How many deck boards do I need?

Divide the depth of the deck by the board width plus the gap to get the rows, then count the boards each row takes. A 16 x 12 foot deck in 5.5 inch boards is 26 rows of 16 foot boards.

What joist spacing do I need?

16 inches on center is standard. Many composite boards need 16 inches or closer, and 12 inches for a diagonal pattern.

How much extra decking should I order?

About 10% for straight boards, and 15% for a diagonal pattern or a picture-frame border.

What size joists do I need?

It depends on the span, the wood and the load. Check the joist span table in your local residential code, which in most of the US is IRC table R507.6.

Deck calculator

Deck boards, joists and screws for a rectangular deck.

Deck boards

0 boards

How it works

rows of boards = depth x 12 / (board width + gap), rounded up
boards         = rows x boards per row, or rows / rows per board when two fit in one length
joists         = width x 12 / spacing, rounded up, + 1
screws         = rows x joists x 2

Boards run along the house, across joists that run out from the ledger to the beam. Each row takes one board when the deck is no wider than a board, two boards when it is wider (with the joint over a joist), and a short deck gets two rows out of each board. Joists are counted including the two at the ends, each the depth of the deck long, plus the ledger and the rim along the width.

Wood decking usually goes down with a 1/8 inch gap and composite with 3/16 or what the maker's install guide says. Most composite boards need joists at 16 inches or closer, and 12 inches when the boards run on a diagonal. Joist size depends on the span, the species and the load: check the span table in the residential code your area uses (IRC table R507.6) before you buy lumber.

Questions

How many deck boards do I need?: Divide the depth of the deck by the board width plus the gap to get the rows, then count the boards each row takes. A 16 x 12 foot deck in 5.5 inch boards is 26 rows of 16 foot boards.
What joist spacing do I need?: 16 inches on center is standard. Many composite boards need 16 inches or closer, and 12 inches for a diagonal pattern.
How much extra decking should I order?: About 10% for straight boards, and 15% for a diagonal pattern or a picture-frame border.
What size joists do I need?: It depends on the span, the wood and the load. Check the joist span table in your local residential code, which in most of the US is IRC table R507.6.

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