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Decking Materials Calculator

Estimate deck boards, joists, and screws from the deck size and board and joist spacing.

  • deck
  • decking
  • boards
  • joists
  • screws
  • patio
  • lineal
  • canada

About Decking Materials Calculator

A deck is mostly a long, repetitive run of identical parts, which makes it easy to under-order and end up one board short with the rest of the pack discontinued. This calculator works out the three quantities that make up a deck's material list — the decking boards laid across the surface, the joists that carry them underneath, and the screws that hold it all together — from the deck's footprint and your board and joist spacing.

It lays the boards along the length and spaces the joists across the width, the usual arrangement, and reports both a board-row count and total lineal metres so you can buy whichever way your timber merchant sells. Give it your board stock length and it converts the lineal figure into a whole-board count. The screw estimate assumes two fixings per board at every joist it crosses.

How to use

Choose metric or imperial and enter the deck's length and width. Set the board width and the gap you want between boards — a small gap lets the timber drain and breathe — and the joist spacing, commonly 400 mm (16″) centres. If you know the length the boards come in, add it to get a whole-board count; leave it blank to work in lineal metres.

The results show board rows and decking length, the joist count and their total length, and an approximate screw count. Add about 10% to the board and lineal figures for cuts and waste — more for a diagonal or herringbone layout — and remember the joist count is for a simple single-span deck. Stairs, picture-frame borders, and angled sections all need extra framing the footprint alone can't predict. Always check joist spacing against your decking's span tables.

Frequently asked questions

  • How many deck boards do I need?

    Divide the deck width by the board's coverage — its width plus the gap between boards — and round up to get the number of rows; each row runs the deck length. A 3 m-wide deck with 140 mm boards and a 5 mm gap needs 3 ÷ 0.145 ≈ 21 rows. Multiply rows by the deck length for total lineal metres, or by the boards-per-row at your stock length for a whole-board count. Add about 10% for waste.

  • What gap should I leave between deck boards?

    A gap of around 3–6 mm is typical so water drains, air circulates, and debris falls through rather than sitting on the deck. Timber boards are often laid tighter because they shrink as they dry, while composite boards follow the manufacturer's specified gap, which can be larger. The gap matters for your board count — wider gaps mean fewer boards — so it's a separate input here.

  • How far apart should deck joists be?

    For most timber decking, 400 mm (16″) centres is standard, giving boards solid support and a firm feel underfoot. Some heavier or thicker boards allow 600 mm (24″), while many composite boards require closer spacing — 300 mm (12″) — especially when laid diagonally. Always follow the span table for your specific decking product, since spacing too wide leads to bounce and sag.

  • How many screws will I need for my deck?

    The usual rule is two screws where each board crosses each joist, so the count is board rows × joists × 2. A 6 × 3 m deck at 16″ centres works out to roughly 21 rows × 16 joists × 2 ≈ 670 screws. Buy a box or two extra — you'll strip or snap a few, and it's worth having spares than stopping the job. Hidden-fastener systems use a different count set by the clip spacing.

  • Does the calculator include the substructure and stairs?

    No — it covers the decking boards, the joists, and the screws for a simple rectangular deck on a single-span layout. Beams, posts, footings, ledger boards, stairs, railings, and picture-frame borders are separate and depend on the deck's height and design. Use this for the surface and joist take-off, then add framing for everything below and around it.

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