How this decking calculator works
Decking is sold by the board, so the job is to work out how many boards cover your deck. Deck boards run across the deck, perpendicular to the joists, so the calculation has two parts. First find the number of rows: the deck width divided by one board width plus the gap between boards, rounded up. Each board occupies its own width plus one gap on the grid, so a 5.5 in board with a 1/8 in gap effectively takes up 5.625 in.
Second, each row is rarely a single board, so multiply the rows by the deck length divided by your board length, rounded up. That gives the bare number of board pieces. Then add a waste allowance — typically 10%, more for diagonal or picture-frame layouts — because cuts waste board ends and some boards arrive cracked, and round up to whole boards. For the frame, the number of joists is the deck length divided by the joist spacing, rounded up, plus one for the end joist; this tool assumes joists span the deck width. Fasteners are a rough estimate based on the deck area.
Reference note: results are a planning estimate. Layout, board direction, picture-frame borders, stairs and the size of your offcuts all change real waste, and the screw figure is a rough rule of thumb that varies with the fastener system. Always check the board manufacturer's span tables and your local building code before buying.
Typical board and joist sizes
| Item | Common size | Notes |
|---|---|---|
| 5/4×6 board | 5.5 in wide | Most common decking board; nominal 6 in, actual 5.5 in. |
| 2×6 board | 5.5 in wide | Thicker board for longer spans. |
| Board gap | 1/8 in (0.125) | Allows drainage and shrinkage; composite often 3/16–1/4 in. |
| Joist spacing | 16 in OC | 12 in for diagonal layouts; 24 in only for some heavy boards. |
Frequently asked questions
- How many deck boards do I need?
- Boards run across the width, so rows = deck width ÷ (board width + gap), rounded up. Each row may need several boards end to end, so multiply by deck length ÷ board length, rounded up, then add a waste allowance for cuts and defects. Enter your dimensions above and the calculator returns the count both before and after waste.
- What gap should I leave between deck boards?
- About 1/8 inch (0.125 in, roughly 3 mm) is typical for pressure-treated boards fitted while still damp, since they shrink as they dry. Kiln-dried or composite boards often use a slightly larger 3/16 to 1/4 inch gap. The gap lets water drain and air circulate, and the calculator adds it to each board width.
- What is joist spacing?
- It is the on-center distance between the framing members under the boards. Common spacings are 16 in on-center for standard decking and 12 in for diagonal layouts or thinner composite boards; 24 in suits only some heavy-duty boards. Closer spacing means a stiffer deck and more joists, so follow the board span tables.
- How much waste should I add?
- About 10 percent is standard for a simple rectangular deck. Add more — around 15 percent for diagonal or picture-frame layouts and angled decks — because every cut wastes part of a board and some boards arrive cracked or warped. The calculator adds whatever percentage you enter and rounds up to whole boards.
- How do I calculate the number of joists?
- Divide the deck length by the joist spacing, round up, then add one for the end joist. A 16 ft deck at 16 in on-center needs ceil(192 ÷ 16) + 1 = 13 joists. This tool assumes joists span the deck width and boards run perpendicular across them; check beams, posts and footings separately.
- Does board direction matter?
- Yes. Deck boards must run perpendicular to the joists so each board is supported at regular intervals, which is why rows depend on the deck width and joist count on the deck length. Running boards diagonally looks good but increases both waste and the number of joists, so add extra material for those layouts.