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Deck joist calculator

Joists = deck length in inches ÷ spacing, rounded up, plus one for the end. A 16-foot deck with joists 16 inches on center has 12 spaces and 13 joists. Each joist spans the deck depth, so a 12-foot-deep deck uses 12-foot joists; DeckCalc rounds up to the next stock length and adds a rim joist across the outer ends and a ledger along the house.

Feet and inches
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      Joist spacing is limited by the decking: 16 inches for 5/4 wood and most composites, 24 inches for 2-inch wood decking, and less when boards run diagonally.

      How the deck calculator works

      DeckCalc assumes the deck boards run parallel to the house and the joists run out from a ledger on the house to a beam near the outer edge. Rows of boards = depth × 12 ÷ (board width + gap), rounded up. Boards = rows × deck length × (1 + waste) ÷ board length, rounded up. Joists = length × 12 ÷ spacing, rounded up, plus one. Screws = rows × joists × screws per joist.

      Deck joist spans (IRC 2021 Table R507.6)

      SpeciesSize12″ o.c.16″ o.c.24″ o.c.
      Southern pine2×69′-11″9′-0″7′-7″
      2×813′-1″11′-10″9′-8″
      2×1016′-2″14′-0″11′-5″
      2×1218′-0″16′-6″13′-6″
      Douglas fir-larch, hem-fir, SPF2×69′-6″8′-4″6′-10″
      2×812′-6″11′-1″9′-1″
      2×1015′-8″13′-7″11′-1″
      2×1218′-0″15′-9″12′-10″
      Redwood, western cedars, ponderosa / red pine2×68′-10″8′-0″6′-10″
      2×811′-8″10′-7″8′-8″
      2×1014′-11″13′-0″10′-7″
      2×1217′-5″15′-1″12′-4″

      No. 2 grade, wet service factor, 40 psf live load, 10 psf dead load, deflection L/360, no cantilever. Higher snow loads need shorter spans.

      Maximum joist spacing for decking (IRC Table R507.7)

      DeckingPerpendicularDiagonal
      1¼″ (5/4) wood16″12″
      2″ wood24″16″
      Composite / plasticPer the manufacturer's span rating (ASTM D7032); 16″ and 12″ are typical

      Results are material estimates. Span values are from the 2021 International Residential Code; your jurisdiction may use another edition or local amendments, and beams, posts, footings, ledger attachment and guards need their own design. Check with your building department before you build.

      Frequently asked questions

      How far apart should deck joists be?

      16 inches on center is standard. Use 12 inches for diagonal decking or where the composite maker requires it.

      How many joists for a 12 ft deck?

      10 joists at 16 inches on center (144 ÷ 16 = 9 spaces, plus one).

      Do I count the rim joist?

      DeckCalc lists the rim joist and ledger separately because they run the other way, along the deck length.

      Is DeckCalc free?

      Yes, with no sign-up or ads.

      Are my measurements saved?

      Yes, in this browser only.

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