Deck planning
How Many Deck Boards and Joists Do I Need?
Divide the deck width in inches by the board width plus the gap to get rows of decking, then add 10 percent. A 12 x 16 foot deck needs about 28 boards that are 5.5 inches wide and 16 feet long, and 13 joists at 16 inches on center.
Open the deck material calculatorCounting deck boards
Boards are laid side by side across the width of the deck. Both 5/4x6 and 2x6 decking are actually 5.5 inches wide. Add the gap between boards, usually 1/8 to 1/4 inch, to get the coverage of one row.
For a 12 foot wide deck: 144 inches / 5.75 inches = 25.04, so 26 rows. With 10 percent waste, buy 28 boards. Order boards as long as the deck so each row is a single piece.
Counting joists
Joists run perpendicular to the deck boards. Divide the deck length in inches by the spacing, round up, and add one. A 16 foot long deck at 16 inches on center is 192 / 16 + 1 = 13 joists.
| Decking | Typical joist spacing | Notes |
|---|---|---|
| Wood decking, laid straight | 16 in on center | Most common |
| Wood decking, laid diagonally | 12 in on center | Boards span farther at an angle |
| Composite or PVC decking | 12 to 16 in on center | Follow the manufacturer guide |
| Stair treads | Closer stringer spacing | Composite often needs 9 to 12 in |
What a board count leaves out
Decking is often less than half of the material cost. A realistic budget also needs the frame and everything that holds it up.
- Rim joists, beams, and blocking
- Posts, footings, and concrete
- Ledger board, flashing, and structural fasteners where the deck meets the house
- Joist hangers, post bases, and deck screws or hidden clips
- Railings, balusters, and stairs
- Stain or sealer for wood decks
Structure and permits
Joist size, beam size, and footing layout come from span tables and local code, not from a board count. Decks attached to a house, and decks more than 30 inches above the ground, usually require a permit, inspections, and guardrails. The American Wood Council publishes a free prescriptive guide, DCA 6, that many building departments reference.
- Board rows = deck width in inches / (board width + gap), plus 10 percent.
- Joists = deck length in inches / spacing, rounded up, plus one.
- Framing, footings, and permits follow span tables and local code.
Run the numbers
The Deck Material Calculator applies this math to your own measurements and shows each step. Best for a first budget before deciding whether to DIY or call a deck contractor.