AstraCalculator

Deck Board Calculator

Enter the deck size and the board you are buying. This counts the rows, the boards in the length you want, and the screws to fix them.

Measured along the direction the boards run.

The name on the label is nominal. The board is narrower than it says.

Wet treated boards shrink as they dry, so they go down tighter. Dry and composite boards need the gap built in.

How the math works

Rows = deck width in inches, divided by one board width plus one gap, rounded up. Boards = rows x the number of stock lengths needed to cover the deck length. A 5/4 x 6 board is 5.5 inches wide, not 6, so a 12 foot wide deck at a 3/16 gap needs 26 rows.

Common questions

How many deck boards do I need for a 16 by 12 foot deck?
About 26 rows of 5.5 inch boards at a 3/16 inch gap. With 16 foot boards that is 26 boards, one per row and no joints, which is why people size decks to the board length they can buy.
What gap should I leave between deck boards?
Three sixteenths of an inch for normal timber. Wet pressure treated boards can go tighter at an eighth, because they shrink as they dry. Composite needs its stated gap for expansion, usually a quarter inch.
How many screws per deck board?
Two at every joist. At 16 inch spacing a 16 foot board crosses 13 joists, so 26 screws per board. Order ten percent over, because some will strip.
Should boards run the long way or the short way?
Whichever avoids joints. Running boards across the short dimension often means one board per row with no butt joints, which looks better and gives water fewer places to sit.

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