Deck boards & joists
Count the deck order the way the yard sells it: joists, decking by stock length with the gap included, and screws.
Result
Joists span the 16 ft direction — whether one 16 ft joist can, and what size it must be, is the span table's call, not this count's.
How is this calculated?
Using the tool
The deck order, counted the way the yard sells it: joists spanning the length at your spacing, decking crossing them with the gap already in the math, boards by the stock length you'll actually buy, and the screw bucket at two per crossing. Structure stays with your plans — this counts the lumber, it doesn't size it.
The count
Two counts run perpendicular to each other, and each one divides its own direction. Joists work like studs turned flat: they span the length, and you count them across the width — one per space plus the starter, rims included, so doubling the rims is extra. Decking crosses them, so a row spans the width and the rows stack along the length, divided by what one row really covers: the board face plus the gap— a 5½" board with a ¼" gap covers 5¾", and that quarter inch is a full extra board on a long deck if you forget it. Rows × boards-per-row gives the order; two screws at every board-joist crossing fills the bucket.