How to use the deck calculator
- Choose Imperial or Metric.
- Enter the deck length (the direction the boards run) and width.
- Enter the board width, the gap between boards and the board length you will buy.
- Enter the joist spacing and the post spacing along each beam.
- Set fasteners per board per joist and a waste allowance.
- Add unit prices in the takeoff table to get a materials budget.
How the estimate works
Rows of boards = ⌈deck width ÷ (board width + gap)⌉. Boards per row = ⌈deck length ÷ board length⌉. Boards = rows × boards per row × (1 + waste). Joists = ⌊deck length ÷ joist spacing⌋ + 1. Fasteners = rows × joists × fasteners per crossing. Footings assume two beams, each with posts at the spacing you enter.
Worked examples
16 × 12 ft deck. 5½ in boards with a 3/16 in gap: 26 rows. 16 ft boards, one per row, plus 10%: 29 boards. Joists at 16 in: 13. Fasteners: 26 × 13 × 2 = 676. Footings with posts every 6 ft on two beams: 6.
4.8 × 3.6 m deck. 140 mm boards with a 5 mm gap: 25 rows; 4.8 m boards plus 10%: 28 boards. Joists at 400 mm: 13. Fasteners: 650.
How to interpret the results
The board count is the main purchase. Joists, beams and footings are typical quantities only; their sizes and spans must come from span tables or an engineer for your loads, timber grade and height above ground.
Tips and common mistakes
- Run boards across joists, never parallel; the deck length is the board direction.
- Use stainless or coated fasteners compatible with treated lumber.
- Plan butt joints over doubled joists or blocking when boards are shorter than the deck.
- Check height rules: raised decks need guards, and many places require permits.
Assumptions and limitations
The calculator handles one rectangle with boards running the long way. Ledger attachment, bracing, stairs and railings are not included. Structural parts must comply with your local building code; consult a professional for raised or heavily loaded decks.