DeckCalc Tip

Deck calculator

Enter the deck size, your decking and the joist spacing. DeckCalc counts the rows of deck boards and how many boards of your chosen length to buy, the joists with their stock length, the rim joist and ledger, and the deck screws. It also picks the smallest joist size that spans the deck depth according to the 2021 International Residential Code (IRC) Table R507.6. The example is a 16 × 12 ft deck with 5/4 × 6 boards and joists 16 inches on center.

Feet and inches
Prices (optional)

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      The boards are assumed to run along the house, with joists running out from a ledger to a beam near the outer edge.

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

      With 5½-inch boards and 3/16-inch gaps, 26 rows of 16 ft boards: 26 boards, or 29 with 10% extra.

      How many joists for a 16 ft deck?

      13 joists at 16 inches on center (192 ÷ 16 = 12 spaces, plus one).

      What does this not include?

      Beams, posts, footings, joist hangers, flashing, stairs and railings. Those depend on your design and local code.

      Is DeckCalc free?

      Yes, with no sign-up or ads.

      Are my measurements saved?

      Yes, in this browser only.

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