Where does the joist span data come from?
CrewMeasure uses the official AWC DCA 6 Table 2 dataset added to the repo with source metadata and validation tests.
Deck Calculator
Estimate deck area, joist span limits, pier concrete, decking board quantities, and baluster spacing.
You'll need — decking boards and — footings for this — sq ft deck.
Deck dimensions
Length, width, and the joist span you're aiming for.
Joist sizing (advanced)
Species, joist size, and spacing feed the verified span-table lookup. Auto mode recommends a size for you.
Loads and footings
Loads stay editable and visible, even when the verified lookup table only covers standard assumptions.
Foundation and posts
How this deck meets the ground, and the posts it stands on. The choice changes the concrete in each hole, the length of post you buy, and whether there is a gravel bed to order.
Decking boards
Board count uses deck area divided by board coverage area with waste applied before purchase rounding.
Baluster spacing
Optional — enter a railing run and baluster width to solve the gap so it never exceeds the max opening (4 in default, the standard IRC guardrail rule).
You'll need — decking boards and — footings for this — sq ft deck.
You'll need
Deck results
Warnings
- ERROR Deck length and width must both be greater than zero.
- ERROR Enter a desired joist span greater than zero.
- ERROR Enter a footing count greater than zero.
- INFO Desired joist span must be greater than zero.
- ERROR Enter a depth greater than zero.
- ERROR Diameter is required for round pours.
- INFO Material estimate only. Footing depth, size, reinforcement, frost depth, and bearing capacity must be verified locally.
- WARNING Pick a post size to put a size and a length on the post line, and — on posts set in concrete — to work out how much of the hole the post fills.
Not counted -- answer these to add them
- Ledger board, ledger flashing, joist hangers and rim joist
- Whether this deck hangs off the house on a ledger or stands free on its own beams. The dimensions are the same either way, and the two builds buy different hardware: a ledger deck buys a ledger, flashing over it and a hanger per joist, and bands the outer end of the joist run only; a freestanding deck buys none of those and bands both ends.
- Post size
- Which nominal post size this deck stands on. Nothing about the deck's dimensions says whether it is 4x4 or 6x6, and the size decides both what goes on the order and — on posts set in concrete — how much of each hole the post fills.
- Beams
- How many beam lines carry the joists, and their spans. Footing count alone does not say — the same footings can be one beam line or two.
Detail values
- Deck area
- — sq ft
- Total load
- — lb
- Max joist span
- Data not available for this combination
- Minimum joist size
- Data not available for this combination
- Span lookup status
- —
- Footing concrete
- — cu yd
- Decking boards
- —
Assumptions
- Verified span table load basis: 40 PSF live load and 10 PSF dead load
- Species group: Southern Pine
- Attachment: not entered — ledger and hangers not counted
- Joist mode: Auto recommend minimum size
- Joist order:
- Decking material: Wood
- Max baluster opening: 4 in (IRC R312.1.3: a guard passes no 4 in sphere)
- Decking gap: not modelled — a board covers its bare face, so a gapped deck counts roughly 2% more boards than it lays
- Ledger flashing: on a ledger-attached deck, counted at the ledger's own length — end laps are not added, because the lap depends on a flashing width this screen never asks for
- Decking fastening: two screws per board at every joist it crosses. Estimated — the ordinary face-screwed pattern, NOT read from a fastening schedule; a hidden-fastener or clip system is counted differently and is not modelled
- Foundation method: Piers with post base — a formed round pier, post standing on a base above grade
- Footing size: not entered — hole diameter and depth are both needed before any concrete can be stated
- Post treatment — cut ends and notches: field-treat every cut end and notch before it goes in. CODE REQUIREMENT — IRC R402.1.2 and AWPA M4; the preservative is applied at the mill and a saw cut exposes untreated wood. Stated, not counted: no product and no coverage rate is sourced here.
- Timber treatment use category: Deck members above ground are specified UC3B and members in ground contact UC4A (AWC DCA 6, Table 1 footnote 2).
Included: decking boards, waste allowance.
Not included: labor, permits, engineering review or stamped plans, fasteners and connectors unless explicitly itemized above.
Formula details
- Deck area: deckLength x deckWidth
- Joist count: ceil((deckWidthInches) / joistSpacing) + 1
On-centre run with a joist at each end -- the same fencepost count framing.ts uses for studs.
- Decking screws: deckingBoardCount x min(joistCount, floor(boardLengthInches / joistSpacing) + 1) x 2
The crossing count is geometry: it comes from the board's own length, because a board shorter than the deck does not reach every joist. The two screws per crossing are not -- that is the ordinary face-screwed pattern, estimated, and NOT read from a fastening schedule. A hidden-fastener or clip system counts differently and is not modelled here.
- Total load: area x (deadLoadPsf + liveLoadPsf)
- Span lookup: Use verified AWC DCA 6 Table 2 row for species, joist size, and spacing.
- Decking board count: ceil((deckArea / boardArea) x (1 + wastePercent))
boardArea is the board's bare face -- the gap between boards is not modelled, so a gapped deck counts roughly 2% more boards than it lays (a 1/8 in gap on a 5.5 in board).
- Footing concrete volume: Reuse round concrete footing calculation from Sprint 1 and multiply by footing count.
- Baluster count and gap: count = ceil(runInches / (balusterWidth + maxOpening)); gap = (runInches / count) - balusterWidth
Solves the gap so it never exceeds the 4 in guardrail opening of IRC R312.1.3 (no opening passes a 4 in sphere), evenly distributed across the full run. The section's stair-triangle and open-stair-side exceptions are not modelled here.
- Rim joist: ledger-attached: deckLength; freestanding: deckLength x 2
The band closes the joist ENDS, and the joists are spaced along the deck's length, so the band runs the length. A ledger deck bands the outer end only -- the house side is the ledger. The two side members are the outermost joists and are already inside the joist count, so they are not banded again.
Material List
About this calculator
Use verified AWC DCA 6 Table 2 span lookups for supported joist combinations, expose your actual loads, and reuse Sprint 1 concrete logic for pier estimates.
FAQ
Why can span recommendations disappear?
The verified table only covers 40 PSF live load and 10 PSF dead load for listed species, sizes, and spacings, so unsupported combinations show a clear no-data note instead of guessing.
Do footing quantities reuse the concrete calculator?
Yes. Deck piers call the same round-pier concrete logic used by the standalone concrete calculator.
What changes when I pick composite decking?
Board length switches to a picklist of standard factory-cut lengths (12/16/20 ft), and a warning appears if your joist spacing exceeds common manufacturer caps for the selected layout — 16 in o.c. for straight, 12 in o.c. for 45° diagonal. These are manufacturer guidelines, not a code table, so always verify against your product's installation instructions.
How is baluster spacing calculated?
Enter your railing run and baluster width and CrewMeasure solves the gap so it never exceeds the max opening (4 in by default, the standard IRC guardrail-opening rule), evenly distributed across the full run rather than leaving a mismatched leftover gap at one end.
Disclaimer
This is a material estimate and general span-reference tool only. Final joist sizing, footing design, connections, loads, and code compliance must be verified locally.