How to Estimate Deck Boards and Joists

    Count rows, linear feet and boards for decking, and see how joist spacing affects the count, with a worked example for a 12 × 12 foot deck.

    Decking is laid in rows across the joists, so estimating it is mostly a matter of counting rows and deciding what board lengths to buy. This guide covers the surface boards; the structure underneath, including footings, beams and railings, has its own code requirements.

    Step 1: Count the rows

    A common 5/4 × 6 deck board is about 5.5 inches wide, and a small gap is left between boards for drainage, so each row takes about 5.625 inches. Divide the deck's width in inches by 5.625 and round up.

    Step 2: Count the boards

    Each row runs the length of the deck. If your boards are 12 feet long and the deck is 12 feet long, you need one board per row. For longer decks, pick lengths that land on joists, and add boards for waste, stairs and a border.

    Worked example: a 12 × 12 foot deck

    Gaps and spacing

    The right gap depends on the material and the manufacturer, and joist spacing depends on the decking and span. Always follow the decking maker's instructions.

    Staining and sealing

    A gallon of deck stain typically covers roughly 150 to 300 square feet of deck, depending on the product and how rough the boards are. Add railings and stairs if you plan to finish them too.

    This guide covers decking boards only. Footings, ledger connections, guards and stairs have structural and safety requirements; check local code and consider a professional.

    Try the calculators

    More guides