Enter the deck’s size and the boards you’re buying. Boards are assumed to run along the deck’s length, with joists running across underneath at the spacing you choose. Add a waste allowance for off-cuts and bad boards.
Rows of boards = deck width ÷ (board width + gap), rounded up. A 4 m wide deck with 140 mm boards and 5 mm gaps needs 4,000 ÷ 145 = 27.6, so 28 rows.
Boards: each row runs the deck’s length. The total length of decking (rows × length) plus the waste allowance, divided by the board length, gives the number of boards. When a row needs more than one board, joins must land on a joist, so allow extra waste for staggered joins.
Joists run across the deck at the chosen spacing, plus one at each end: (length ÷ spacing, rounded up) + 1. Common spacings are 450 mm or 16 in for softwood and composite, and 400–600 mm depending on board thickness.
Screws: two per board at every joist it crosses, plus 10% spare. Oil or stain: about 10 m² per litre (400 ft² per US gallon) per coat; check the tin.
Divide the deck width by the board width plus gap to get the number of rows, multiply by the deck length for the total length of boards, add 10% waste, and divide by the length of the boards you’re buying.
Around 3–6 mm (1/8–1/4 in) for most timber, so water drains and boards can swell. Wet treated pine is often laid tight because it shrinks as it dries. Composite boards usually have a specific gap in their instructions.
Commonly 450 mm (about 18 in) or 16 in (406 mm) centres for 19–25 mm thick boards, closer for thin boards, diagonal decking or composite. Always check the board manufacturer’s maximum span.
Two screws per board at every joist. For a 6 × 4 m deck with 28 rows and 15 joists that’s about 28 × 15 × 2 = 840, plus about 10% spare, so buy two boxes of 500.
Usually parallel to the longest side, or at right angles to the house wall, so fewer joins are needed. This calculator assumes boards run along the length you enter; swap length and width to lay them the other way.
