Deck calculator
Enter the deck footprint and the boards you plan to use. The calculator counts decking boards by length, joists by spacing, and the screws or hidden fasteners to attach it all, with waste built in.
How the deck calculator works
Deck boards run in one direction, so the calculator divides the deck width by the board width plus gap to find how many rows you need, then divides the length by the board length you plan to buy to find boards per row. Multiplying the two gives total boards, and waste covers cuts and the occasional warped board.
Joists run perpendicular to the decking, spaced 16 inches on center for most lumber and some composites, 12 inches for diagonal patterns or softer composites. The count is the deck length in inches divided by spacing, plus one for the end. Fasteners are figured at two screws per board per joist, or one clip per board per joist for hidden systems.
Formula
rows = width in inches ÷ (board width + gap)
boards = rows × (length ÷ board length), each rounded up, then × (1 + waste)
joists = length in inches ÷ spacing + 1
Example: a 16 × 12 ft deck with 5.5 in boards and 1/4 in gaps needs 12 × 12 ÷ 5.75 = 25 rows. With 16 ft boards that is one board per row, 25 boards, or 28 with 10 percent waste. Joists at 16 in on center: 192 ÷ 16 + 1 = 13 joists, 12 ft long. Screws: 25 × 13 × 2 = 650.
Choosing materials
- Pressure-treated pine is cheapest. Leave 1/4 in gaps because wet boards shrink as they dry.
- Cedar and redwood look better and resist rot naturally but cost two to three times more.
- Composite never needs staining and uses 1/8 in gaps, but most brands require 12 or 16 in joist spacing; check the manufacturer's span table.
- Buy boards in the longest length that avoids seams. A seam in the middle of a deck needs a double joist or blocking underneath.
- Footings for the posts are easy to size with the concrete calculator.
Common questions
How many deck boards do I need for a 12x16 deck?
About 25 boards of 5.5 in decking in 16 ft lengths, or 28 with waste. If you use 8 ft boards instead, double the count to 50 plus waste and plan the seams over joists.
How many screws per deck board?
Two screws at every joist. A 16 ft board over joists 16 in apart crosses 13 joists, so it takes 26 screws. Budget about 350 screws per 100 square feet of deck.
What joist spacing do I need for a deck?
16 inches on center for 5/4 and 2x wood decking laid straight. Use 12 inches for diagonal decking, and check the span chart for composite, since many brands require 12 or 16 inches.
How much does a deck cost in materials?
Pressure-treated lumber decks typically run $15 to $25 per square foot in materials for the deck surface, framing, and railing. Composite surfaces add $8 to $15 per square foot. Enter your local board prices above for the decking and joist portion.