Field Guide
How Many Deck Boards Do I Need? Boards, Joists & Screws
A deck is a grid: joists run out from the house, and deck boards run across them. Count the rows of boards across the depth, the joists along the length, and the fasteners where each board crosses each joist.
1. How to Calculate Decking (The Math Explained)
Here's the exact process this calculator uses, with a deck 16 ft along the house and 12 ft deep as the example.
-
1
Count the board rows
Each 5½ in board plus a 3/16 in gap covers 5.6875 in of the depth.
Rows = 144 in ÷ 5.6875 in = 25.3 → 26 rows -
2
Convert rows to boards
Multiply by the deck length, add waste, and divide by the board length.
Linear ft = 26 × 16 ft = 416 ft With 10% waste = 457.6 ft Boards = 457.6 ÷ 16 = 28.6 → 29 boards -
3
Count joists and screws
Joists at 16 in on center, plus one to finish the run. Two screws where each board crosses a joist.
Joists = 192 in ÷ 16 + 1 = 13 joists × 12 ft Screws = 26 × 13 × 2 = 676 → 710 with 5% extra
2. Choosing Deck Lumber
| Board | Size | Good to know |
|---|---|---|
| Pressure-treated pine | 5/4×6 or 2×6 | The lowest cost. Let it dry before staining; expect some checking and twisting. |
| Cedar or redwood | 5/4×6 or 2×6 | Naturally rot-resistant and stable; needs a finish to keep its color. |
| Composite or PVC | 1×6 nominal (5½ in wide) | Low maintenance. Follow the maker's joist spacing, gap and fastener rules. |
3. Building Tips
Flash the ledger. Bolt the ledger to the house framing (never just the siding) and flash above it so water can't get behind.
Use rated hardware. Joist hangers and screws must be rated for treated lumber (hot-dip galvanized or stainless steel).
Size it from a span table. Joist, beam and post sizes come from the spans and local code; the AWC's DCA 6 guide is a common reference.
Pro tip: plan the board length
Pick a board length that covers the deck in one piece, or that splits evenly. Joints should be staggered and always land on a joist, ideally a doubled one.
4. Frequently Asked Questions
How many deck boards do I need for a 16×12 deck?
With the boards running along the 16 ft side, the 12 ft depth is 144 in. Each 5½ in board plus a 3/16 in gap covers 5.6875 in, so you need 26 rows. 26 rows × 16 ft = 416 linear ft; with 10% waste that's 457.6 ft, or 29 boards of 16 ft.
How many joists do I need for a deck?
Divide the deck length in inches by the joist spacing and add one. A 16 ft deck at 16 in on center needs 192 ÷ 16 = 12, plus 1, or 13 joists. Each joist spans the depth of the deck, so a 12 ft deep deck uses 12 ft joists.
How many screws do I need for decking?
Plan on two screws wherever a board crosses a joist. A 16 × 12 ft deck has 26 rows crossing 13 joists: 26 × 13 × 2 = 676 screws, or about 710 with 5% extra. Hidden clip systems use about one clip per crossing; check the brand's coverage.
How far apart should deck joists be?
16 in on center is standard for wood deck boards laid straight. Diagonal decking needs 12 in, and many composite and PVC boards require 12 in for some colors, profiles or in commercial use. Always follow the board manufacturer's span rating.
What size joists do I need for a deck?
Joist size depends on the span (the distance between supports), the spacing and the wood species. 2×8 and 2×10 joists are the most common. Use a span table such as the American Wood Council's DCA 6 or your local code to size joists, beams and posts.
How much gap should be between deck boards?
Most composite boards need about 3/16 in between boards; follow the maker's instructions. Wet pressure-treated boards are often installed tight because they shrink as they dry, while kiln-dried wood gets a 1/8 to 3/16 in gap. A gap lets water drain and the deck dry out.
What length deck boards should I buy?
Choose a length that covers your deck with as few butt joints as possible, or that splits evenly: 16 ft boards suit 16 ft and 8 ft decks, 12 ft boards suit 12 ft and 6 ft decks. On longer decks, stagger the joints and land every joint on a joist (or a doubled joist).