Enter the size of your deck, the width of the boards and the joist spacing. The calculator works out the deck boards, joists, rim and ledger, and screws you need.
Extra and cost
Joist and beam sizes depend on the span and your local code. This calculator counts pieces; it doesn't size them.
What the calculator counts
- Deck boards: run along the house, in rows across the width of the deck.
- Joists: run out from the house, one at each spacing plus one at the end.
- Rim and ledger: the ledger board against the house and the rim board along the outside edge.
- Screws: two wherever a deck board crosses a joist, plus 5 percent spare.
Posts, beams and footings depend on your design and local code, so they aren’t included. For post footings, use the concrete footing calculator.
Deck formulas
- Rows of boards = deck width (in) ÷ (board width + gap), rounded up
- Boards per row = deck length ÷ board length, rounded up
- Joists = deck length (in) ÷ joist spacing, rounded up, plus 1
- Screws = rows × joists × 2
Worked example
A 16 ft × 12 ft deck with 5½ inch boards, 3/16 inch gaps and joists 16 inches apart:
- Rows: 144 ÷ 5.6875 = 25.3, so 26 rows
- Boards: 26 boards of 16 ft, or 28 with 5 percent extra
- Joists: 192 ÷ 16 = 12, plus 1 = 13 joists, each 12 ft long
- Rim and ledger: 32 ft
- Screws: 26 × 13 × 2 = 676, or about 710 with spare
Deck board sizes and gaps
- A “6 inch” deck board is usually about 5½ inches wide, both in wood and composite. Use the actual width.
- Wood boards are usually gapped about ⅛ to ¼ inch, depending on how wet they are when installed.
- Composite boards have a gap set by the maker, often around 3/16 inch. Check the installation guide.
Joist spacing
16 inches on center is the most common. Composite boards and boards laid on a diagonal often need 12 inch spacing. The decking maker’s instructions and your local code set the limit.
Tips to save boards
- Pick a board length that matches the deck length, such as 16 ft boards for a 16 ft deck, to avoid joints.
- Where boards must join end to end, the joint has to land on a joist. Stagger joints from row to row.
- If you’re using hidden fasteners instead of screws, follow the clip count in the maker’s guide.
Frequently asked questions
How many deck boards do I need?
Divide the deck width in inches by the board width plus the gap, round up to get the rows, then multiply by the boards needed for each row.
How many joists do I need for a 12 × 16 deck?
With joists 16 inches apart running out from a 16 ft wide house wall, 13 joists.
How many screws per deck board?
Two screws at every joist. A 16 ft board crossing 13 joists needs 26 screws.
How many linear feet of decking do I need?
Multiply the number of rows by the deck length. The calculator shows this too.
Does this size the joists?
No. Joist size depends on the span, spacing, wood and loads. Use the span tables in your local code or your plans.
Results are estimates for planning. Decks need to meet your local code. See How We Calculate for the formulas we use.
Guides for this project
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