The deck calculator takes plan dimensions and a joist spacing and returns the area, the joist count and the number of decking rows, and totals them at the prices you enter. It sizes the platform you will walk on.
It does not design the structure. Footings, beams, posts, the ledger connection, guards and stairs come from a prescriptive deck guide or an engineer, and they are what make two similar-looking deck quotes differ by thousands of dollars.
How to use it
- 01
Enter length and width
Use the plan dimensions of the walking surface. Length is the dimension the joists are spaced along; the decking boards run parallel to it, and their rows are counted across the width. If your joists run the other way, swap the two numbers.
- 02
Choose the joist spacing
The residential code caps it by the decking (Table R507.7). Under 1¼-inch wood deck boards laid perpendicular to the joists, joists go no more than 16 inches on center where each board crosses three or more joists and 12 inches where a board rests on only two; under diagonal boards, the limits are 12 and 8 inches. Two-inch wood boards allow 24 inches on center, the widest in the code's wood decking table, or 18 inches where diagonal boards rest on only two joists. Plastic composite boards, or their packaging, carry a label with their maximum span, and the joists go no farther apart than it. The joists' own span then shrinks as the spacing widens; the deck span guide has the code's tables.
- 03
Pick a surface
Pressure-treated wood, cedar or composite. The choice goes into the Project Brief and changes nothing in the count: the framing is the same under any surface, and you price the decking you picked.
How the math works
Joists
Joists = the length in inches divided by the spacing, rounded up, plus one. Rounding up means the last bay is never wider than the spacing you chose. A 16-foot length at 16 inches on center is 12 bays and 13 joists.
Decking rows
Rows = the width in inches divided by 5.5, rounded up. That is the dressed face width of a nominal 6-inch board, and the formula leaves no gap between boards. Real decks have a small gap, which means slightly fewer rows, so treat the count as a ceiling. A 12-foot width needs 27 rows. The American Wood Council's prescriptive deck guide (DCA 6) goes further than the code: it has every decking piece bear on at least three joists, which rules out short offcuts at the ends of rows.
What is not counted
Footings, posts, beams, rim joists, blocking, joist hangers, fasteners, the ledger and its flashing, stairs and guards. Those belong in the structural scope, and the Project Brief keeps them as separate lines so contractors price the same job.
Pricing it
Decking = rows × the deck length × a price per linear foot of board, because every row runs the full length; joists = the joist count × a price per joist. A 16-by-12 deck has 27 rows of 16 feet, or 432 linear feet of decking. Installed = the area × a quoted price per square foot. No price is filled in. The one published deck price, an estimate for a single 16×20 deck, is on the deck cost page, and its publisher's terms keep it out of calculators. Carpenters earned a median $29.12 an hour in the U.S. in May 2025, $36.50 in California and $23.51 in Texas (BLS), a wage rather than a contractor's rate. Beams, posts, footings, hardware, railing and stairs are not counted and belong on the quote as their own lines.
Where the number goes wrong
- Ignoring height. A walking surface more than about 30 inches above grade is the model-code pattern that brings guards, and often a permit, into the project.
- Spacing joists for the wrong decking. Under wood boards the code allows 24 inches on center only for 2-inch boards, or 18 inches where diagonal boards rest on only two joists; 1¼-inch boards need 16 inches or less, and a plastic composite board's label gives its own maximum span.
- Leaving stairs out of the plan. Stairs add framing, footings, guards and labor that the area number never shows.
- Measuring to the outside of a proposed railing instead of the walking surface.
- Treating a ledger as a shortcut. Attaching to the house adds flashing, fastener and lateral-tie details that many building departments want to see drawn.
Questions people ask
How many joists does a 16-by-12 deck need?
With joists spaced along the 16-foot length at 16 inches on center, 13. At 12 inches on center, 17. Add rim joists and blocking on top of that count.
What joist spacing should I use for composite decking?
No wider than the maximum span on the boards' label. The residential code requires plastic composite deck boards, or their packaging, to be labeled with their allowable load and maximum span, tested to ASTM D7032, and caps the joist spacing under them by it. Its own figures are for wood: 16 inches on center for 1¼-inch boards across three or more joists.
Does a deck need a permit?
Not under the model code for a deck of 200 square feet or less, no more than 30 inches above grade at any point, not attached to the house and not serving the required exit door. Austin and Los Angeles set their own lines; the deck permit brief compares them.
How deep do deck footings go?
At least 12 inches below undisturbed ground under the model code, and where the deck is attached to a house on frost-protected footings, below the local frost line, on solid rock or by another approved method. Austin and Los Angeles both put their frost line at 12 inches. A free-standing deck needs no footings when its joists rest on grade over their entire length, or when its joists bear directly on precast concrete pier blocks at grade with no beams or posts, the deck is 200 square feet or less, and its surface is no more than 20 inches above grade within 36 inches of its edge.