🪵Deck Calculator

About this tool

Enter the deck's length (the direction the boards run) and width, the board width and gap, the length of the boards you'll buy, the joist spacing and a waste allowance. You'll get the deck area, boards to buy, the joist count and length, and the screws or hidden clips needed. Add a price per board to estimate the decking cost.

How decking quantities are worked out

Each row of decking covers one board width plus the gap between boards, so the number of rows is the deck width divided by that coverage, rounded up. A 12 ft (144") deck with 5½" boards and 3/16" gaps needs 26 rows. Each row needs the deck length divided by the board length, rounded up: with 16 ft boards on a 16 ft deck, that's one board per row. Buying matching lengths avoids butt joints, while longer decks need staggered joints that land on a joist. Add 5–15% for cuts, defects and mistakes (10% gives 29 boards here, and diagonal patterns need more). Joists run across the boards at regular centres. 16" (400 mm) is standard for most timber and composite decking, and 12" (300 mm) is common for diagonal boards, thinner boards or some composites, so check the maker's span table. A 16 ft deck at 16" centres has 13 joists. Face-screwing uses two screws per board at every joist, which comes to 676 here, so buy a 700-screw box. Hidden clips need about one per board per joist. Honest limits: this covers the decking and joists only. Beams, posts, footings, ledger fixings, stairs, railings, fascia and flashing depend on the height and your local building code. Raised or attached decks may need permits and an inspection. The fence calculator, the concrete calculator and the stair calculator are related.

Frequently asked questions