How many deck boards do I need?

Divide the run of the deck (perpendicular to the boards) by the board pitch, which is the face width plus one gap, to get the number of board rows. Then divide the span each row covers by the length of one board to get boards per row. Multiply the two and add a waste allowance. This calculator does all of that automatically.

What is the right joist spacing for decking?

Most timber decking uses 400 mm (16 in) joist centres; lighter-duty or thicker composite boards may allow 600 mm (24 in), and boards laid diagonally usually need 300 mm (12 in). Tighter spacing means a stiffer, less bouncy deck but more joists. Always check your board manufacturer's span table.

How big a gap should I leave between deck boards?

A 5–6 mm gap is typical for timber so boards can drain and expand; composite boards often specify a wider expansion gap, sometimes 6–10 mm end-to-end. The gap matters because it is part of the board pitch — a wider gap means fewer boards across the same deck.

How many screws does a deck need?

A common rule is two screws per board at every joist crossing. The calculator multiplies board rows by joist count by screws-per-joist. For hidden-fastener composite systems you instead buy clips per board metre, so treat the screw figure as a guide for traditional face-fixing.

Why should I add a waste allowance?

Cuts at the ends, around posts and at the perimeter, plus the odd warped or split board, mean you never use 100 percent of what you buy. A 10 percent allowance is a sensible default; add more for complex shapes, mitred corners or picture-frame borders.

Does this calculator include posts, beams and footings?

No. It estimates the deck surface — boards, joists and screws — only. Support posts, beams, ground footings, fascia boards and balustrade are project-specific, so price those separately on top of the figure shown here.

Deck Building Calculator

A deck building calculator that turns a simple rectangle into a real shopping list: how many decking boards, how many joists, roughly how many screws, and what it all costs. It is built for anyone planning a garden deck, raised platform or balcony floor who wants a fast, honest material estimate before walking into the builders’ merchant — without spreadsheets or guesswork.

How it works

You give the tool five things: the deck length and width, the board face width, the gap between boards, the board length you can buy, and the joist spacing measured centre to centre. Everything can be entered in metric (metres and millimetres) or imperial (feet and inches), and the tool converts behind the scenes so the maths is always consistent.

From there it works in three stages. First the boards: the deck has a “run” direction (across which board rows accumulate) and a “span” direction (the way each board length points). The number of board rows is the run divided by the board pitch — the face width plus one gap — rounded up. The number of bought boards per row is the span divided by a single board’s length, rounded up. Multiply the two for the raw board count, then add your waste allowance for cuts and defects.

Next the joists: joists run perpendicular to the boards and are spaced at fixed centres along the span, so the count is the span divided by the spacing, plus one for the closing joist. Each joist is as long as the run.

Finally the screws: with traditional face-fixing you drive a set number of screws into each board at every joist it crosses, so the total is board rows times joists times screws-per-joist. Multiply each quantity by your unit prices and you get a material cost estimate, with every step shown in the working panel so you can check it.

Worked example

Say you are building a 5 m × 4 m deck with 144 mm boards, a 5 mm gap, 3.6 m board lengths and 400 mm joist centres, with boards running along the length.

Board pitch is 144 + 5 = 149 mm. The 4 m run gives 4000 ÷ 149 = 27 board rows.
Each row must span 5 m, and one board is 3.6 m, so you need 2 boards per row — 27 × 2 = 54 boards. Add 10 percent waste and you buy about 60.
Joists span the 5 m length at 400 mm centres: 5000 ÷ 400 = 12, plus one end joist makes 13 joists, each 4 m long.
Screws at two per crossing: 27 rows × 13 joists × 2 = 702 screws.

Formula note: boards ≈ ⌈run ÷ (boardWidth + gap)⌉ × ⌈span ÷ boardLength⌉ × (1 + waste); joists ≈ ⌊span ÷ spacing⌋ + 1; screws ≈ rows × joists × screwsPerJoist.

Every figure is computed in your browser — none of your numbers are uploaded or stored.