Decking is sold by the piece, so the useful answer is not square footage — it is how many boards of a given length to put on the truck. This calculator counts rows of boards across the deck, pieces per row, and adds your waste factor. It also estimates joists at 16" (400 mm) on centre and the screws needed at every crossing.
Method
- Rows = deck width ÷ (board width + gap), rounded up.
- Pieces per row = deck length ÷ board length, rounded up.
- Boards = rows × pieces × (1 + waste).
- Screws = rows × joists × 2.
Worked example
A 16 × 12 ft deck with 5-1/2" boards and 3/16" gaps: each row takes 5.6875", so 12 ft (144") needs 26 rows. With 16 ft boards there is one piece per row, so 26 boards plus 10% = 29 boards. Joists at 16" o.c. over 16 ft = 13 joists, and 26 × 13 × 2 ≈ 710 screws.
Square it before the first board
An out-of-square deck shows up immediately in the last board, which ends up tapered. Square the ledger and rim joists with the 3-4-5 calculator and confirm diagonals with the diagonal check. Our deck squaring guide shows the full sequence.
Watch: how it is done
Video by Colorado Custom Covers & Decks on YouTube. More in our video library.
Frequently asked questions
What gap should I leave between deck boards?
Around 3/16" (5 mm) is common for kiln-dried and composite boards. Wet pressure-treated boards are often butted tight because they shrink as they dry. Follow the board maker's instructions.
What joist spacing do I need?
16" on centre is standard for most wood and composite decking laid perpendicular. Diagonal decking and some composites need 12".
Should I buy longer boards to avoid joints?
Yes, when possible. Full-length boards look better and need fewer butt joints, and each joint must land on a joist, often with blocking.
Results are planning estimates. For structural work, confirm requirements with your local building authority and a qualified professional. See our disclaimer.