How Much Decking Do I Need?

Board count, with the maths shown

A nominal 6 inch deck board is 5.5 inches wide in practice. The Southern Pine Council says to avoid lumber wider than a nominal 6 inches (5 1/2 inches actual) for decking. Trex recommends a 3/16 inch gap between boards. Each board therefore uses 5.5 + 0.1875 = 5.6875 inches of deck width.

Linear feet of board per square foot of deck is 12 / 5.6875 = 2.11. With no gap it is 12 / 5.5 = 2.18. This is our arithmetic, not a figure published by a manufacturer.

Worked example: a 12 by 16 foot deck is 192 square feet. Board needed: 192 x 2.11 = 405 linear feet. In 16 foot boards that is 405 / 16 = 25.3, so 26 boards. Allowing about 10 percent for cuts and mistakes (our rule of thumb, not a sourced figure) brings it to about 28 boards.

Joist spacing

Trex says joists should generally be no more than 16 inches on center for standard applications, and 12 inches on center if the decking is laid diagonally. The Southern Pine Council rates both 5/4 and 2 by 6 southern pine decking to span up to 24 inches when laid across the joists, though it says 16 inches is often recommended for 5/4 boards to limit bounce, and diagonal decking needs joists 16 inches apart. Always check the spacing in the instructions for the exact boards you buy.

Joist count for the example: a 16 foot run at 16 inches on center is 16 x 12 / 16 = 12 spaces, so 13 joists.

Gaps between boards

• Side to side: Trex recommends 3/16 inch for its composite boards, and up to 3/8 inch in heavily wooded, damp areas.
• End to end: Trex recommends 1/8 inch when installing above 40 F and 3/16 inch below 40 F.
• Wood boards shrink as they dry, so follow the lumber supplier's gap advice. The 2.11 factor above changes if you use a different gap: 12 / (5.5 + gap).

How many screws

Trex recommends two screws per joist crossing. The Grabber Deck Fastener Estimator gives 3.5 screws per square foot for 6 inch boards on joists 16 inches apart, and 2 per square foot at 20 inches.

For the 192 square foot example, 3.5 per square foot is 672 screws, so buy about 700. Our own count (boards per foot x joists per foot x 2 screws) comes to about 3.2 per square foot, a little lower, so the manufacturer figure includes some margin.

Common mistakes

• Forgetting the gap when you work out board coverage.
• Using joist spacing wider than the board maker allows, especially for composite or diagonal layouts.
• Buying exactly the calculated boards. Cuts at the ends and bad boards are always lost.
• Not checking the actual board width. Boards sold as 6 inch are 5.5 inches wide.

Frequently asked questions

Sources: Southern Pine Council: Building a Southern Pine deck, Trex: How to space deck boards, Trex: How to install deck joists, Trex: Decking installation guide 2022, Grabber: Deck fastener estimator technical notes. Read 7 November 2026.