How the deck board count is worked out
The calculator treats the deck as a rectangle with the boards running parallel to the length you enter and the joists running across the width. Rows of boards across the width = width in inches ÷ (board width + gap), rounded up. Boards per row = length ÷ board length, rounded up, because a 14 foot run still takes a 16 foot board. Multiply the two, add the waste percentage, and round up again to get the boards to order.
Joists are spaced along the length at the on-centre distance you pick: length in inches ÷ spacing, plus one for the end. Each joist is as long as the deck is wide. Screws are two per board row at every joist, with the same waste factor; most deck screws come in boxes of 350 or buckets of 1,750.
Choosing the inputs
Use the actual face width of the board, not the nominal size: a 5/4 x 6 or 2 x 6 is 5.5 inches, a 2 x 4 is 3.5 inches, and composite boards are usually 5.5 inches. The gap for wet pressure-treated lumber is often zero because it shrinks; for kiln-dried or composite boards use 1/8 to 1/4 inch. Pick the board length that minimises cut-offs for your run; two 8 foot boards on a 16 foot run leaves no waste but adds a butt joint over a joist.
What this does not cover
The count is decking, joists and screws only. Beams, posts, footings, the ledger board, rim joists, joist hangers, blocking, stairs and railings need their own takeoff. Diagonal and herringbone layouts use the same board area but cut far more waste, so raise the waste to 15% or more. Always confirm joist spacing against the decking manufacturer's span table and your local building code before you order.