🪵 Deck Board Calculator

Enter your deck dimensions, board width and gap spacing to find out how many decking boards to buy.

Deck Length (ft)

Deck Width (ft)

Board Width (in)

Gap Between Boards (in)

Standard Board Length (ft)

How the board count is worked out

Decking is counted across the deck, not by area. Each row occupies its own width plus one gap, so the deck width in inches is divided by that pitch and rounded up to give the number of rows. Every row is as long as the deck, and the total length is then divided by the board length you buy.

rows          = ceiling(deck width(in) / (board width + gap))
linear feet   = rows x deck length(ft)
boards        = ceiling(linear feet / board length)

With the defaults - a 16 x 12 ft deck, 5.5 in boards, a 1/4 in gap - the pitch is 5.75 in, so 144 in of width takes 26 rows. That is 416 linear feet, or 35 twelve-foot boards.

Buy more than the linear feet

The board figure is bare arithmetic with no allowance in it, and decking loses material in ways the sum cannot see:

Boards have to break over a joist. A 12 ft board cannot reach across a 16 ft deck, so each row needs a butt joint cut back to the nearest joist centre - and the offcut is usually too short to reuse.
The last row rarely lands flush. 26 rows at 5.75 in is 149.5 in against 144 in of deck, so that row gets ripped down the length and the strip is scrap.
Cupped, crowned and split-ended boards get culled on the pile, and pressure-treated stock is the worst for it.

Add 5 to 10% to the linear feet - here that is 37 to 39 boards rather than 35 - and more again for a diagonal or herringbone layout, where every board is cut at an angle at both ends and 15% is closer to the mark.

Nothing on this page says anything about the frame underneath. Joist size, span and spacing are set by your local building rules and by the board manufacturer's span table, and those override any figure here. Composite and 5/4 boards in particular often require 12 in centres rather than 16 in, which does not change the board count but does change the deck you are screwing them to.

Frequently asked questions

How many deck boards for a 16x12 deck?

26 rows of 5.5 in boards at a 1/4 in gap, which is 416 linear feet. That is 26 sixteen-foot boards with no joints, or about 39 twelve-foot boards once you allow for butt joints and waste.

What gap should I leave between deck boards?

About 1/4 in for wood that arrives dry, and closer to 1/8 in for wet pressure-treated stock, which shrinks as it dries and opens the gap on its own. Composite boards follow the manufacturer's figure, which is usually wider because they move with temperature rather than moisture.

Should I buy 12 ft or 16 ft decking boards?

Match the board length to the run if you can. Boards that span the deck in one piece use less material overall and leave no butt joints, which are the first place water sits and the first place a deck starts to rot.