Deck Calculator: Boards, Joists and Screws

It adds framing and fasteners to the board count. Alongside the deck boards, it works out how many joists run across the width at your chosen spacing plus one closing joist, and how many screws you need at two per board over joist crossing. A simpler board calculator returns only the surface board count.

About 1/8 inch, or 0.125 in, is common for kiln dried lumber that has already shrunk, which is the default. Wet pressure treated boards can be butted nearly tight since they shrink as they dry, so they often close up to a gap on their own. Composite boards follow the manufacturer's spec, which can be wider.

Sixteen inches on center suits most residential decks and standard composite boards, and it is the default here. Twelve inches adds stiffness for diagonal patterns or thinner boards, while some composites require it. The tool counts joists as the width in inches divided by the spacing, rounded up, plus one closing joist at the far edge.

It assumes two screws wherever a board crosses a joist, so screws equal boards times joists times two. In the 16 by 12 foot example that is 57 boards over 10 joists at two each, which gives 1140 screws. Hidden fastener clips follow a different count, so switch to the clip maker's coverage if you use them.

Around 10 percent covers saw cuts, board ends and the occasional cull board, and it applies to the board count only. Diagonal or herringbone layouts waste more off each angled cut, so raise it to 15 percent or more for those. Waste does not change the joist or screw counts, which come from the deck geometry.

Board width and length change the board count, because each board covers its width plus the gap, times its length, so wider or longer boards fill more area. That in turn changes the screw total, since fewer boards mean fewer crossings. Board size does not affect the joist count, which depends only on the deck width and joist spacing.

Deck length, width and board length are in feet, while board width, gap and joist spacing are in inches. Results are in square feet and whole units. There is no metric mode, so convert any meter based plans to feet and inches before entering them.

No. It covers decking boards, joists and screws only. Beams, posts, footings and ledger connections depend on span tables, soil and local code, so size those from an approved deck plan or a structural reference rather than this estimate.

Yes. Every calculation runs in your browser with no sign in and no upload, so your deck dimensions never leave your device, and the tool keeps working offline after the page loads.