Deck Calculator

Enter your deck size and height to get a joist size from the IRC span table, beam, post and footing counts, decking boards, fasteners, concrete bags and a cost estimate. Every price is editable.

1. Deck size

ft
ft
in
ft

Joists run from the house out to the beam, across the width. Boards run along the length.

2. Decking

%

3. Framing

in

Footing depth must reach below your local frost depth and onto firm soil. Ask your building department. In the Houston area frost is not a concern, but expansive clay is.

4. Prices (typical ranges; replace with your local quotes)

$/lf
$/lf
$/lf
$/lf
$/lf
$/lf
$/lf
$/lf
$/lf
$
$/post
$
$
$
$/ft
$
$
$/ft

Defaults are rough U.S. home-center prices for pressure-treated lumber, so check yours. Composite pricing varies a lot by brand.

Your deck

— sq ft

  • Joists—
  • Beam—
  • Posts—
  • Footings—

Materials list

ItemQtyPriceCost
— materials

Code checks

    How this deck calculator works

    Joists: the tool checks your joist span (the distance from the house to the beam, less the overhang) against IRC Table R507.6 for the lumber species group and spacing you pick. The table assumes No. 2 grade lumber, a 40 psf live load plus 10 psf dead load, and wet-use conditions. If one span is too long, it adds another beam line.

    Beams and posts: beam sizes and post spacing come from IRC Table R507.5 for built-up beams. The values in this tool are for southern pine No. 2. If your lumber is another species, the allowed spans are shorter, so confirm with the American Wood Council guide or your inspector. Posts are 4×4 up to 8 ft above grade and 6×6 above that.

    Footings: each footing carries a share of the deck set by the post spacing times half the joist span plus the overhang. That area picks a round footing diameter from published R507.3 footing sizes for your soil. A footing must sit below the frost line on undisturbed soil and posts must be tied to the footing with a post base, not left in contact with concrete.

    Decking: each board covers its width plus a small gap (1/8 in wood, 3/16 in composite). The tool counts rows, adds your waste percentage, and rounds up to whole boards. Screws are two per board per joist. Hidden clips are one per board per joist.

    Safety rules to know

    This tool is a planning aid, not a design. Clear spans, loads and connections for your site are the responsibility of the person who builds it, so verify with your local building inspector.

    Sources

    Estimates only — not engineering, legal or code advice. Check requirements with your local building department and a licensed professional before you build.

    Common questions

    What size joists do I need for a 12 ft deep deck?

    With southern pine No. 2 lumber at 16 in on center, 2×8 joists can span 11 ft 10 in and 2×10 joists can span 14 ft 0 in (IRC Table R507.6). A 12 ft deep deck with a 1 ft overhang past the beam has an 11 ft span, so 2×8s work. With no overhang the span is the full 12 ft and you need 2×10s.

    How many footings does a 16 × 12 ft deck need?

    With this tool's defaults (attached deck, 1 ft overhang, 2×8 joists at 16 in) the beam is a double 2×12, which can span 8 ft 7 in between posts, so the beam needs 3 posts and 3 footings. Each footing carries about 52 sq ft, which calls for an 18 in round footing on 1,500 psf soil.

    At what height does a deck need a guard?

    A guard is required when the walking surface is more than 30 in above the ground, measured within 36 in of the edge (IRC R312.1). The guard must be at least 36 in tall with openings that stop a 4 in sphere. Your inspector may set a different rule.

    How many deck boards do I need for a 16 × 12 ft deck?

    Each 5/4×6 or 2×6 board covers about 5-5/8 in with its gap, so the 12 ft width takes 26 rows. At 16 ft long that is 416 ft of board, or 458 ft with 10% waste. That is 39 boards of 12 ft.

    How far apart should deck joists be?

    Closer spacing allows longer spans and stiffer decks. Southern pine 2×8 joists span 13 ft 1 in at 12 in, 11 ft 10 in at 16 in and 9 ft 8 in at 24 in. Most 5/4 wood boards and composite decking need 16 in on center or less. Check your board maker's span chart.

    Do I need a ledger board, and how is it attached?

    A ledger attaches the deck to your house. It is bolted through the house rim, with flashing above it, and needs lateral-load connectors (IRC R507.9). If you do not want to attach to the house, a freestanding deck uses an extra beam line instead. The tool shows both.

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