How Many Deck Boards Do I Need?

Quick answer: divide your deck's width by the board width plus the gap to get the number of rows, multiply by the deck length to get total linear feet, then divide by the stock length you're buying. A 16 ft × 12 ft deck in 5.5-inch boards with a 1/8-inch gap works out to about 26 rows, ~416 linear feet, or roughly 29 sixteen-foot boards with waste. The deck calculator does all of this — here's how it works.

Start with the board width (the real one)

The whole count hinges on how many board widths fit across your deck, and deck boards are narrower than their name suggests:

• A "6-inch" board (5/4×6 or 2×6) is actually 5.5 inches wide
• A "4-inch" board is 3.5 inches
• Composite boards are usually 5.25–5.5 inches

Add the gap you'll leave between boards, divide the deck width by that, and round up. With 5.5-inch boards and a 1/8-inch gap, each board occupies about 5.625 inches, so a 12-foot-wide deck needs 144 ÷ 5.625 ≈ 26 rows.

From rows to boards

Each row runs the full length of the deck, so:

Total linear feet = rows × deck length

For our 16-foot deck, that's 26 × 16 = 416 linear feet. Lumberyards price decking by the linear foot, so this is the number to quote. To get a board count, divide by your stock length and round up — 416 ÷ 16 ≈ 26, plus 10% waste ≈ 29 boards.

Buy the length that wastes the least

A yard stocks 8, 10, 12, 16, and 20-foot boards. The deck calculator shows the board count for every length side by side so you can pick the one that spans your deck with the fewest butt joints and least offcut. A 16-foot deck is most efficient in 16-foot boards; odd dimensions sometimes waste less split into two shorter lengths.

Don't forget the gap and the joints

• Gap: install pressure-treated boards tight (about 1/8 inch) because they shrink as they dry; give kiln-dried and composite boards 1/8–1/4 inch to move with the weather.
• Joints: where boards meet end-to-end, stagger those seams from row to row so they don't line up — it's stronger and looks far better.
• Waste: add about 10% for cuts, and 15% for a 45-degree diagonal layout.

Beyond the boards

The deck calculator also estimates the joists (at your on-center spacing) and the face screws to fasten the deck, so you can price the whole framed surface, not just the decking. Looking at it from the material angle instead? The decking calculator focuses on linear feet and square feet, and the deck board calculator on the board count itself.