This decking estimator turns a deck's dimensions into a materials list: how many deck boards you need, the joists and spacing beneath them, the fasteners, and an approximate cost. It gives you a realistic shopping list and budget before you order timber.
From area to boards
The number of deck boards depends on the deck width, the board width, and the gap left between boards for drainage and expansion (commonly 3–6 mm). The estimator divides the span to be covered by the board width plus gap, then works out how many board lengths are needed to run the deck's length, adding an allowance for waste and for staggered end joints.
Joists are sized and spaced to carry the decking and the load above it — typically 400 mm (16 in) centres for standard boards, closer for thinner or diagonally laid boards. The estimator counts joists across the span and the fasteners needed to fix every board to every joist.
Allowing for the real job
A good estimate includes waste for cuts and defects, extra fasteners (you always drop some), and the picture-frame or border boards if you plan them. Laying boards diagonally increases both board and joist quantities. Treat the figures as a well-founded starting point and round up at the merchant.
A 5 m × 3 m deck uses 144 mm boards with a 5 mm gap, laid across the 3 m width.
- Effective board coverage = 144 + 5 = 149 mm per board.
- Boards across 5 m length = 5000 ÷ 149 ≈ 34 rows.
- Each row spans 3 m, plus ~10% waste for cuts and staggered joints.
Around 34 board rows at 3 m, plus joists at 400 mm centres and fixings — add 10% for waste.
