How the deck calculator works
Boards across the width, joists underneath, screws at every crossing. The calculator divides the deck width by the board width plus gap to get the boards across, multiplies by the number of board lengths needed to cover the deck length, then counts joists at your spacing plus one. Screws are 2 per board at every joist. A 12 ft x 16 ft deck with 5.5 inch boards needs 26 boards with 16 ft boards (52 with 8 ft boards), 10 joists at 16 inch spacing, and 520 screws.
Longer boards mean fewer joints and a cleaner deck. Leave the 0.125 inch gap between boards for drainage and expansion — without it, boards cup and trap water.
Building the fence to match? Use the fence calculator.