Support: Deck Material Calculator

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Common questions

How many deck boards do I need?

Divide the deck's width by one board's face width plus the gap to get the rows, then multiply by the boards per row for the length you buy, and add about 10% for cuts. A 16 by 12 ft deck in 5 1/2 inch boards with a 3/16 inch gap is 26 rows of one 16 ft board, so 29 boards with waste. The calculator does this for any size.

How many joists for a 16 ft deck?

At 16 inches on centre, 16 ft divided by 16 inches is 12, plus one for the far end: 13 joists, plus two rim joists. At 12 inches it is 17. Joists run the way the deck is wide, so their length is the deck's width.

How many screws per deck board?

Two at every joist the board crosses. A board 16 ft long over joists at 16 inch centres crosses 13 joists and takes 26 screws; a 16 by 12 ft deck with 26 rows takes about 676, so buy 700 to 750. Hidden clip systems take one clip per crossing.

How wide is a 5/4 × 6 deck board?

5 1/2 inches, the same as a 2 × 6. The 6 is the nominal size before planing. Composite boards are made to 5 1/2 inches too. The calculator uses the actual width, and Other width takes any measurement.

What gap should I leave between deck boards?

About 1/8 to 1/4 inch (3 to 6 mm) for dry timber and composite, or as the board maker states. Wet pressure-treated boards are often laid tight because they shrink as they dry. The gap changes the row count, so set it before you buy.

Does diagonal decking need more boards?

Yes. It covers the same area but every board end is a mitre cut and the offcuts are triangles, so allow at least 15% extra, and the joists usually need to be closer, 12 inches on centre. Tick Boards laid diagonally and the calculator uses 15% or your own figure if higher.

Is this calculator free?

Yes, completely free, with no sign-up. The numbers are worked out in your browser and never sent to us.