Deck Calculator
Estimate the deck boards, joists, and screws your deck needs from its size and board width.
Results are estimates for planning. Joist spans, beam sizing, and footings must follow your local building code — check before you build.
How to Measure for a Deck
Decking is counted as rows of boards running across the joists.
Deck size
Measure the deck length and width in feet. Boards run along the length; each row is as wide as one board plus the gap between boards.
Boards and joists
Enter the board width and the gap, plus the length the boards come in. Joist spacing is the centre-to-centre distance — 16 inches is standard.
Common Mistakes to Avoid
- No waste allowance. Cut-offs and defective boards are normal. Add about 10% so you don't run short.
- Forgetting the board gap. A 1/8 inch gap across many rows changes the row count.
- Treating boards as one length. If the deck is longer than a board, each row needs more than one board end to end.
- Under-buying screws. Two screws per board at every joist adds up fast — roughly 350 per 100 sq ft.
Frequently Asked Questions
Divide the deck width by the board width plus gap for the number of rows, then multiply by boards per row. Add about 10% waste.
About 1/8 inch for drainage and airflow. Wet pressure-treated boards can be butted tight since they shrink as they dry.
Usually 16 inches on center. Use 12 inches for a stiffer deck or for boards run at 45 degrees.
Roughly 350 deck screws per 100 sq ft — two at each board-joist intersection.
Yes — about 10% for cut-offs and defects, closer to 15% for diagonal or picture-frame layouts.