Enter your deck size, board width and the gap between boards, and get the number of decking boards, the total board run, and how many joists you need. Works in feet/inches or metres/millimetres.
Deck area
24.0 sq ft
Total board run
6.0 ft
Joists
5
Boards run along the length of the deck and stack in rows across its width. Each row takes up the board width plus one gap, so the calculator works out the number of rows, multiplies by the boards needed per row, and adds wastage:
rows = ceil( deck width ÷ (board width + gap) )
boards = ceil( rows × ceil(deck length ÷ board length) × wastage )
joists = floor( deck length ÷ joist spacing ) + 1
It also reports the total linear run of board - handy when decking is priced by the metre or foot rather than per board.
This example follows the unit system you pick in the calculator above.
A 20 ft × 13 ft deck, 12 ft boards 5½ in wide with a ¼ in gap, 10% wastage, joists at 16 in:
Most decks are rectangular, but you can set any section to a right-angled triangle - handy for a wedge that follows a fence line or a corner cut-off. The board run tapers row by row, so the calculator measures each row’s length separately and adds them up; the area is ½ × leg A × leg B. Build an L-shape or angled deck by adding several sections.
The edge treatment controls what happens at the perimeter:
| Board type | Typical width | Suggested gap |
|---|---|---|
| Softwood (wet treated) | 120–145 mm | 3 mm (closes as it dries) |
| Kiln-dried hardwood | 90–140 mm | 5–6 mm |
| Composite | 135–150 mm | Per maker (often 5–8 mm) |
A 10ft × 10ft deck with 5.5-inch boards and a 1/4-inch gap needs about 21 rows of board. With 10ft boards that is 21 boards, or about 24 including a 10% wastage allowance. Enter your deck size, board width and gap above for the exact count, plus the joists.
Leave about 3–6 mm (1/8–1/4 inch) between boards for drainage and air flow. Timber that is still damp will shrink as it dries, so use a smaller gap for wet treated timber and a larger one for kiln-dried or composite boards. The calculator includes the gap so the board count is accurate.
Joists are typically 400 mm (16 inches) apart on centre for standard residential decks, or 300 mm (12 inches) for boards laid diagonally or for thinner composite boards that need more support. Wider 600 mm (24 inch) spacing is only for thick boards rated for it. Pick your spacing above to get the joist count.
Add 10% for boards running straight along the deck, and 15% for a diagonal or picture-frame layout where every board is cut at an angle. The allowance covers end cuts, the occasional split or bowed board, and keeping a spare.
It gives you the joist count for the surface frame based on your spacing, plus the total linear run of decking. It does not size the supporting posts, beams or footings, which depend on the deck height and ground conditions - check your local building rules for those.
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