Deck Calculator

Decking is bought by the linear foot, so the useful number is how many rows of board it takes to cross the deck. Each row occupies its actual board width plus the gap — a nominal 2 × 6 is really 5.5 inches, so at a 1/8-inch gap each row covers 5.625 inches, not 6. Using nominal widths is the single most common way deck orders come up short.

Twelve feet is 144 inches; at 5.625 inches per row that is 26 rows. Each row spans the 16 ft length, so you need 416 linear feet of decking, or 458 with a 10% allowance — about 29 sixteen-foot boards. Joists running across the width at 16 inches on center come to 13 joists of 12 ft, plus the rim and ledger boards.

Pressure-treated lumber is usually sold wet and shrinks as it dries, so it is often laid tight or near-tight and opens up on its own. Kiln-dried and composite boards don't shrink, so they get a deliberate 1/8 to 3/16-inch gap for drainage and expansion. Composites also expand lengthwise with heat, which is why their instructions specify end gaps that vary with installation temperature — follow the manufacturer, not habit.

Sixteen inches on center is standard for most decking run perpendicular to the joists. Go to 12 inches for a diagonal layout, for thinner boards, or where a composite manufacturer requires it — many void their warranty above 16 inches, and some demand 12 for a 45-degree run. Tighter spacing means more joists and more hangers, so decide the pattern before you order framing.

A decking total says nothing about the structure beneath it: posts, footings, beams, joist hangers, ledger flashing, and hidden fasteners or screws at roughly 350 per 100 sq ft. Framing hardware and fasteners must be rated for treated lumber, since modern treatments corrode ordinary steel quickly. Ledger attachment and footing depth are code-governed and inspected in most jurisdictions — check before you dig, not after.