How to use the deck calculator
- Enter the deck size: the side along the house and how far the deck sticks out from it.
- Enter the actual board width, not the nominal size. A 5/4 x 6 or 2 x 6 deck board is 5 1/2 inches wide.
- Set the gap between boards from your decking instructions, and any overhang past the rim at each end.
- Untick any board lengths your supplier does not stock. The calculator finds the combination that wastes least.
How the materials are calculated
Rows of boards = (deck depth + gap) ÷ (board width + gap), rounded up
Row length = deck width + 2 × overhang
Boards = pieces per row × rows × (1 + spare allowance), rounded up
Joists = deck width ÷ joist spacing, rounded up, + 1
Face screws = rows × joists × 2
For each row the calculator tries every combination of the lengths you ticked and keeps the one with the shortest offcut. If two combinations waste the same, it prefers fewer pieces, because every joint needs a joist under it and extra fasteners.
Worked example
A deck 16 feet along the house and 12 feet deep, with 5 1/2 inch boards and 1/8 inch gaps, has (144 + 0.125) ÷ 5.625 = 25.62 rows, so 26 rows. A 16 foot board covers each row with no offcut, so you need 26 boards before spares. At 16 inches on center the frame takes 192 ÷ 16 + 1 = 13 joists, and the decking needs 676 face screws. The last row is 3 3/8" wide, so one board is ripped to fit.
Choosing board lengths
Lumber yards price boards by the foot, but offcuts are money you throw away. Designing the deck around a stocked length, such as a 16 foot deck for 16 foot boards, removes almost all waste. When a deck is longer than the longest board, a combination such as 12 plus 8 feet for a 20 foot run can still leave no offcut. The table of single lengths shows what happens if your supplier only has one length in stock.
What this calculator does not size
It counts joists but does not choose their size. Joist and beam sizes, post spacing, footings, ledger fastening and guard posts all depend on spans, loads and your local code. The American Wood Council's DCA 6 guide sets out a prescriptive design for typical single-level decks, and your building department can tell you which rules apply where you live.
Frequently asked questions
How many deck boards do I need for a 12 by 12 deck?
With 5 1/2 inch boards and a 1/8 inch gap, a 12 by 12 foot deck has 26 rows of boards. One 12 foot board covers each row, so you need 26 boards plus a few spares for any that are split or bowed.
How big a gap should I leave between deck boards?
The American Wood Council deck guide (DCA 6) says to space wood decking about 1/8 inch apart. Trex recommends 3/16 inch between the long edges of its composite boards, more under heavy tree cover. Always follow the instructions for the board you buy.
Which way do deck boards run?
Deck boards run across the joists. On a deck attached to a house the joists usually run out from the house, so the boards run parallel to the house wall. This calculator assumes that layout.
How many screws do I need per deck board?
The calculator assumes two face screws per board at every joist it crosses, or one hidden fastener per board per joist. Check the fastening schedule from your decking manufacturer, as some hidden fastener systems differ.
What joist spacing should I use?
Many decks are framed at 16 inches on center. Boards laid on a diagonal and some composite or thinner boards need closer joists. The maximum spacing for your decking is in the manufacturer's instructions and in DCA 6.
Can I join two boards in the same row?
Yes, as long as the joint lands on a joist and both ends are fastened. Stagger the joints so they do not line up in neighboring rows. The cut plan above shows which combination of lengths leaves the least waste.