Deck Calculator

Plan a deck down to boards, joists and footings, and see why one extra foot matters.

Your deck

The deck

Boards are assumed to run along this dimension. Matching it to a stock board length of eight, twelve or sixteen feet is what keeps the offcut pile small.
The direction the joists span. This is what decides how many rows of decking there are.

The boards

Actual, not nominal. A 1x6 deck board is 5.5 inches wide, and using six would leave you roughly one board short in every twelve.
A quarter inch is normal. Wet pressure treated timber shrinks as it dries, so fit those tighter, around an eighth, and let the gap open up on its own.
A row longer than one board needs a butt joint, and a joint has to land on a joist. Buying the longest board that covers the run in one piece avoids that entirely.

The frame

Sixteen inches is the default for boards laid square. Diagonal decking, and most composite boards, need twelve. Check what the board manufacturer requires before framing.
A rough grid, not a structural answer. Beam size, joist span and local code all change it, and a deck more than a little way off the ground should be checked against the code before you dig.
Ten percent on a plain rectangle. Go to fifteen for a deck with angled corners or a picture frame border, where the offcuts are larger and more numerous. 10.00%
Prices
The price for one board at the length selected above. Composite costs several times what treated pine does, which is where most of the difference in a deck quote comes from.
Priced at the length that spans the width. Beams, hangers, fixings and footings are not counted here.
Decking boards to buy
29 boards
Rows of decking 26 rows
Joists 13 joists
Footings 9 footings
Deck area
192 sq ft
Decking in linear feet
416 ft

What it costs

Material cost $831
Cost per square foot $4.33

Every figure here is a planning estimate, not a professional takeoff. Yields, waste factors and code requirements vary by product, supplier and jurisdiction, so confirm quantities against the manufacturer’s data and your local building department before you order.

Generated by Mathterial

A deck is two counts stacked: the boards you walk on, and the frame that carries them. This deck calculator works out both, along with the footings, and it exposes the single decision that moves the bill more than any other.

Match the deck length to a stock board length

Decking is sold in fixed lengths. A board that has to span further than one stock length needs a second board, and the offcut is rarely long enough to start another row. So the cost does not rise smoothly with the size of the deck. It steps.

A 16 by 12 foot deck built from 16 foot boards takes 29 boards, about $831 in decking and joists. Add one foot:

  • 16 by 12: 29 boards, about $831.
  • 17 by 12: 58 boards, about $1,498.
  • 24 by 12: 58 boards, about $1,566.

That extra foot nearly doubles the decking bill, and the next seven are almost free. Every row on a 17 foot deck needs a second board for its last foot, and the rest of that board is offcut. If the design has landed at 17 feet, you are paying for a 24 foot deck either way, so either pull it back to 16 or build the deck you have already bought.

The frame underneath

Joists run across the width at fixed centers along the length, with one closing each end, so a 16 foot deck at 16 inch centers takes 13 joists. Spacing is a structural decision rather than a budget one:

  • 24 inch centers: 9 joists. Only where the decking is rated for it.
  • 16 inch centers: 13 joists. The standard for most solid wood decking.
  • 12 inch centers: 17 joists. Required by many composite boards, and by any diagonal layout, since a diagonal board spans further between joists than a square one.

The American Wood Council’s DCA 6 is the prescriptive guide most residential decks are built to, and it requires every piece of decking to bear on at least three joists. Check the decking manufacturer’s span table before settling this. Composite laid at 16 inch centers when it wanted 12 will feel soft underfoot within a season, and the fix is rebuilding the frame.

Board width changes the row count

A 2x6 measures 5.5 inches and needs 26 rows across a 12 foot deck. Narrower 3.5 inch boards need 39 rows and 43 boards, which is more material, more fasteners and considerably more labor for the same area.

The quarter inch gap between boards is not optional. It carries drainage and allows for seasonal movement, and it is counted above as part of the row pitch.

Footings and what this page leaves out

A 16 by 12 deck at 8 foot footing spacing needs 9 footings on a three by three grid. For the concrete going into them, the post hole concrete calculator subtracts the post and gives you the bag count.

Beams, ledger boards, hangers, fasteners, railings and stairs are all separate quantities and all subject to code. This page sizes the deck surface and its joists, not the structure that carries it to the ground.

Deck calculator FAQ

How many boards do I need for a 16 by 12 deck?

About 29 boards using 16 foot lengths of 5.5 inch decking with a quarter inch gap, which is 26 rows plus a 10% allowance. Building the same deck one foot longer takes 58 boards, because every row then needs a second board.

How far apart should deck joists be?

Sixteen inch centers for most solid wood decking, 12 inches for many composites and for any diagonal layout. A 16 foot deck takes 13 joists at 16 inch centers and 17 at 12. Always check the decking manufacturer span table.

Why does making a deck slightly longer cost so much more?

Because decking comes in fixed lengths. A 17 foot deck built from 16 foot boards needs two boards per row instead of one, so the board count doubles from 29 to 58 while the deck grows by only a foot.

How many footings does a deck need?

A 16 by 12 deck at 8 foot footing spacing needs 9, set out on a three by three grid. Spacing depends on beam size and species, and footing depth is governed by your local frost line.