Deck Calculator

You give it the deck length and width, plus your board size, and it works out the materials. It lays courses of decking across the width, dividing by one board width plus the gap, then counts the boards from your board length and the total linear feet. It also estimates the joists at your spacing and a rough footing count. Everything runs in your browser, so you'll see the numbers shift the moment you change a dimension or a gap.

First find the courses by dividing the deck width in inches by one board width plus the gap between boards, then round up. Multiply the courses by the deck length to get the total linear feet of decking, and divide that by the length of the boards you'll buy to get the count. A 12 by 16 deck with 5.5-inch boards and a quarter-inch gap needs about 26 courses, which is 416 linear feet, or 26 sixteen-foot boards.

Most decks set joists 16 inches on center, which suits standard 5/4 decking and composite boards, while 12-inch spacing stiffens the deck for diagonal patterns or thinner boards. The joist count is the deck length in inches divided by the spacing, plus one to close the far end. This tool defaults to 16-inch spacing, so drop it to 12 for a sturdier frame, and the joist count updates right away.

A small gap, often an eighth to a quarter inch, lets water drain and gives the boards room to expand and contract with weather, which keeps them from buckling. Pressure-treated boards that arrive wet shrink as they dry, so some builders set them tight and let the gap form naturally, while dry or composite boards get a deliberate gap. The calculator folds your chosen gap into the course count, so the spacing is built into the estimate.

It depends on the beam layout and span, but a common approach puts footings every 6 to 8 feet along each beam line, with most decks running two beam lines. This tool gives a rough footing count from the deck length, which is a starting point, not a structural design. For a deck attached to a house or over 30 inches high, check your local code and have the framing plan reviewed before you dig.

Yes, it's completely free, with no sign-up, and it runs right in your browser, so nothing you enter leaves your device. Type the deck size, set the board width and gap, and read the boards, joists, and footings in a tap. Bookmark it for planning a deck build, pricing materials, or a weekend project, and you'll have an accurate starting estimate whenever you need one.