Deck Material Calculator

What a deck is made of

A deck is four systems stacked together: footings in the ground, joists that span between beams, decking boards on top, and the fasteners holding it all together. The board count is the number most people get wrong, because it depends on the board width plus the gap — a 12-foot-wide deck needs about 26 boards at 5.5 inches wide with a quarter-inch gap. Joists are spaced 16 inches on centre for wood decking and 12 inches for most composite decking, which changes the joist count by a third. Footings depend on your soil and your local code, and the estimate here — two per joist — is a starting point, not a structural design.

How the numbers work

Boards equal the deck width in inches divided by the board width plus the gap, rounded up, then increased by the waste allowance for cuts and defects. Joists equal the deck length in inches divided by the spacing, plus one for the rim joist at the end. Screws are estimated at two per board-joist intersection, which is the standard fastening pattern. The cost line multiplies each quantity by the unit price you enter — set the prices to zero if you only want quantities.

Before you build

Most jurisdictions require a permit for a deck above a certain height or size, and the footing depth must go below the local frost line — that depth varies from a few inches in warm climates to 4 feet or more in the north. Composite decking usually needs joists at 12 inches on centre, not 16, so check the manufacturer's span table before you order. And buy boards from the same batch: colour varies between production runs, and a deck built from two batches shows it.

Frequently asked questions

How many deck boards do I need for a 12×16 deck?

About 26 boards at 5.5 inches wide with a quarter-inch gap, before waste. With a 10% waste allowance that is about 29 boards. Enter your exact size above for your deck.

What joist spacing should I use?

16 inches on centre is standard for wood decking. Most composite decking requires 12 inches on centre because it is more flexible. Always check the decking manufacturer's span table — spacing is a structural requirement, not a preference.

How deep do deck footings need to be?

Below the local frost line, which varies widely — a few inches in warm climates, up to 4 feet or more in cold ones. Your local building code states the required depth, and an inspector will check it.