How to use the deck calculator
- Enter the deck length in the direction the boards run and the width across the boards.
- Enter the board face width (actual, not nominal), the gap between boards, and the board length you plan to buy.
- Choose a joist spacing for the screw estimate and add a waste allowance.
How the calculation works
Each row covers one board width plus one gap. The number of rows is the deck width plus one gap, divided by (board width + gap), rounded up. Each row needs enough boards end-to-end to cover the deck length. Total boards = rows × boards per row, then the waste allowance is added.
Fasteners are estimated at two screws per board at every joist, a common face-screwing pattern. Hidden-fastener systems use clips instead; follow the manufacturer’s count.
Formula
- L
- Deck length along the boards
- W
- Deck width across the boards
- b, g
- Board face width and gap
Worked example
16 × 12 ft deck with 5½ in boards and 3/16 in gaps
Width = 144 in. Rows = (144 + 0.1875) ÷ (5.5 + 0.1875) = 25.35 → 26 rows. With 16 ft boards, 1 board per row → 26 boards, 29 with 10% waste.
Joists at 16 in: ⌊192 ÷ 16⌋ + 1 = 13; screws = 26 × 13 × 2 = 676.
Assumptions and limitations
- Covers rectangular decks only; for L-shapes, calculate each rectangle separately.
- Picture-frame borders, stairs, fascia and railings are not included.
- This is not a structural design. Joist size and spacing, beams, posts, footings, ledger attachment and fastening must follow your building code and the decking manufacturer’s span limits.
Frequently asked questions
How many deck boards do I need for a 12 × 12 deck?
With 5½ in boards and 3/16 in gaps, a 12 ft width needs 26 rows. With 12 ft boards that is 26 boards, or about 29 with 10% waste.
What gap should deck boards have?
Gapping depends on the material and its moisture content. Many wood and composite boards use about 1/8-1/4 in; always follow the manufacturer’s instructions.
Should deck boards run parallel or perpendicular to the house?
Boards usually run perpendicular to the joists, which typically means parallel to the house wall. Enter the deck length in the direction the boards run.