledger boards and fasteners are not included.
Frequently asked questions
How far can a 2x8 joist span?
It depends on species, grade, spacing and load, so there is no single number — for residential floor loads a 2x8 might run roughly 10 to 13 feet at 16 inch centres, but that range is wide enough that you must check the span table that applies to your lumber and jurisdiction rather than trust a rule of thumb.
Should joists be 16 or 24 inches on centre?
Sixteen inch centres is the residential standard for floors: less bounce, and it suits standard subfloor thickness. Twenty-four inch centres saves lumber and is common with engineered I-joists and thicker subfloor, but on dimensional lumber it makes floors feel noticeably springy.
Do I need blocking between joists?
Code usually requires blocking or bridging at the ends and at intervals on longer spans — often every 8 feet. Beyond code, it stops joists twisting and measurably reduces bounce, so it is worth doing even where you could skip it.
What is a rim joist?
The board running perpendicular across the ends of the joists, closing off the floor system. It keeps the joists from rolling over, ties the framing together and gives you something to attach sheathing and siding to. On a deck it is the outer frame you see.
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