DeckCalc Tip

Deck screw calculator

Face-screwed deck boards normally get two screws at every joist they cross. Screws = rows of boards × joists × 2. A 16 × 12 ft deck with 5½-inch boards and joists 16 inches on center has 26 rows crossing 13 joists, which is 676 screws. Buy a little extra for stripped or bent screws. DeckCalc uses your own screws-per-joist number if you use hidden clips, which usually need one clip per board at each joist.

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      Use coated or stainless screws rated for pressure-treated lumber, and stainless near salt water.

      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 screws per square foot of decking?

      About 3.5 with 5½-inch boards at 16-inch joist spacing; 676 screws for a 192 sq ft deck.

      How many screws come in a 5 lb box?

      It depends on the length and gauge; 2½-inch deck screws are often around 350 to 450 per 5 lb. Check the box.

      How many screws per board per joist?

      Two for boards up to about 6 inches wide, three for wider boards.

      Is DeckCalc free?

      Yes, with no sign-up or ads.

      Are my measurements saved?

      Yes, in this browser only.

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