Key Takeaways
- Board count depends on board width and gap, not just square footage
- Framing (joists, beams, posts) often costs more than the decking boards
- A 10 percent waste allowance covers cuts and angled edges for rectangular decks
- Joist spacing of 16 inches on centre is standard for most residential decks
- Always check local building codes for post and beam spacing requirements
Square foot is a unit of area equal to a square measuring 1 foot on each side, commonly used in the US for measuring deck surfaces.
Joist spacing is the distance between the centres of adjacent joists, measured in inches. Closer spacing means more joists and a stronger deck surface.
Building a deck starts with knowing how much material you need, and that means more than just square footage. The number of boards depends on the board width, the gap between boards, and how much waste the shape creates. The framing underneath, which is what holds everything up, adds joists, beams and posts to the list, and those numbers add up faster than most people expect.
Here is how to work through each part of the material estimate.
The deck area is the starting point
Multiply the length by the width. A deck that is 12 feet by 16 feet covers 192 square feet. That number is the basis for everything else, but it does not tell you how many boards to order.
Board count depends on coverage per board
A board covers more area than just its width times its length, because the gap between boards takes up space. The effective coverage per board is the board width minus the gap, multiplied by the board length.
For a standard 5.5 inch board with a 3/16 inch gap:
- Effective width = 5.5 - 0.1875 = 5.3125 inches = 0.4427 feet
- Coverage per 12 ft board = 0.4427 x 12 = 5.31 square feet
A 192 square foot deck needs roughly 36 boards before waste. Add 10 percent for cuts and angled edges and you are at about 40 boards.
Framing is the hidden cost
The decking boards are what you see, but the joists, beams and posts underneath are what hold everything up. Getting the framing count right matters because adding material later means tearing up boards you already laid.
Joists run perpendicular to the boards. At 16 inch spacing, a 12 foot wide deck needs joist count equal to 12 x 12 / 16 + 1 = 10 joists. Each joist runs the full 16 foot length of the deck.
Beams run parallel to the joists and support them. At 8 foot spacing, a 16 foot long deck needs ceil(16/8) + 1 = 3 beams.
Posts support the beams. With 8 foot post spacing and a 12 foot beam length, you need ceil(12/8) + 1 = 3 posts per beam, for a total of 3 x 3 = 9 posts.
The waste allowance covers real losses
Cutting boards to length wastes material at every cut. Angled edges, curved sections and boards that do not quite reach the end of a run all contribute. Ten percent is a reasonable allowance for a straightforward rectangular deck. An irregular shape or a deck with many angles may need more.
Zero percent waste does not mean zero waste. Even with a zero percent allowance, rounding up to whole boards means you will have a small amount left over.
A caution worth reading
The numbers above are shopping estimates, not engineering specifications. For anything structural or anything your local building department has an opinion about, use what they require. This site produces material quantities, not engineering drawings.
Once you have your dimensions, the deck material calculator turns them into a complete material list with boards, joists, beams and posts.
Editorial note: This article is reviewed for accuracy and updated regularly. All calculations use standard industry formulas. For the latest density figures, consult your supplier directly.
Frequently asked questions
How many deck boards do I need for a 12 by 16 foot deck?
Using standard 5.5 inch wide boards at 12 feet long with a 3/16 inch gap and 10 percent waste, a 12 by 16 foot deck (192 square feet) needs about 40 boards. The exact number depends on the board dimensions you choose and the waste allowance.
How far apart should joists be on a deck?
Most residential decks use joist spacing of 16 inches on centre, which is the standard for most decking materials. Heavier loads or thinner decking may call for 12 inch spacing. Check your local building codes, which may specify maximum joist spacing for the decking material and thickness you are using.
How many posts does a 12 by 16 foot deck need?
With 8 foot post spacing, a 12 by 16 foot deck typically needs 9 posts: 3 beams (one at each end and one in the middle) with 3 posts per beam. Post spacing affects the strength of the frame, so follow your local building codes.
How do I calculate the beam count for a deck?
Divide the deck length by the beam spacing and round up, then add one. A 16 foot deck with 8 foot beam spacing needs ceil(16/8) + 1 = 3 beams. Each beam runs the full width of the deck.