How to use the deck calculator
- Enter the deck length in the direction the boards will run, and the deck width.
- Enter the actual board width, not the nominal size. A 5/4 × 6 deck board is usually about 5.5 in wide.
- Enter the gap between boards recommended by the board manufacturer.
- Choose the board length you plan to buy and the joist spacing, then keep a 10% waste allowance for cuts and defects.
The formulas
Rows of boards = deck width ÷ (board width + gap) (rounded up) Total board length = rows × deck length Boards = total board length × (1 + waste %) ÷ board length (rounded up) Joists = deck length ÷ joist spacing + 1 (rounded up) Screws ≈ rows × joists × 2Worked example
A 16 × 12 ft deck with 5.5 in boards, 1/8 in gaps, 16 ft boards and joists 16 in on center:
- Rows: 12 ft = 144 in ÷ 5.625 in = 25.6, so 26 rows.
- Total board length: 26 × 16 = 416 ft. With 10% waste: 457.6 ft ÷ 16 = 28.6, so 29 boards.
- Joists: 16 ft = 192 in ÷ 16 = 12, plus 1 = 13 joists, each 12 ft long.
- Screws: 26 × 13 × 2 = 676 screws.
Tips
- Composite decking and diagonal board patterns often require closer joist spacing, such as 12 in. Always follow the decking manufacturer's span table.
- Choose board lengths that match the deck length to reduce butt joints and waste.
- Decks attached to a house usually need a permit. Check your local building code for footings, beams and ledger requirements.
Frequently asked questions
How many deck boards do I need for a 12 × 16 deck?
With 5.5 in boards, 1/8 in gaps and 16 ft boards running the long way, you need 26 rows, or about 29 boards including 10% waste.
What is the standard joist spacing for a deck?
16 in on center is the most common spacing for wood decking. Some composite boards and diagonal layouts require 12 in. Check the board manufacturer's instructions.
How many screws per deck board?
Face-screwed boards typically get two screws at every joist. Hidden fastener systems use clips instead, so follow their packaging.
Does this include railings, stairs or footings?
No. This calculator covers deck boards, joists and screws. Plan railings, stairs, beams and footings separately and check local code.