How Many Deck Boards Do I Need? Formula & Quick Chart

Decking is sold in standard boards, usually 5½ inches wide, and in lengths from 8 to 20 feet. To count how many you need, work out how many rows of boards fit across the deck, multiply by the deck length to get the total length of board, add waste, and divide by the length you plan to buy. A 12 × 12 ft deck takes about 29 boards of 12 ft.

The formula

1. Rows of boards. Divide the width of the deck (the direction the boards stack up) by the board width plus the gap. Round up. A 5½ in board with a 1/8 in gap takes up 5.625 in, so a 12 ft (144 in) wide deck needs 144 ÷ 5.625 = 25.6, which is 26 rows.
2. Total length. Rows × deck length. For 26 rows on a 12 ft deck, 26 × 12 = 312 ft.
3. Add waste. 10% is typical for straight boards, and 15% for a diagonal pattern or a picture frame border. 312 ft × 1.10 = 343 ft.
4. Divide by the board length. 343 ÷ 12 = 28.6, so buy 29 boards of 12 ft.

The deck board calculator does this and also counts joists and screws.

Boards needed for common deck sizes

With 5½ in boards, 1/8 in gaps and 10% waste. The first number is the deck length (the way the boards run), and the second is the width.

Choosing the board length

• Match the deck length where you can. A 16 ft deck and 16 ft boards means no butt joints, which are weaker and less attractive. But the longest boards cost more per foot and are harder to carry and straighten.
• If boards are shorter than the deck, the joints should land on a joist with space for both ends (use a double joist, or two joists side by side), and be staggered between rows so they do not line up. This needs extra boards for the offcuts.
• Do the sums at several lengths. A 12 ft deck can be done in 12 ft boards with almost no waste. A 13 ft deck in 12 ft boards needs a second board per row and wastes most of it, unless you can use the offcuts elsewhere, while 14 ft or 16 ft boards waste far less. Pick the length with the least offcut.

Gaps, composite and wood

Gap size depends on the material. Pressure-treated wood is often damp when installed and shrinks as it dries, so installers lay it nearly flush (about 1/8 in, or a nail's width). Kiln-dried and naturally dry woods, and most composite and PVC boards, need a gap of 3/16 to 1/4 in to allow for expansion and for water to drain. Always follow the manufacturer's installation guide, which may specify the gap and the temperature range for installation.

Joists and spacing

Joists are the frame under the boards. Spacing is typically 16 in on center, which suits standard decking. Many composite boards and any diagonal layout need 12 in. Wider spacing (24 in) is only for decking rated for it. The calculator counts joists as the length divided by the spacing, plus one for the end. How long and how large the joists must be, and how far they can span, is set by your local building code (use span tables), the lumber species and grade, and the load.

Screws and fasteners

Each board needs two screws or nails at every joist it crosses. With 5½ in boards and 16 in joists, this is roughly 350 to 400 fasteners per 100 ft² of deck. Use screws rated for exterior and pressure-treated lumber (coated or stainless steel). Hidden fastener systems are neater, but cost more and take longer.

What isn't included here

• Framing: beams, posts, footings, ledger boards, flashing, joist hangers and bracing.
• Railings and stairs: required by code for decks above a certain height (often 30 in), with specific heights and baluster spacing.
• Fascia and trim: often the same boards cut to size.
• Permits: most decks need a building permit, and many need an inspection.

A deck is a structure that people stand on, often well above the ground. Have the design checked by your local building department, or hire a professional for anything attached to a house or more than a few feet high.