Deck Board Calculator

How to calculate deck boards

Deck boards are counted in rows. Each row covers the width of one board plus one gap, so the number of rows depends on the distance across the boards, and the boards in each row depend on the distance along them (the run) and the length you buy.

• Rows = distance across the boards (in) ÷ (board width + gap), rounded up
• Boards = rows × boards per row
• Boards to buy = boards × (1 + waste % ÷ 100), rounded up
• Linear feet = boards to buy × board length

The last row is usually ripped (cut lengthwise) to fit, so it counts as a whole board. Use the actual width of the board, not its name: a 2×6 and a 5/4×6 deck board are both 5 ½ inches wide, and a 2×4 is 3 ½ inches. Measure the deck to the outside of the frame and add any overhang past the rim joist or fascia.

Boards per row. When the run is no longer than a board, each row takes one board, and a board at least twice as long as the run gives two rows. When the run is longer than a board, the row needs a butt joint over a joist. The calculator fills each row with full boards plus one shorter piece and cuts as many short pieces from a board as fit, so a 20-foot run with 16-foot boards takes one board and a quarter per row (four 4-foot pieces come from one board). The best-fit suggestion picks the length with the fewest butt joints, and then the least offcut.

Diagonal decking. On a 45° layout every row is a different length, so the calculator works from area instead: linear feet = deck area × 12 ÷ (board width + gap), and boards = linear feet ÷ board length. It assumes offcuts from the long middle rows fill the short corner rows, and the default waste rises to 15 percent to cover the angled cuts. It suggests the shortest board that spans the longest row without a joint.

Joists and fasteners. Joists = run (in) ÷ joist spacing, rounded up, plus one for the end joist. Every board crosses every joist, and each crossing takes two face screws or about one hidden clip. A butt joint puts two board ends on the joist, so it adds another two screws.

Example: a 12 × 16 ft deck

A deck 16 feet along the house and 12 feet out, with 5/4×6 boards running the 16-foot way, 3/16-inch gaps, 16-foot boards, joists 16 inches on center and 10 percent waste:

1. Board plus gap: 5.5 + 0.1875 = 5.6875 in.
2. Rows: 12 ft = 144 in, and 144 ÷ 5.6875 = 25.32, so 26 rows.
3. Boards: one 16-foot board per row, so 26 boards (416 linear feet on the deck).
4. With waste: 26 × 1.10 = 28.6, so 29 boards, or 464 linear feet.
5. Joists: 192 in ÷ 16 = 12 spaces, so 13 joists.
6. Fasteners: 26 × 13 × 2 = 676 face screws, or about 338 hidden clips.

With 12-foot boards each row is a 12-foot board plus a 4-foot piece, and one board gives three pieces: 26 + 9 = 35 boards, 39 with waste, and 26 butt joints to support. The 16-foot boards are the better buy.

Laid diagonally on joists 12 inches apart with 15 percent waste, the same deck has 42 rows and about 405.1 linear feet of board. The longest row is 16.97 feet, so the calculator suggests 20-foot boards: 25 of them.

Deck boards for common deck sizes

5 ½-inch boards (2×6 or 5/4×6) with 3/16-inch gaps, running the long way, 10 percent waste included:

The 10 × 10 and 12 × 12 decks take the same number of 12- and 16-foot boards, but the 16-foot boards leave a long offcut on every row. For a quick estimate by area, use linear feet per 100 square feet (no waste added):

Deck screws and hidden clips

The same decks with 16-foot boards and joists 16 inches on center, end joists included:

Choose screws long enough to reach about 1 ½ inches into the joist: 2 ½ inches for 5/4 boards (1 inch thick) and 3 inches for 2× boards (1 ½ inches thick). Use screws rated for treated lumber, coated or stainless, because the copper in pressure-treated wood corrodes plain steel. Screws are sold by count or by the pound, and the count per pound changes with length and gauge, so check the label before you convert. Hidden-clip systems come with their own coverage figures and starter clips; follow the maker's numbers when they differ.

Which way deck boards run, and joist spacing

Deck boards always run across the joists. On a deck built off a ledger, the joists run out from the house, so the boards run parallel to it; that is the "along the length" setting when the length is the side against the house. Boards laid at 45° take more cuts, more waste and closer joists.

Joist spacing sets how stiff the floor feels and is limited by code. The International Residential Code, which most US jurisdictions adopt with local changes, lists maximum joist spacing for decking in Table R507.7 of its 2018 and 2021 editions: 1 ¼-inch-thick wood (5/4 deck boards) may span 16 inches with boards square to the joists and 12 inches on a diagonal, and 2-inch-thick wood (2×6) 24 inches and 16 inches. Composite decking follows the span on its manufacturer's label. Most builders use 16 inches on center for straight runs and 12 inches for diagonals; check your local code and the decking instructions before you frame.

The joist count here includes both end joists. It does not include doubled joists or blocking at butt joints, beams, stair stringers or the ledger.

Gapping pressure-treated and composite decking

Side gaps let water drain, leaves fall through and air dry the boards.

• Wet pressure-treated lumber often comes from the yard still damp from treatment and shrinks in width as it dries. Many builders lay it tight or with a ⅛-inch gap and let the gap open up over the first season.
• Dry wood, such as pressure-treated lumber stamped KDAT (kiln-dried after treatment), is already near its final size, so gap it at installation, about ⅛ to ¼ inch.
• Composite and PVC boards grow and shrink in length with temperature more than wood does. Manufacturers specify the side gap, the end gap between boards and the gap at the house, often with a chart by installation temperature, and their hidden clips set the side gap. Follow the install guide, because the warranty can depend on it.

Tips for ordering deck boards

• Match the board to the run. A 16-foot run with 16-foot boards needs no joints at all. If the run is longer than your longest board, plan where the joints go before you order.
• Stagger the butt joints. Do not line up joints on the same joist in neighboring rows. Many builders double the joist or add blocking at each joint so both board ends get full bearing, and many composite makers require it.
• Count the extras separately. Picture-frame borders, fascia, stair treads and railing caps are not in this count. Add them by length, and raise the waste for mitered borders.
• Buy a few spares. Some boards will be crooked, split or badly checked. The waste allowance covers them, but keep the leftovers for repairs.
• Pricing by the board foot. Some lumberyards quote decking by the board foot instead of by the piece; the board foot calculator converts your board count.