Deck calculator

Divide the deck width across the boards, in inches, by the board width plus the gap to get the number of rows, then multiply by how many boards it takes to run the length of each row. A 12 foot wide deck with 5.5 inch boards and a 3/16 inch gap is 26 rows. If the boards run 16 feet on a 16 foot deck, that is 26 boards before extra for cuts.

Divide the deck length in inches by the joist spacing, round down and add one. At 16 inches on center, a 16 foot deck has 13 joists. The right spacing depends on the decking: many composite and PVC boards call for 16 inches or less, and diagonal decking often needs 12 inches, so check the board maker's instructions.

Face screwing usually puts two screws through each board at every joist, so multiply board rows by joists by two. A deck with 26 rows over 13 joists takes about 676 screws, plus a few extra for waste. Hidden fastener systems have their own counts per square foot on the package.

Common guidance for wood decking is around 1/8 to 1/4 inch between boards so water drains and the wood can move, with 3/16 inch a frequent choice. Composite and PVC boards list their own gaps based on temperature and board length, so follow the manufacturer.

No. Joist sizes, beam spans, post spacing, footing depth and how the ledger attaches to the house come from span tables and local building code, and many towns require a permit and inspection for a deck. This tool is a material count for the surface and joists.