Carpentry Math
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Free deck calculator

Deck Calculator

Enter the deck size to see how many boards, joists, posts and screws to buy.

Framing and waste

How to use the deck calculator

  1. Enter the deck length (the way the boards run) and width (out from the house).
  2. Pick the decking and the board length you will buy.
  3. Open Framing and waste to change joist spacing, board gap, beams and post spacing.

How the deck boards are worked out

Rows = deck width ÷ (board width + gap), rounded up

Boards = rows × deck length ÷ board length × (1 + waste)

Joists are counted from the deck length at your spacing plus the end one. Joist, beam and ledger sizes depend on span, wood species and local code, so check them against your code’s span tables before you build.

Related calculators

Deck steps: stair calculator. Railings: baluster spacing calculator. A fence around the yard: fence calculator.

Frequently asked questions

How many deck boards do I need?

Divide the deck width by the board width plus the gap to get the rows, multiply by the deck length for linear feet, then divide by the board length and add 10% for waste. A 16 × 12 ft deck with 5 1/2" boards and 3/16" gaps needs 26 rows, 416 linear feet, or 29 sixteen-foot boards.

How far apart should deck joists be?

16" on centre is standard for 5/4 wood decking laid square to the joists. Many composite boards need 12" on centre, and diagonal decking needs closer spacing too — check the board maker’s instructions.

How much gap should I leave between deck boards?

About 3/16" for dry lumber and composite. Wet pressure-treated boards can be butted tight or gapped 1/8": they shrink as they dry.

How many screws do I need for a deck?

Two screws where each board crosses each joist. For a 16 × 12 ft deck with 26 rows and 13 joists that is about 680 screws, or two boxes of 350.

How many posts and footings does a deck need?

It depends on the beam size and span, but posts about 8 ft apart along each beam is common. A free-standing deck has two beams; a deck attached to a ledger board on the house often has one.