Deck Board Calculator
Work out how many deck boards it takes to cover your deck, accounting for board width, the gap between boards, and a waste percentage.
How it works
Boards run the length of the deck, so what matters for the count is the width they have to span. We convert the deck width to inches, then figure out how far one board plus its gap reaches across.
A common composite or 5/4 board is about 5.5 inches wide, and boards are spaced with a small gap — a quarter inch is typical — so water drains and the wood can breathe. Each board effectively covers its own width plus that gap.
Divide the span by the width-plus-gap and round up to get the number of rows. A 12-foot-wide deck with 5.5-inch boards and quarter-inch gaps needs 26 rows. Then we add your waste percentage for bad cuts and defective boards.
Frequently asked questions
What gap should I leave between deck boards?
An eighth to a quarter inch is standard. Pressure-treated lumber that's still wet will shrink as it dries, so people often butt those tighter, while dry or composite boards get the full gap since they won't shrink much.
Does this count the length of each board?
No — it counts how many rows of board it takes to cover the deck's width. Buy each row long enough to run the deck length, and if your deck is longer than the boards you can get, plan for butt joints over a joist.
How much waste should I add?
Ten percent covers the usual bad cuts, split ends, and the occasional warped board you set aside. Add more if your deck has angles, a picture-frame border, or a lot of stair treads that eat into full-length boards.