How many deck boards do I need?

Nominal size is not actual size

Decking is counted by rows, not by area. Because boards run the full length of the deck, the number of rows is set entirely by the width: divide the width in inches by the board width plus the gap, and round up. A 12 foot wide deck with 5.5 inch boards and a quarter inch gap needs 26 rows. Multiply rows by the deck length and you have your linear feet.

The trap is board width. A 2x6 is not 6 inches wide — it measures 5.5 inches, because nominal lumber sizes describe the rough-sawn dimension before drying and planing. A 2x4 is 3.5 inches, a 2x8 is 7.25. Using the nominal number instead of the actual one will leave you roughly 8% short, which on a large deck is several hundred dollars of missing boards. Composite decking does not follow the same convention at all, so measure a sample.

Gaps are not optional. Pressure-treated lumber arrives wet and shrinks as it dries, so it is often laid tight and opens up on its own. Kiln-dried and composite boards do not shrink much and need a deliberate quarter inch gap for drainage and expansion. Without it, water sits on the deck and the boards cup.

Two things this count does not include: the framing underneath, and the fact that every butt joint must land on a joist. If your deck is longer than the boards you can buy, you are not just adding boards — you are planning where the seams fall, and usually adding blocking or a doubled joist to land them on.

A worked example: a 20 × 14 deck

Take a deck 20 feet long and 14 feet wide, with 5.5-inch boards, a quarter-inch gap and 10% waste. The width sets the rows: 168 inches ÷ 5.75 inches is 29.2, so 30 rows. Each row runs the full 20 feet, so you need 600 linear feet. With 10% waste that is 660 feet, or 42 boards at 16 feet.

Here is the catch the board count does not show: a 16-foot board does not span a 20-foot deck. Every row needs a joint, and every joint has to land on a joist. A clean way to do it is to buy 20-foot boards if your yard stocks them, or to combine 12- and 8-foot boards and stagger the joints so no two neighboring rows break on the same joist. Plan the joint pattern before you order, then count boards by length.

Fasteners add up fast too. With two screws per board at every joist and joists 16 inches apart, each 20-foot row crosses 16 joists, so the deck takes about 30 × 16 × 2 = 960 screws. Buy a little over a thousand and you will not run out on the last row.

Actual board widths

*With a quarter-inch gap. Composite brands vary by an eighth of an inch or so, so measure a sample board before you finalize the order.

Mistakes that cost money

• Using the nominal width. Counting a 2x6 as 6 inches leaves you about 8% short. Use 5.5.
• Ignoring the manufacturer's joist spacing for composite. Many composite boards need joists 12 inches apart instead of 16, and closer still for diagonal patterns. Changing the frame after the boards arrive is expensive.
• Lining up the butt joints. Joints that all fall on the same joist make a visible seam and a weak line. Stagger them across at least three joists.
• Forgetting the picture-frame border. A border board around the edge adds rows and changes the length of every field board. Count it separately.

What it costs

As a rough guide, a 16-foot pressure-treated 5/4x6 deck board often sells for $15 to $25 in the US, and composite boards of the same length commonly run $40 to $80. For the deck above, 42 boards come to roughly $630 to $1,050 in pressure-treated wood or $1,680 to $3,360 in composite, before framing, fasteners and hardware. Lumber prices move a lot, so price your exact list at a local yard.