How many deck boards do I need?

Divide the deck width by the board width plus the gap to get the number of rows, then work out how many stock boards each row takes. A 16 × 12 ft deck with 5½ in boards and a ¼ in gap needs 12 × 12 ÷ 5.75 = 26 rows; at 16 ft stock that is one board per row, so 26 boards. If the deck were 20 ft long you would need two boards per row and accept the offcut.

Take it from the code table, not from a rule of thumb. 16 in on centre is the most common for solid-sawn framing with dimensional decking, 12 in is often required for composite or for boards run diagonally, and 24 in only works with specific decking products and spans. The right answer depends on species, grade, joist size, span and whether there is a cantilever — which is exactly why it is an input here rather than something this tool decides.

No, and deliberately. Joist and beam sizing, post sizing and footing depth are prescriptive-code questions (IRC R507 plus whatever your jurisdiction amends) that turn on species, grade, load and frost depth. A calculator that produced a number from a length and a width would be inventing engineering. This tool counts material for the spacing and sizes you specify; the sizes come from the tables or your designer.

It depends on the moisture in the board. Wet pressure-treated lumber shrinks as it dries, so it is normally laid tight or with about ⅛ in and opens up on its own. Kiln-dried and composite boards are already at their working size and are usually set at around ¼ in. Composite manufacturers publish a required gap for both width and butt joints — follow theirs, since it affects the warranty.

This tool already builds in the real source of deck waste by counting boards per row rather than dividing total linear feet, so a 20 ft deck on 16 ft stock is charged for the offcut it actually produces. On top of that, add material for stairs, railing, picture framing and any angled or herringbone pattern — those can add well over 10% and none of them are in the rectangle you measured.