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Deck Calculator

Enter the size of a rectangular deck to estimate the deck boards, joists, posts and screws it takes. Set the board width, gap, the board length you plan to buy and the joist spacing to match your design.

Quick examples

Deck boards run this way; joists run across it.

A nominal 6 in deck board is 5½ in wide.

16 in is standard; 12 in for diagonal decking or many composites.

1 for a deck hung from a house ledger; 2 or more if freestanding or deep.

%

Covers end cuts and defects in the deck boards.

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Deck boards29
Deck area
192 ft²
Decking length (before waste)
416 ft
Joists
13
Posts
3
Deck screws (2 per joist crossing)
676

Saved setups

Save a set of inputs you reuse — your usual rate, your loan, your room sizes — and load it back in one tap.

    Your recent calculations

    Results you calculate here are kept on this device so you can come back to them.

      Formula

      Rows of decking = deck width ÷ (board width + gap), rounded up
      Deck boards = rows × deck length × (1 + waste %) ÷ board length, rounded up
      Joists = deck length ÷ joist spacing, rounded up, + 1
      Posts = beam rows × (deck length ÷ post spacing, rounded up, + 1)
      Screws = rows × joists × 2

      How to use it

      1. Enter the deck length in the direction the boards run, and the width.
      2. Enter the actual board width and the gap between boards.
      3. Choose the board length you will buy and the joist spacing.
      4. Set post spacing and the number of beam rows, then read the counts.

      Worked examples

      A 16 × 12 ft deck in 5½ in boards with a ⅛ in gap, 16 ft boards, joists at 16 in and posts every 8 ft

      Deck area
      192 ft²
      Decking length (before waste)
      416 ft
      Deck boards
      29
      Joists
      13
      Posts
      3
      Deck screws (2 per joist crossing)
      676

      A 12 × 10 ft deck with a ¼ in gap, 12 ft boards, joists at 12 in and posts every 6 ft

      Deck area
      120 ft²
      Decking length (before waste)
      252 ft
      Deck boards
      23
      Joists
      13
      Posts
      3
      Deck screws (2 per joist crossing)
      546

      What the estimate covers

      This is a materials count for the decking surface and a simple frame: joists running across the boards, and posts under each beam row. It does not size the joists, beams or footings, and it leaves out the ledger, rim joists, blocking, hangers, railings and stairs.

      These figures are planning estimates. Local building codes and a qualified professional govern the actual design. Joist and beam spans depend on lumber species, grade and load, and most decks attached to a house need a permit.

      Useful numbers

      A nominal 5/4×6 or 2×6 deck board is 5½ in wide. Leave about ⅛ in between boards; wet pressure-treated lumber is often butted tight because it shrinks as it dries. Joists are normally 16 in on center, or 12 in for diagonal decking and many composite boards. A common rule of thumb for screws is about 350 per 100 sq ft of deck.

      Questions people ask

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

      With 5½ in boards and a ⅛ in gap, a 12 ft width takes 26 rows. Running 16 ft long with 10% waste, that is 29 boards 16 ft long.

      How many joists does a 16 ft deck need?

      At 16 in on center, 13 joists: 192 in ÷ 16 = 12 spaces, plus one.

      How far apart should deck posts be?

      Commonly 6–8 ft along a beam, but the allowable spacing depends on the beam size, the joist span and your local code.

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