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Deck Calculator

Boards, framing, stairs, and railing — everything for a rectangular deck.

5.5" is standard 5/4×6

1/4" typical

Decking boards, joists, screws, stairs, and railing for a rectangular deck — the full material list, not just the boards.

How it works

The number of board rows is the deck's width divided by the board width plus the gap between boards, rounded up, with 10 percent added for waste. Joists are the deck's length divided by the on-center spacing (16 inches is typical), plus one, because a run of joists needs one more than the number of spaces. Screws come out at roughly 350 per 100 square feet of decking.

Example

A 12 × 16 foot deck with 5 1/2-inch boards and a 1/4-inch gap needs about 34 rows of board, and about 670 deck screws — call it one 5-pound box.

Tips & common mistakes

  • Leave a gap between boards. Wet treated lumber shrinks as it dries and will open its own gaps; dry boards need a deliberate one so water can drain rather than sit and rot the deck.
  • Buy your boards a few days early and let them sit where the deck will be. Lumber that acclimates on site moves less after you have screwed it down.
  • Check every board before you fasten it and put the crown up. A board with a slight bow, laid crown-down, becomes a puddle.
  • Do not skimp on the fasteners. Treated lumber will corrode ordinary screws, and the deck that fails is almost always the one with the wrong screws in it.

Frequently asked questions

How many deck boards do I need for a 12x16 deck?

About 34 rows of 5 1/2-inch board across a 12-foot width, once you allow a 1/4-inch gap between boards and 10 percent for waste. Multiply by the length you are buying to get your board count.

How far apart should deck joists be?

16 inches on center is standard and safe for most decking. Go to 12 inches if you are laying boards diagonally, or if you are using a composite that specifically calls for it.

What gap should I leave between deck boards?

About 1/4 inch for dry boards. If your lumber is wet pressure-treated stock straight off the pile, you can lay it much tighter — it will shrink and open its own gap as it dries.