Deck Calculator — Boards, Joists & Fasteners

Decking quantity comes from coverage width, not nominal width. A 5/4 × 6 board is actually 5-1/2 inches wide, and with the standard 1/4 inch drainage gap each board covers 5.75 inches of deck. A 12 by 16 foot deck with boards running the 16 foot direction needs 12 × 12 ÷ 5.75 ≈ 25 boards, plus waste for cuts, picture framing and any diagonal layout.

Joists are counted like studs: divide the deck width by the spacing and add one. Sixteen inches on center works for standard perpendicular decking with pressure-treated boards. Twelve inches on center is required for most composite decking run at 45 degrees, and many composite manufacturers also require 12 inch spacing for stair treads. Always check the manufacturer's published span table, because ignoring it voids the warranty on nearly every composite product sold in the US.

Below the frame, footing count comes from beam spans. Most residential decks land on footings every 6 to 8 feet along each beam line, sized and dug below the local frost depth. A 12 by 16 deck typically uses four to six footings. Each 12 inch diameter footing 42 inches deep takes about 2.75 cubic feet of concrete, or roughly five 80 lb bags.

Hardware is not the place to save money. Pressure-treated lumber is corrosive to standard steel, so every hanger, screw, bolt and connector in contact with it must be hot-dip galvanized or stainless. Ledger attachment is the single most common cause of catastrophic deck failure in the United States: use through-bolts or structural screws into the rim joist, install proper flashing above the ledger, and never rely on nails or lag screws driven into siding alone.

Pressure-treated pine remains the budget option at $2.50 to $4.00 per square foot for material, and it is what most of the country builds. Cedar and redwood sit at $5 to $9, and capped composite runs $8 to $16 per square foot of decking material depending on the collection. Add framing lumber, footings, hardware and railing and installed cost lands between $30 and $60 per square foot in most US markets in 2026.

The long-run comparison usually favors composite for anyone staying in the home. Pressure-treated decks need cleaning and re-sealing every two to three years at $0.75 to $1.75 per square foot each time, while composite needs washing only. Over a 25 year horizon, the maintenance difference typically outweighs the higher upfront material cost — though a treated frame under composite boards is still the standard build and still needs periodic inspection.