Deck Board Calculator

How many deck boards do I need?

A 12 foot wide deck takes 26 courses of 5 1/2 inch decking with a 1/8 inch gap between boards. The arithmetic is 144 inches plus one gap, divided by 5.625 inches per course, rounded up. Each course is the deck length divided by the board length: a 16 foot deck takes one 16 foot board per course, a 12 foot deck takes two 12 foot boards.

Where this goes wrong in practice is the board width. The board sold as a 2x6 measures 5 1/2 inches across. Run 6 inches through the same sum and 144 divided by 6 is 24 courses — two short of the 26 the deck actually needs. Two courses on a 16 foot deck is 32 linear feet, or two boards, and the mistake only shows up when the last row sits against a 9 inch strip of exposed rim.

The gap is part of the row count

An eighth of an inch sounds like nothing. Across a 24 foot wide deck it is 51 gaps, which is 6 3/8 inches of extra width — enough to change the course count by one, and enough that the last row is either a full board or a rip cut.

The gap does work you cannot see. Water that lands on the deck has to get off it, and boards laid tight trap it on the joists, which is where rot starts. Pressure treated wood also moves: boards straight off the rack are wet, and as they dry they lose a little width, so a deck laid with an eighth inch gap ends up with a wider one by the end of the first summer.

Composite is different again. You are not choosing the gap, the fastener clip is: hidden clip systems set 3/16 to 1/4 inch, and the manufacturer voids the warranty if you butt the boards. If you are using a clip system, pick that option here rather than the default — it changes the row count by one or two on a wide deck.

What length board to buy

Board length decides how many pieces you carry, not how much deck you get. A 20 foot deck with 16 foot boards takes two per course and throws away 12 feet of offcut on every course; the same deck with 20 foot boards takes one, and the offcut is nothing.

Long boards are heavy and awkward. A 20 foot treated 5/4 board runs about 30 pounds and behaves like a wet noodle in the air; two people can carry one, one person cannot. Twenty foot stock also arrives on a longer truck and some yards only deliver it with a minimum order. Sixteen is the practical ceiling for a two person crew, which is why it is the default here.

Stagger the joints or the deck reads as stripes

Every butt joint has to land on a joist — the board ends carry no load otherwise, and a joint floating between joists will lift within a season. That constraint is what forces the layout: on a 16 foot deck with 8 foot boards, each course is a full board plus a half, and if every course breaks at the same joist the deck gets a visible seam running its whole width.

Shift alternate courses by one joist bay so the breaks stagger. The cost is more cuts and a little more waste, which is why the waste box on this page starts at 10% and not 5%. On a 12 foot deck with 8 foot boards, the offcut is 4 feet, and a 4 foot piece is only useful as blocking.

Boards also come with a crown, one face slightly convex. Lay them crown up so the deck sheds water toward the gaps rather than cupping it, and run the grain of adjacent boards the same way or the light will catch them differently in patches.

Diagonal and picture-frame layouts cost extra boards

A 45 degree layout adds roughly 10% to the material and this page adds it for you. The reason is geometric rather than mysterious: the courses are longer than the deck is, and both ends of every course get a mitre that cannot be reused anywhere else. Picture framing the perimeter adds another run of boards around the edge plus eight mitred corners, and 15% is a fair allowance for the two together.

Diagonal also changes the framing under it. A board running at 45 degrees spans further between joists than the same board running square, so the code span table drops one step: 16 inches becomes 12, 24 becomes 16. If you plan to lay diagonal, decide before the joists go in, not after.

Joist spacing decides the board, not the other way round

Two things follow. First, the number of joists is the number of bays plus one: a 16 foot wide deck at 16 inches on center takes 13 joists, not 12. Second, if the deck has to carry a hot tub or a heavy grill, the decking span is not the limiting number — the joists and beam are, and those need their own sizing.

Fastening: two screws per joist crossing for 5/4 and 2x6, set about an inch in from each edge. Predrill composite and hardwood near the ends or the board splits. Hidden clips use one screw per joist plus a starter screw at the beginning of each course.

Boards are only one line of a deck takeoff. The joists and rim are framing lumber, which the lumber calculator sizes; the post footings are concrete, which the concrete calculator turns into cubic yards; and how deep those footings have to sit is set by the frost depth your state enforces. Do the footing depth first — it changes the tube length and the bag count, and it cannot be fixed after the concrete sets.

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Last updated: 2026-10-29